Iris

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winndigital
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Joined: Jul 15 2003

Can someone explain to me more about the iris on camaras and when best to use it

harlequin
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Joined: Aug 16 2000

my understanding is that the iris allows more or less light to travel thru the lens and hit the cmos chip.

therefore you 'open up' the iris to let in more light and 'close down' the iris to lessen light entering.

when would you use it:

1. less light on a very bright day to stop everything getting 'washed out'
2. more light when shooting a 'twilight scene'

many cameras work better if you leave them to work out the iris settings , but , if you want to use the manual setting , play around and see what it does for you.

Gary MacKenzie

sepulce@hotmail.com ( an account only used for forum messages )

Thinkserver TS140 , 750ti Graphics card  & LG 27" uws led backlight , Edius 8

Humax Foxsat HD Pvr / Humax Fox T2 dvbt

tom hardwick
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Joined: Apr 8 1999

Good answer Gary. The beauty of our camcorder viewfinders and side screens is that they're wysiwyg pieces of kit. You're seeing on the screen all the settings you've made - to white balance, shutter speed, focus and so on. So when you twiddle the dial to vary the aperture, you can see instantly the effect you're having on the look of the finished footage.

The iris on cameras is just like the iris in our eyes. On bright days the iris closes down so as to avoid over exposing our retinas, and in the same way the iris in a camcorder closes to avoid overloading the sensitive CCD.

Many manufacturers these days are shying away from using diaphragm blades to control the aperture - see my review on Sony's PDX10 in this month's CV for more on this. I'm in the process of testing Canon's new MVX3i and this camera uses the same idea of having multiple ND filters to control the exposure on the chips rather than having closing diaphragm blades.

Why? Because small apertures combined with short focal lengths on tiny chips = blurry pictures. Diffraction quickly robs sharpness, so manufacturers would rather up the shutter speed and add more ND than close the blades to f8 or 11.

Now you Panasonic MX350/500 owners will be writing in saying , "Hey, my footage is sharp even at f16". But it ain't all it seems. Hitting 'Display' on replay of the tape is only giving you a theoretical aperture reading. Most likely as not a reading of f11 is nothing more than f4 with gobs of ND added to soak the light. We're easily fooled, then quickly pleased.

tom.

winndigital
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Joined: Jul 15 2003

Thank you all for your input and help in explaining the iris

MrWhite
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Joined: May 18 2003

What, you gotta be kidding me. I’ve never heard such a load of old amateur twaddle, sure the iris is used for exposure up to a very small point, but that’s only the half of it, and the smallest half at that. It’s main purpose (at least to a professional cameramen) is for “Depth of Field” you know, the effect which gives your scenes a particular feel and mood i.e. a shallow or deep “Depth of Field” will drastically effect the way a shot looks.

Tom, how you can say Gary's answer was a good one is unbelievable, sorry Gary, but your answer is typical of an uneducated amateur filmmaker that doesn’t have a clue what he is talking about. My guess is that you haven’t been to filmschool or any professional BBC cameraman courses, maybe just a couple of IOV courses, but last time I spoke to the chap that organises the IOV shooting courses he did tell me that the first half of the day on "DV camcorder technique" courses usually gets wasted as the lecturer try to convince the sorry students that you “Have to use the camcorder on full manual mode to get professional results” PERIOD!!

Go and rent some Hollywood blockbusters and notice how the background is out of focus on close intimate “Two Shots” (that’s two people). This emphasizes the actor and actress and not the picture on the wall in the background.
On the other hand you would require a very deep depth of field (say F16/F22 at the shorter end of the zoom range) if you had say two people in the foreground and there was someone spying on them in the background, here you would want the spy in the background in sharp focus too, unless of course you intended pulling off a “Pull Focus” shot, but that’s another story.

The sharpest aperture setting for most professional lenses is generally around the mid point, this would normally be around F4 or F5.6. Most professional lenses will perform more accurately in this range than they do at the extreme wide open or closed down settings, this is known as the ‘sweet spot’. By setting your lens to it’s mid-point will produce better colour contrast and sharper definition.

A small aperture will loose saturation and a wide aperture will loose sharpness so by having your all-singing-all-dancing MiniDV camcorder set to the so-called “Auto-Iris” will give you nothing but garbage. Imagine if you are shooting in the sun, the cameras auto setting will close the iris down giving you deep depth of field with loss in colour saturation and if you are shooting in poor light it will open up giving you a narrow depth of field with a soft image.

Who knows, maybe you’re happy with these inferior shots, or maybe you think that the shots look fine, trust me I haven’t even seen any of these shots taken on “Auto Iris” but I know they are crap, go show them to a professional and see for yourself.

For master shots a setting of F5.6 or F8 is nice as it gives a nice depth of field that’s not too deep or too shallow. For a tight shot or a nice intimate two shot, F4 or F2.8 will throw the background out of focus nicely giving the image a more three dimensional look as it separates the actors from it.

