Pull Focus

16 replies [Last post]
Roger Maytum
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Joined: Apr 7 1999

I like to use the pull (or rack) focus technique. trouble is - on my Panasonic NV-DX100 there is no lever on the focus ring. It's really difficult to achieve. Has anyone solved this problem? At present, I can't think how I might add some sort of lever to the focus ring. Thanks.

Roger Maytum

pcwells
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Joined: Jun 10 1999

Okay, pulling focus isn't easy. Firstly, the shot has to be carefully planned for it to work properly. Secondly, the camera person needs to keep an eye on the lens *and* the viewfinder! Video monitors are a great invention and hiring someone as a focus puller is usually the only way to get quick results.

I can't suggest a substitute stick, but marking up the focus ring with a chinagraph pencil or thin strips of tape to denote start and end points is one way of getting half way there - although a video monitor is essential if you want to see what you're doing.

The problem with most consumer and prosumer cameras is that the focus ring isn't calibrated in feet or metres and won't stop turning when it's reached its limit, so any carefully placed focus marks can easily become meaningless.

Good Luck!

Pete

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[This message has been edited by bcrabtree (edited 27 June 1999).]

Alan Roberts at work
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Joined: May 6 1999

The problem with focus rings on our camcorders is that they don't directly drive the lens. They interfere with the servo instead. This means that if you carefully calibrate a shot by marking the ring with fixed points, and then turn the camera to autofocus, or power off, the marks won't be in the right place next time you try manual. Also, the depth of field is pretty huge with small image formats, so you need to have lens fully open to restrict depth of field, and still might not have it short enough for dramatic effect.

There's no real answer to this, other than bigger image formats or f0.3 lenses

tom hardwick
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Joined: Apr 8 1999

The TRV900 Sony has a neat switch up near its focus ring. It goes Auto, Manual, Infinity. You can be filming in the manual focus mode, closeup, say, and immediately jump to infinity focus. Or you can go the other way, being in the manual focus mode on the subject at 10m, and flicking to auto to pull focus to another subject at a metre.

As Alan says, you'll have to do all this at full telephoto and wide apertures, or the audience will miss the point. And in reality the 900 has its greatest differential focus at 51mm and f2,8.

If you tell a 35mm stills man these figures and ask for his opinion on your differential focus options, he'll giggle. And it's EXACTLY the same for us with our 1/4" chips; the laws of optics cannot be denied - we'll struggle for blurry backgrounds.

tom.

Alan Roberts at work
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Joined: May 6 1999

Drama in the Beeb typically uses Super 16 or Digibeta. S16 is 13.6mm diagonal, Digibeta is nominal 2/3" i.e. 11mm diagonal, so a Digibeta camera has to open up about 1/2 stop to get the same DOF as S16.

I've done a few quick, back-of-an-envelope, sums for 1/4" format (4.1mm diagonal). For the same DOF as Super 16, the aperture must be 4.1/13.6=0.315 in linear dimensions. So the favourite f2.8 to f4 used in Super 16 becomes f2.8*0.315=f0.88, and f4*0.315=1.26. We haven't got a chance

Colin Barrett
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Joined: Jun 5 1999

The problem of pulling focus on a "prosumer" cam is that the lenses on most of them are pretty crummy compared to their pro counterparts. The DX100 (and DX110) is a good example of cams whose manufacturers assume that they'll be operated in auto mode. The lenses are not designed for critical use, either. All you have to control the focus in manual mode is a pokey little thumbwheel - not the best thing for accuracy. Having used the DX110 a few times (in reviewing a piece in CV's sister publication Camcorder User), I was able to get some fairly neat pulls forward and back - but the week after I was doing this I then used a DigiBeta with a £12k Canon W/a lens, and the difference isn't worth thinking about!!

In other words, it can be done, but it's a fiddle. (You can't even do a decent p/f on the Canon XL-1, despite the pro looking lens - a horrible bit of glass actually).

Keep tryin'.

Colin Barrett
Regular feature writer for Computer Video.

Chad
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Joined: Apr 19 1999

Not sure whether this will help at all but I have a cable tie attached round my focus ring. I find this make it a lot easier to adjust and I guess it would make quite a good reference point.

I think I'm a bit out of my depth here so ignore me if I sound dumb!!!