You should choose an aperture setting that gives you the depth of field you desire for the shot, then if you need more light, bring in some lighting equipment or reflectors, and if you need less light, then cut the light entering the lens by using neutral density filters. This is how professionals set the exposure, by lighting the set or using ND’s, not using the silly “Auto” function, this is for wedding videographers who always use the silly excuse of “but I don’t have time to do it manually as the light changes too quick”, what a joke, I can change the exposure quicker and more accurately than the cameras so-called “Auto” function ever could, believe me, just go and try it and once you get used to it and see the difference you will never turn back.

Cinematography is about controlling light physically, not automatically, anything shot in auto mode will look bad, take the unmistakable shift in focus as the auto-focus searches for something to lock onto for example, or the nauseating displacement in the depth of field as the auto-iris compensates for the changes in light levels. These examples are all unmistakable signs of cheap video and lazy people and indicates that the programme or movie should not be taken seriously. By all means use the automatic iris button temporarily to get the basis for your exposure setting, but then always keep the camera in its full state of manual.

[This message has been edited by MrWhite (edited 04 January 2004).]

[This message has been edited by MrWhite (edited 04 January 2004).]

PaulD
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Joined: Aug 31 2002

Hi
Essentially correct Mr White (are you a descendant of the esteemed Sir Thomas Bouch?), but it isn't necessary to be trained on a BBC Cameraman course to make good DV movies - I reckon its better to learn how to tell a good story visually more than to concentrate on shooting everything at f5.6.
The auto iris function of modern cameras does a pretty good job in most circumstances, allowing the story to be told, and without the full resources of a full lighting crew its not really practical to shoot everything at a pre-set aperture in the average movie project undertaken by subscribers to this forum.
Regards :–)

[This message has been edited by PaulD (edited 04 January 2004).]

MrWhite
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Joined: May 18 2003

That's fair enough Paul. I just wanted to point out the professional way of using the Iris control, as it wasn’t fully explained. Even though a lot of people won’t have the resources to use it the proper way I believe people should still have the full knowledge and not just the instruction manual version of what a function does. You have to know what the rules are before you can go and break them right?

In hindsight I suppose any movies is better than no movie.

But please don’t use this quote as an excuse for shoddy and lazy camera technique.

harlequin
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quote:
Tom, how you can say Gary's answer was a good one is unbelievable, sorry Gary, but your answer is typical of an uneducated amateur filmmaker that doesn't’t have a clue what he is talking about. My guess is that you haven’t been to filmschool or any professional BBC cameraman courses, maybe just a couple of IOV courses, but last time I spoke to the chap that organises the IOV shooting courses he did tell me that the first half of the day on "DV camcorder technique" courses usually gets wasted as the lecturer try to convince the sorry students that you “Have to use the camcorder on full manual mode to get professional results” PERIOD!!

Cheers Mr White.

Ok , so which course would you suggest i do ?

Sony seemed to think that the course Sony Professional UK ran about 10 years ago was a pretty good course , we had the production company who made Sony's M7 advert showing us how to use the Sony M7 cameras.
And a BBC editor showing us how to a/b edit on Sony Hi-Band.
I obviously need to get up to date with technology.

Should i trade in our Sony DSR370PK1's as well since i can't possibly know how to use a manual lens ?

Maybe i should tell the local people i've done videos for that you are capable of doing a better job , but , your new email address and lack of website make it very hard for me to pass your info onto them.

You are correct , i'm not a professional videocameraman , i have never claimed to be.

Gary MacKenzie

sepulce@hotmail.com ( an account only used for forum messages )

Thinkserver TS140 , 750ti Graphics card  & LG 27" uws led backlight , Edius 8

Humax Foxsat HD Pvr / Humax Fox T2 dvbt

rbarry
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Joined: Mar 27 1999

I think Mr white may well be underestimating Tom's credentials. Also, this web board (or the magazine it represents) is not a hardened professional cinematographers site, and nor is the magazine a trade magazine. It is a community largely made up from amateurs, aided by the good nature of a minority of professionals. However, the majority of postings come from this minority, as they are the ones in the know. The beauty of this community is that it appears to be recognised that the majority of members are "laymen" (like me), and answers to questions are generally given in easy to understand layman's terms.
I was under the impression that depth of field control has a greater effect at the further end of the magnification (focal length) of a zoom lens, for example pulling focus. Obviously there are many different techniques available to the camera operator, the well known and at the time ground-breaking original Jaws movie, when the chief of police witness's his first shark attack from the beach. A combination of camera zoom, physical movement of the camera toward the actor, focus, and aperture correction.

To claim that aperture settings are largely concerned with depth of field is not right. It depends on tecniques and style, the majority of times, when a change in aperture is made, it is not to change the depth of field, but more to adjust to ambient light levels.

busbyvideo
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Joined: Feb 7 2002

Dear Mr White

YOUR QUOTE

"This is how professionals set the exposure, by lighting the set or using ND’s, not using the silly “Auto” function, this is for wedding videographers who always use the silly excuse of “but I don’t have time to do it manually as the light changes too quick”, what a joke,"

As much as i appreciate your expert knowledge here, i find your above statement to be very offencive to myself and all other Wedding Videographers - Which i believe to be the bulk of all the forum users. We are not all BBC trained but i believe that many BBC cameramen would not want to tackle a Wedding shoot, as they would not be able to work under the pressure of a 1 man crew without a take 2 or 3 to fall back on.

To be a professional is not just a matter of qualifications, but also an attitude, and the way you treat your customers and members of the public. We are all trying to help each other here. Although the previous answers were not as technically accurate as yours, i am sure they were sufficient enough to answer the question. The last thing we want here is to put off members from helping others. Its very easy to be critical. It may be true that we don't always dot the eyes and cross the tees. Perhaps your answer is more suitable for a more advanced forum.

------------------
Mike

Mike

Alan Roberts at work
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Joined: May 6 1999

I'm not going to quote or misquote anyone else here, but to start from scratch.

The iris is a volume control on the light entering the camera; open it to get more light, close it to get less. When the iris is most open you get the least depth of field, when most closed you get the greatest depth of field. And that's it.

There's another way to control the amount of light entering the lens, and that's neutral density filters; they simply absorb light rather than passing it, so they reduce exposure without changing the depth of field.

Both these processes operate on the light rather than on the camera.

Once the light has got to the camera sensor(s), there are two more ways to control the signal level. One is the exposure period, the other is gain.

The exposure period is the duration of a field or frame (depending on whether you're shooting interlaced or progressive). the longest exposure period gives the greatest exposure because the light falls onto the camera for the longest, and vice versa. Shortening the exposure period reduces the video signal level and encreases the sharpness of each exposure, useful if you want gritty/jerky pictures.

The gain is simply a volume control after all this has been done. If you operate with high gain you can get signals in low light, but the noise will be greater because you've amplified the noise as well as the video signal.

Experienced photographers use all of these tricks, balancing each against the others for artistic purposes. It's up to you to use the tricks to get the pictures you want, the camera can't think for you.

MrWhite
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Joined: May 18 2003

Thanks for that last comment Mike, no offence intended and I am sorry if it came across this way. Also to rbarry for the post before.

You are both right, I forgot for a moment were I was posting, of course CV magazine is aimed at the amateur end of the market and my post would have been better suited to a forum like Bplus or Televisual.

Either way the best way by far to learn is by “Doing It” which most folk on here are doing.

Gary I don’t know your full credentials, I am basing my comments on your original post, which to me simply seemed way off the mark for someone who is “Professional”. But then maybe you only ever intended to answer the question in a literal way as the instruction manual would have i.e. what the iris does electronically and not creatively?

Alan McKeown
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Joined: May 9 2001

An Iris, in the context of a light attenuator, is a continuously variable aperture, usually approximately circular.

An important point to note about a “camera” iris is that it is located within the lens. It determines the effective diameter of the lens and hence (in inverse relationship) the depth of field.

An iris located outside, in front of the lens, would still act as a variable optical attenuator (in the sense that the light energy reaching the camera sensors per unit time would be reduced) but it would affect the picture with a circular masking effect. This effect will be familiar from old movies as an “iris wipe”.

Hope this helps to explain why only neutral density filters are available as an external means of attenuating the imaging light.

Alan

[This message has been edited by Alan McKeown (edited 05 January 2004).]

Alan McKeown
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Joined: May 9 2001

rbarry,

You wrote:
“I was under the impression that depth of field control has a greater effect at the further end of the magnification (focal length) of a zoom lens, for example pulling focus.”

If I understand you correctly, this impression is mistaken.

For practical purposes, long focal length lenses give just the same depth of field as short focal length lenses, for constant F-number and constant focussed-object magnification (which is the fair way to make the comparison).

The only way to reduce the depth of field (for a given focussed-object magnification and with a given camera imaging-sensor size) is to increase the relative aperture of the lens (ie. decrease the F-number).

Alan

busbyvideo
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Joined: Feb 7 2002

Apology accepted Mr. White.

It was late in the evening / early morning when i got back from a long shoot and your lambasting of Wedding Videographers struck a nerve.

I am sure we can all contribute to this board without belittling others that are, after all, spending their own free time trying to help fellow videographers.

------------------
Mike

Mike

Z Cheema
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Joined: Nov 17 2003

Boingg And Zebedee said time for bed

Jim Bird
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Joined: Sep 15 2000

Hi,

I think the top professionals would be using a film camera and a lens we could only dream about.

If you’re filming in the thick of the action, be it in the Iraqi war or the wedding ceremony, I do not think a cameraman would have much time to F about with the manual f stops.

I think getting usable footage would be far more important than looking for the perfect picture and missing half the action.