Chad!

Christian Lett
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Joined: Apr 26 1999

I found that zooming in slightly and moving the camera back a bit will reduce the depth of field enough to blur backgrounds and extreme foregrounds. Even 35mm cameras have a big D.O.F. at a wide angle but it's still a miles better situation than camcorders.

I'm determined to lose the video look somehow!

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Christian J. Lett
[email=clett@nationalexpress.co.uk]clett@nationalexpress.co.uk[/email]

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

tom hardwick
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Joined: Apr 8 1999

Let's get technical (and somewhat pedantic). You'll often hear it said that wideangles give more depth of field than telephoto lenses, but get this: it isn't so, it just appears so.

Here's the test, and here's the truth. You set your 35mm camera on a tripod, 28mm wideangle in place. Focus on your garden gate, set f4 (say), expose film.

Now substitute 100mm lens (or zoom to this focal length). Focus on garden gate, use f4 again (the light hasn't changed), expose film. Let's say the 35mm frame now exactly encompasses the gate, hinge to latch.

Now comes the interesting and revealing bit. In the darkroom, print the 100mm shot 7x10"ish. Then greatly enlarge the neg shot with the 28mm so that the 7x10" paper contains the same information, ie this print also encompasses hinge to latch.

What have we got? Well, we now have 2 identical photographs of our garden gate. Identical pictures, but the blowup from the 28mm neg will be more grainy and consequently less sharp.

But the interesting thing is that the depth of field in the two shots is absolutely identical - the flowers before and beyond the gate show exactly the same circle of confusion characteristics.

So when you hear it brandied about that wideangles-give-greater-depth-of-field, you'll know that it cannot possibly be so. The laws of optics won't and don't allow it.

tom

Alan Roberts at work
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Joined: May 6 1999

Tom is right, for a given image size and format, the depth of field is fixed by the stop, not the focal length of the lens. However, in common usage, a wide angle lens is mostly used to get a "bigger" picture, and under those circumstances the DOF is geater. But if you get in close to get the same image size as with a longer lens, the DOF will be the smae for the same stop.

BTW, the DX100 doesn't have a thumbwheel for focus, it's a ring on the lens (at least mine has). But the comments about quality are right, focus tracking is poor.

Christian Lett
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Joined: Apr 26 1999

Yes, I realise now that my statement was incorrect about depth of field vs. focal length. But this still leaves us at the beginning - with our huge D.O.F where only things in the extreme foreground are going to be affected by a pull focus.

So are there any "work arounds" we can employ to get the desired effect or are we camcorder users just clutching at straws?

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Christian J. Lett
[email=clett@nationalexpress.co.uk]clett@nationalexpress.co.uk[/email]

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

Tarun
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Joined: Apr 29 1999

Have you heard of ND filters folk?

Tarun
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Joined: Apr 29 1999

That is the problem here no "work arounds" but cribbing and techie details... in my mind from underdeveloped country, all we look for are "work arounds"...
Okay, go on full tele, use lowest fno. possible on the camera(2, 2.8 etc.), get camera as close as possible to the object, and turn the focus ring and watch the viewfinder and you have it - s.f.
And camera work is traing your eyes, mind and hands to coordinate or wait for some preprogrammed s.f.

Alan Roberts at work
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Joined: May 6 1999

Just to restate, NDs will cut down the light and allow you to open up the lens to get the shortest DOF. If the lens is already fully open an ND is of no more use, you're still stuck with the DOF given by the fully open lens. The real problem is that small camcorders don't have f0.8 lenses

Tarun
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Joined: Apr 29 1999

Most of the broadcast cams also do not have it, but then how do they manage it?

Alan Roberts at work
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Joined: May 6 1999

They have 2/3" sensors (11mm diagonal), or Super16 (13.6mm diagonal). So the equivalent of f4 in TV or S16 is F1.26 in 1/4" (4.12mm diagonal), and f2.8 becomes f0.8, to get the same DOF in each format.

Nigel Longman
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Joined: Apr 28 1999

I created a simple focus-pull effect in Premiere by putting the background video on one track and the foreground image with a suitable alpha channel on a superimpose track. By applying a time-varying blur filter to each track in opposite directions a pseudo focus-pull effect was obtained.

Regards, Nigel