The automatic features on the modern camera are pretty dammed good, with my Canon XM1 if I get in a tizz, I can switch it to full auto then flick it back to manual and correct the settings from there if time allows.

When Alfred Hitchcock the famous director was asked the difference between Movies and Television he replied, A film crew has to work a whole day to create 9 seconds of usable footage whilst a Television crew has to work a whole day and gets 9 minutes of usable footage.

Everything is relative and one does only what is required no more.

I can tell you now being an amateur, it takes me 9 seconds to get 9 minutes of usable footage, I just love my NLE’s slow motion effects.

Jim Bird.

MrWhite
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Joined: May 18 2003

Each to their own I suppose. Personally I never use any of the cameras auto functions and I would recommend that everyone superglues any button or switch that says "Auto" on their camcorder to the “Off” position.

As for wedding and war cameraman not having time, this is just a cop out, the people who say that obviously haven’t tried and are just too lazy to even give it a go.

Learning to use the camcorder in fully manual mode is kind of like learning anything else, take the piano for example, you wouldn’t expect to play a Mozart Rondo at “Presto” speed the very first time you sit in front of the keyboard would you?

Another example is typing, if you learn to type with a programme like “Mavis Beacon Teaches Typing” for example you (yes everyone on here) will be up to 30 wpm within 8 weeks if you practice for half an hour per day.

So lesson one, go put your camcorder on full manual and adjust the iris yourself and I guarantee after 3 or 4 weeks of this you won’t turn back, either that or carry on doing it “The Wrong” way and forever be very very limited in what you can achieve with your Cinematography skills.

Expensive lenses have absolutely nothing to do with it, it’s all about been creative and that means “You” been creative and NOT the camcorder.

Fact is I know I’m right so I don’t really care what anyone else thinks, I’m just trying to educate and pass on a little knowledge, but (at least as far as this forum goes) I will no longer “Cast my pearls before swine”.

[This message has been edited by MrWhite (edited 06 January 2004).]

[This message has been edited by MrWhite (edited 07 January 2004).]

Alan Roberts at work
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Joined: May 6 1999

OK, I can't keep out of this any longer.

The auto functions are there to help, not to take over. Any professional operator will easly learn to use or abuse them, and to take full advantage of any facility that makes his/her work easier. Life isn't always about getting the best out of the kit, it's often about getting the footage on tape andthen geting the hell out of there. Whatever tools the kit has to help that process are to be praised and used like there's no tomorrow. If a camera can be powered up, pointed and recording in 3 seconds in auto than that's what he/she'll use in a crisis if it takes 5 seconds to do it the "right" way.

Don't knock the facilities or the people who use them. I've had access to some of the best video kit ever produced, and for many years at that. I now how it all works in very intimate detail, that's my job. I advise professionals on an almost daily basis. And they go out and produce fine dramas, music, naturla history programming to a standard that I can only dream of producing myself.

Yet, when I go out with a camcorder, I leave most of its functions on auto, because I can't be bothered to fiddle with it. If I'm on holiday and taking video snapshots, the last thing I want to do is to have to set up the camera correctly, if there's an auto function that gets most of the way there, I'll use it. Occasionally, looking at the footage, I can see that I could have got it better manually, but by and large the auto functions do as they should and it all works.

I know quite a few top level programme-makers who use auto functions, because they work. I know an awful lot who use auto-iris even on HDTV lenses that cost over £40k on cameras that cost £70k, and they make startlingly good programmes. The trick is to know what tool to use each time. Auto iris is good for getting the lens in range, so press the auto button on the lens to get it nearly right, then fine tweak; that's a perfectly correct and acceptable use of facilities, speeding up shooting and getting better results. I also know quite a few who still insist on using a tape measure to focus; they make good programmes as well, but they make more mistakes and it takes longer. Both approaches work and work well.

there's no "right" or "wrong" way to use video kit, there's only the way you're happiest with; provided it makes the pictures you want, no-one has any right to complain.

There, my 2 pen'orth.

tom hardwick
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Joined: Apr 8 1999

I've just re-read Gary's reply (second posting in this flaming list) and I still can't see what MrWhite is complaining about, and I still stand by my, "Good answer Garry" reply. Moreover, I contend that MrWhite is talking nonsense as his claim that, "Imagine if you are shooting in the sun, the cameras auto setting will close the iris down giving you deep depth of field with loss in colour saturation" is simply not true of modern domestic camcorders. Have a read of the Sony PDX10 test in this month's CV to find out why.

I'm pleased that MrWhite mentions the "sweet Spot" as it shows he's obviously reading my posts. The "Sweet Spot" is a tom hardwick expression I'll modestly claim. But I'll take issue with this. He says, "On the other hand you would require a very deep depth of field (say F16/F22 at the shorter end of the zoom range)". Again, I would advise anyone who is using a camcorder equipped with chips smaller than 1/3" to stay well clear of such apertures - especially so at the wide end of the zoom, where diffraction will cause severe degredation of the image. But again, it's almost impossible to set such apertures manually on modern consumer camcorders, and the PDX10 test will explain why. The MX500, TRV950 and the MVX3i are all the same in this regard.

tom.

MrWhite
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Joined: May 18 2003

Who wrote the test on the PDX10 I wonder, why would I want to read that, I don't buy CV anyway only Broadcast, Televisual, Lighting Cameraman, Showreel, Zerb, Television Lighting and The Producer. The PDX10 doesn’t even have what I call a lens or an Iris setting option.

The “Sweet Spot” is a Scott Billups expression, at least that’s the person who I associate it with, certainly not anyone from on here. That expression has been around for donkeys years.

I would still highly recommend anyone reading this not to believe all the twaddle been expressed by the untrained folk on here and go and buy the following book:

Digital Moviemaking 2nd Edition by Scott Billups ISBN: 0-941188-80-9

That includes you Tom, you might even learn something.

You asked me what course you should go on, well to be honest most courses (at least in the UK) are run be self-taught amateur running around with PD150 and other similar amateur cameras that haven’t got a “Reel” lens that you can use manually, any lens that has a servo focus that doesn’t stop at infinity can’t be used for professional applications PERIOD !!! As for Premiere, well that says it all really doesn’t it

So as for which training course, don’t bother with any of them, but, what I would recommend is that you Tom phone up one of the professional film studios or locations in the UK i.e. Pinewood or the Eastenders set in Borehamwood and ask very kindly if you can go along and sit very quietly to one side on the set and promise you won’t make a sound or get in the way, if you ask in the right tone they might say yes. Then just sit there for 8 hours and watch and listen. I can guarantee that after this you will be totally inspired and you will learn more in those 8 hours that you would the first year of film school at Beaconsfield, or 50 IOV courses.

Oh yes and you won’t here their DP shout to the cameraman “Stick that camera on Auto for me mate”.

So stop pretending to be a professional until you actually know what you are talking about.

Of course if you know what “Astigmatism” is I take all this back. If you don’t know I’ll give you a clue, it is to do with lens imperfection and how the iris can correct it.

harlequin
harlequin's picture
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Joined: Aug 16 2000

quote:
Of course if you know what “Astigmatism” is I take all this back. If you don’t know I’ll give you a clue, it is to do with lens imperfection and how the iris can correct it.

interesting.
i have astigmatism and a simple iris won't fix it , that requires a corrective (toric)lens.

quote:
I don't buy CV anyway

so why do you hang out round here ?
is it because you need help from here ?

Gary MacKenzie

sepulce@hotmail.com ( an account only used for forum messages )

Thinkserver TS140 , 750ti Graphics card  & LG 27" uws led backlight , Edius 8

Humax Foxsat HD Pvr / Humax Fox T2 dvbt

MrWhite
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Joined: May 18 2003

You're quite right Gary, why am I wasting my time.

"I'M OUT OF HERE"

CV now deleted from favorites.

rongrover
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Joined: Jun 1 2002

It really is a pity that bitterness and rudeness and sarcasm, especially the latter, has to come into these forums, but, sadly there is alot of it.

Personaly I have learnt alot from what has been discused by all concerned in the above postings and it will be a pity to lose anyone.

Everyone has a style and manner of writeing and it can upset some people. ( I certainly have been upset by some remarks made against me ) but in the end it is the value of being able to gain and sometimes give information that matters.

I for one certainly value the comments made by Mr White (but I will never forgive him for not letting me buy that JVC camera!!!) as I do everyone else. So I hope Mr White, you don't pack your bags and go.

I appeal to everyone, please try to cut out the ill feelings and sarcasm. There are other ways to put your points accross if you don't agree with someone.

All the best, Ron.

tom hardwick
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Joined: Apr 8 1999

If you're still with us MrWhite I can tell you that I agree 100% with you when you say:

... Read Digital Moviemaking 2nd Edition by Scott Billups ISBN: 0-941188-80-9
That includes you Tom, you might even learn something.

We can all - every single one of us - learn something new every day - and for me that's what makes life so very interesting. You also say:

"...recommend is that you Tom phone up one of the professional film studios or locations in the UK i.e. Pinewood or the Eastenders set in Borehamwood and ask very kindly if you can go along and sit very quietly. I can guarantee that after this you will be totally inspired and you will learn more in those 8 hours..."

Yes, I can agree with you there. So might I have the opportunity and impunity to disagree with you too? You say "... any lens that has a servo focus that doesn’t stop at infinity can’t be used for professional applications PERIOD !!! As for Premiere, well that says it all really doesn’t it"

I'm sure there are many who frequent these pages who most certainly use the PD150 and Premiere in a professional application. I assure you that being professional and earning one's living are much more to do with experience and attitude than about infinity stops and Avid.

And I'll gently say that Gary's right again, in that astigmatism cannot be cured by the iris. I'm sure you're just egging us on here, aren't you? :)

tom.

Christian Lett
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Joined: Apr 26 1999

I'm afraid I won't be losing any sleep over the departure of this MrWhite. If he has a point to make (which he obviously does) there is surely a better way than the obnoxious, holier-than-thou attitude displayed by him throughout this thread.

I think it's a great shame, as I agree with what he says regarding using manual settings, but I cannot afford the kit he's talking about (Sorry, MrWhite, I only have a TRV900) so I guess I'll never produce anything of merit! I will however be reading the book he recommends.

As he states quite clearly anyway, he is far too professional to be hanging out on-line with the likes of us amateurs. Apologies for this post, but I couldn't not say anything after his blunt remarks to those people who DO know what they're talking about (namely Tom, Gary and Alan Roberts - who I know IS a professional). Apologies also to Winndigital, who started this thread with a simple enough and relevant question and had to put up with all of this.

[This message has been edited by Christian Lett (edited 08 January 2004).]

Christian Lett After Effects and Maya Artist www.quarterlightpictures.com

Mad_mardy
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Joined: Oct 19 2000

what a pr*t
bunch of amateurs indeed, yes there are amateurs, its predominatly a amateur message board, but there are lots of professionals here too. unfortunatly the industry seems to full of people like him. shame really

System 1: AMD X6 2.8, M4A79 Deluxe, 4GB DDR2, ATI HD4870 1GB DDR 3, 2TB total drive space, Matrox RTX 2, Premiere Pro CS4

System 2: AMD X2 5600, M2NPV-VM, 2GB DDR2, Geforce 8600GT 256 DDR 3, 450GB Total drive space, RTX100 with Premiere Pro 2

Camera's: JVC HD200, JVC HD101, 2X Sony HC62

Alan Roberts at work
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Joined: May 6 1999

I reckon that 36 years researching the business makes me a professional, and I still think he's a bigoted prat. Anyone who spouts dogma like that should be ignored, clearly he has a soapbox and a need to stand on it to shout. Fine, let him. Somewhere else.

busbyvideo
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Joined: Feb 7 2002

Its a pity we have lost someone with Mr. Whites experience. But i think its not iris but his attitude he has to adjust.

He called us "uneducated amateurs"

Well, as a fully qualified Lecturer, i am certainly more qualified to discuss who is educated than he is. As a teacher, you would never dream of lambasting your pupils for not being 100% accurate, it would destroy their confidence. A politely put correction would have saved all this unpleasantness.

------------------
Mike

Mike

Martin B
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Joined: Aug 16 2000

I don't regard it as a pity at all!!

I don't care how experienced he is (or at least claims to be!) he rarely makes a contribution without taking the opportunity to add a snide remark. Maybe that's why he needs to change his name so often.

I'm just disappointed that I and others still rise to the bait.

Martin

rongrover
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Joined: Jun 1 2002

Well put Mike.

All the best, Ron.

red
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Joined: Oct 1 2000

Mr White you have probably put off winndigital from asking a question for life.

Can't you see that our pro's and experts tailor their answers to the question?

If you were anywhere near as good as you seem to think you are you would do the same.

They would have gradually led winndigital through the process and he'd have the answer to his questions.

What are you saying anyway, depth of field, aperture control, soft focus, pull focus, etc, etc. Run through the manual have we??
Hardly rocket science. My 16yr old has done all that at college.

You sound like you've just finished a camera course yourself, what a prat!

Dear winndigital, please excuse the above and keep asking the questions. Many people can and will answer though Tom and Alan are a bit more eloquent than most and Gary is the best for a quick answer if you need it.

Alan Roberts at work
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I often find that I start giving an answer but run out of time to do it properly, so tend to abbreviate the science. In the words of someone famous (who?) "I haven't the time to write a short document". Seriously, though, once you start getting really deeply into a subject, you find that there are no simple answers to even simple questions, they all have caveats and exceptions, that's why I tend to ramble more than some do. Right now, I'm struggling with writing a chapter for a book (not mine, this is another one) on how to get the best out of cameras; they want 10 pages, I know I can do it in 100, but what do I have to leave out? Ho hum.

[This message has been edited by Alan Roberts at work (edited 12 January 2004).]

adgroberts
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Been off the forum for a few weeks so was unaware about all this. Intriguing in retrospect. As usual I agree with Ron, it's always a pity to loose someone knowlegeable. Returning to the original topic, the auto vs manual debate certainly started many years before camcorders. I always felt my auto Olympus OM10 was inferior to my pals manual OM1, but in considering Iris function it is best to consider the biological eye. Human in general but consider how amazing and diverse both eagle and fly eyes are; both perfect for their purpose. While the eye by automatic means will control the focus and depth of field allowing something 10 cms away (if you are young) to be in as good focus as the moon, the depth of field in one situation is 1cm and infinity in the other. The human CPU (brain) controls which of these extremes (or inbetweens) it wishes. The film maker or videographer has to second guess which "setting" for the "eye" (camera) the viewer will wish. This is the difficulty, and probably the difference between a bad film and a great film is the filmmakers ability to predict what the viewer wants instinctively.
The other great advantage the eye has is that there are two of them. Remember the comparason of a photograph with a Viewmaster binocular picture? Anyone heard of any binocular camcorders coming on to the market??
Regards
Tony Roberts

adgroberts

Alan Roberts at work
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Ahem, don't confuse focusing distance with depth of field, I can go on about this at length but don't propose to.

And as for binocular camcorders, James Cameron has been doing exactly that for a few years now, even in HDTV, and under-water. He used Sony HDW900 camcorders though, not exactly consumer kit

busbyvideo
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Hi Alan

Can you give a brief comparison of focusing distance with depth of field

Mike

Jim Bird
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Hi,

Astigmatism aberrations are similar to comatic aberrations for goodness sake, however these artifacts are not as sensitive to aperture size and depend more strongly on the oblique angle of the light beam. The aberration is manifested by the off-axis image of a specimen point appearing as a line or ellipse instead of a point. Depending on the angle of the off-axis rays entering the lens, the line image may be oriented in either of two different directions, tangentially (meridionally) or sagittally (equatorially). The intensity ratio of the unit image will diminish, with definition, detail, and contrast being lost as the distance from the center is increased.

Big deal.

Jim Bird.

busbyvideo
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Thanks Jim

As clear as a muddy lake.

------------------
Mike

Mike

Alan Roberts at work
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Joined: May 6 1999

Let me have a go

Focusing distance is the distance from the lens in the object field (i.e. on the scene side of the lens) at which a point is reproduced as a point on the image plane (i.e. on the other side of the lens. Conventionally these distanmces are called U and V (but I can never remember which way round) and the equation that joins them together is:

1/U + 1/V = 1/F

where F is the focal length of the lens. If U goes to infinity, i.e. the object is at infinity, then the rays entering the lens are parallel to the optical axis as they enter, and will exit converging at a point, the focus, at distance F from the centre of the lens. That's how little boys burn holes in paper.

Depth of field is the range of distances in the object field, within which any point is reproduced on the image plane as a disc of diameter equal to or less than the "disc of confusion". The magnitude of the
disc of confusion represents the smallest item in the image that can be distinguished, in television it's about 1.5 to 2 times the size of a pixel.

I can drone on a lot more if you want me to.

Jim Bird
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Hi Mike,

-------------------------------------------

Thanks Jim
As clear as a muddy lake.

------------------
Mike

-----------------------------------------------

Clear as mud Mike , Well I have some good news for you, if Mr. White is such an expert he will be able to clarify the my post and put it into basic English for you.

Want you Mr. White?

And, thank you Alan for making light of an optical issue.

Jim Bird.

busbyvideo
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Thanks Alan

A very informative answer, which confirmed my own beliefs. Clear and to the point.

-----------------------------------------

Hi Birdy Boy

Perhaps you should take a leaf out of Alans book on exposition of information. And as for involving Mr. White?, no thanks. I'll stick with alans explanation for now.

(Your not Mr. White, are you?)

Mike

Alan Roberts at work
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Knock it off, Jim's well known here and is just as much an expert in his own field as many others of us are. We've seen some of his pictures.

If my explanations are easier to understand, it's because I've had a lot of years formulating them, it's what I do. I don't supply information to baffle, just to clarify, like I said at the end of the message, I can go on for an awfully long time explaining things,l but I normally stop when I thinky you've got enough, I rarely go deeply into things like Jim did unless someone's specifically asked for the info.

Jim, don't take any of that the wrong way, it's just that you overwhelmed with information without explanation. But I now know who to come to when I need more on lenses.

Alan McKeown
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Joined: May 9 2001

Jim, it is standard practice to acknowledge the author(s) if the text is not your own.

http://micro.magnet.fsu.edu/primer/java/aberrations/astigmatism/

Alan

busbyvideo
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Joined: Feb 7 2002

No offence intended Jim - Hence the smiley.

I thought you were tacking the p*** with your answer, like you swallowed a dictionary.

Surely, you did'nt expect anyone to understand all that jargon.

------------------
Mike

Mike

Alan McKeown
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My dictionary gives, as one of the definitions of “jargon”:

“A twittering of birds”

Alan

busbyvideo
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I'm sure Jim was only trying to help, but the answer given had no relation to the question asked.

"Astigmatism aberrations are found at the outer portions of the field of view in uncorrected lenses, and cause the ideal circular point image (Airy pattern) to blur into a diffuse circle, elliptical patch, or line, depending upon the location of the focal plane"

This relates to an eye disorder, as described in:-

Davidson, Michael, W. (2003)
Molecular Expressions. http://micro.magnet.fsu.edu/primer/java/aberrations/astigmatism/

And

Vargas-Martin, Fernando (1998)
Correction of aberations in the human eye. http://iriso.fcu.um.es/publications/JOSA98_Sept_vargas/JOSA_1998v15n9p2552.pdf

I'm not trying to cause any trouble here, and Alan is quite right to stick up for a friend who is being critisised (not) but the answer, in jest or not, did not answer my question.

As a P/T college lecturer, we often see text like this. A student does a search on the net for info relating to a question, but not being familiar with the subject, hands in a report that, although containing words related to the subject, dosent actually answer the question.

Once again, thanks for all the guys help here, its much apreciated. If however you are unsure, its best to say this or nothing at all. If you are replying in jest - no problem. Just let us know that this is the case. I like a joke like everyone else.
Once again, keep it lighthearted. No offence intended to anyone. Sometimes misunderstandings occur due to the technology we are using here (The Forum) its difficult to show emotions and moods. I'm happy with the outcome - I got my answer. Thanks to all involved.

------------------
Mike

Mike

Alan Roberts at work
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Actually, Jim's text does apply, in that it explains how the disc of confusion (DoC) can get distorted or enlarged, and that in turn affects depth of field (DoF), because it's the dimensions of the DoC that define the sums for DoF of a lens. Note, however, that it doesn't take into account the remeinder of the imaging system, right up to the eye, and that's usually what defines the DoC rather than just the lens. This the DoF of the entire systenm depends on the DoC for the entire systems and not just that for the lens. And that means it varies with vieweing distance and with your eyesight as well. Bur I wasn't going to go into all that

This is all getting a bit abstruse though, and quite a long way from the origins of this thread.

I meant no defence of Jim, nor any attack. I was simply calling for a little calm in what looked like it might have become a slanging match.

busbyvideo
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Thanks again Alan.

We are all here to share knowledge and experience, and must be willing to accept any corrections or criticisms in a professional manner.

I would never(intentionally)cause or suffer any slagging or slanderous remarks on this forum. We are all comrades here. But would boldly repel anyone who goes out of their way to upset the harmony we have here.

Have a nice day :-}

------------------
Mike

Mike

Alan Roberts at work
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Nicely put

Anyway, I just had a look at that URL and it's completely explained something that's had me puzzled for a while:-

If I take a focus chart of the "starburst" variety (a radial set of trangles meeting at the middle, so that the circular pattern alternates black/white with angle) and focus my camera on it, I get the very centre blurred into grey and the rest of the pattern focused nicely. If I now take the lens out of focus gradually, the centre gret blob gets bigger and bigger until it starts to revela the original pattern at the centre, sharp but phase inverted (i.e. black where it was white and vice versa). At this stage there's an annular grey doughnut separating the more-or-less still sharp outer from the phase inverted inner. If I carry on defocusing, the centre goes grey again, and then sharp again but back in ohase wioth the original, and there are then two grey rings in the pattern. And so on.

I was convinced that this is an exposition of a Fourier transform but couldn't see what it might be. Now I know, it's the Airey pattern

Thanks Jim, I may never have worked that one out without that reference and the wonderful demo.

[This message has been edited by Alan Roberts at work (edited 28 January 2004).]

Alan McKeown
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Quote:
“Thanks Jim, I may never have worked that one out without that reference and the wonderful demo.”

But Jim did not provide that reference!!

( No offence, Jim.

Alan

Alan Roberts at work
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But he found it, and quoted it, and then the URL got posted and I looked at it. Basically, I don't care where information comes from as long as it's correct and it explains things properly. What really took me back is the animated graphic, that really showed me what's going on, and for that, thanks to this thread because I wouldn't have found it otherwise.

There, will that do ?

Jim Bird
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Joined: Sep 15 2000

Hi,

Apologies to every one who I may have offended with my post, including Mr. White I would like to add.

It was not intentionally trying to offend anyone and apologies to any person who was offended.

Was I guilty of plagiarism? I’m not so sure, anyway it was a very cool description of Astigmatism, there are some extremely good well worded explanations of this term Astigmatism, on the internet and I especially choose the coolest one for the benefit of our Mr. White.

I genuinely believe Mr. White was not trying to offend anyone either.

Mr. White obviously, with his undoubted skills and experience is a net contributor (giver) to this site, he does not come here to learn and he comes here to give us the benefit of his knowledge.

I guess Mr. White has become a little frustrated with the replies and the reaction he has received to his post and this frustration has lead to anger.

I think Mr. White is could be over qualified and unable to communicate with a few of us here and so, has become dissatisfied with things around here, which is a real shame.

Last but not least, THANK YOU Alan (at work) for risking your reputation on my behalf, it that was very good of you, I appreciate that very much.

I will try and repay your faith in me by trying to behave a little better in the future.

Cheers.

Jim Bird.

winndigital
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Joined: Jul 15 2003

Thank you all very much for your replies over the last month. It has been very educational and very interesting to read, all the information posted and may I say quite humerous at times. once again thak you.

red
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Joined: Oct 1 2000

Amen