How do I use the Zebra

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George Lemonofides
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Joined: Nov 28 2000

My Sony 900E has a zebra facility which I understand helps me avoid over exposure. However like most camcorder user manuals, there is no explanations or help in how to use advanced features

Can any one walk me through the process of using this feature effectively

George L

tonyco
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Joined: Jul 7 1999

Hi George,
the Zebra facility basically indicates in the view finder areas of the image that are overexposed, it is normall to see some zebraing in strong highlights. This is basicly a guide, to help you with exposure, it has come from pro cameras and is there because the view finder doesn't always give you a true idea of your actual exposure.
I would always suggest setting up the brightness and contrast of the view finder against a video monitor. Basically you plug your camera up to a video monitor and adjust the view finder until you get a good match with the image on the screen.

Hope this helps

Tony Cobley www.infomedia-hps.co.uk

[This message has been edited by tonyco (edited 03 December 2000).]

Regards

Tony Cobley
www.infomedia-hps.co.uk

Alan Roberts at work
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Joined: May 6 1999

To fill in and expand on this.....

The zebra function was originally done for broadcast and pro cameras to when it became possible to disconnect them from the cabled ccu (camera control unit) that had an engineer and terst kit attached to it. In studios, it was (is) fine for the cameraman to point, focus and compose and for the engineer to do the rest. But when camcorders came along, they had to be battery operated and stand-alone, so there's no-one around to help.

Exposure control is tricky to get right in a tiny viewfinder, so the zebra pattern is useful as a meter. It superimposes a strobing/flashing pattern in those parts of the picture that are at a prescribed video level, neither above nor below, at that level. High end cameras usually have two zebras with different patterns, and the operator can tweak both. So it's usual to have one set to about 65 or 70% (caucasian flesh tones under normal lighting) and the other to 100% (peak white, clip level) or maybe just a little higher. So, just by looking in the viewfinder, the cameraman has accurate metering of those two levels, and can adjust iris, gain, lighting, shutter, to get the effect he wants.

Consumer cameras have one zebra (if you're lucky), set to about 100% with no adjustment. Use it to make sure you're not overexposing (and thus clipping). If it's adjustable, decide whether getting good skin tones or control over clipping is more important, and set it accordingly.

Either way, the zebra function is the most accurate light meter you'll ever find, because it's calibrated for your specific camera. Trust it and you shouldn't go far wrong (in exposure terms, there are lots of other tigger traps to fall into).

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tom hardwick
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Joined: Apr 8 1999

As always an authoritative answer from AR@W. I might add that newcomers to zebras are often frightened by their presence and tend to back off. Probably wise in the game reserve, but un-necessary in the viewfinder.

A hint. Leave the camera on auto exposure but with zebra turned on and note how often the camera would happily film with quite a lot of this "over exposure" if left to it's own devices. When you switch to manual exposure (which you should almost invariably do by the way) dial in the same sort of percentage of zebra stripes as you've seen in the auto mode. Remember that zebras tend to make most people back off, and as a result they end up with large areas of the frame under exposed.

tom.

Alan Roberts at work
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Joined: May 6 1999

Good advice from Tom as usual (and, no, this isn't a mutual admiration society). Don't be afraid to experiment, how do you think we get to know about all this?

I'll admit that I'm in a slightly priviledged position in that I've got the long-term loan of an old video waveform monitor, which now sits permanently on the composite output of my DV Walkman (hooked to the PC as the DAC for DV editing), so I can check signal levels at all times. For DV shooting, I use a DX100, and routinely leave it in auto exposure. Despite what Tom says, it has not yet given grossly wrong exposure except in odd circumstances, more of that below. It always manages to keep the peak exposure about right, certainly within 5%. So, I've learned to leave well alone unless I know there's going to be a problem. And the problems do happen.

Experiences of wrong exposure resulting from auto control:
1 Backlit subject, no front light to speak of. Solution: move the subject, don't increase exposure as that would add gross noise.
2 Subject reaonably well lit, but much brighter background in shot, usually a window causing the metering to expose for the window. Solution: move the subject to lose the window (cheap), or put lots of neutral density filter over it to equalise lighting levels (expensive), or Polaroid material on windows and lens and rotate until it looks right (very expensive).

And that's it, I've never had any other exposure problems. But, I'm always happy to lift the signal a little in NLE to get it just right. For amateur shooting, I take the line that I'd rather get it in the can and sort it later than spend too much time getting locations exactly right, because I know I can always do a bit of judicious tweaking in NLE. But, for perfect results, and if I were trying to sell the pictures, I would follow Tom's advice to the letter, that's the professional way.

The penalty for not getting exposure right in the camera is that you'll have to tweak it in NLE, and the only controls you've got are gain and gamma, both of which modify the noise on video. Get it right using scene light and you'll get around that.

However, there's one fly in the ointment, clipping. If you've overexposed, you can't get it right by winding the gain down because everything above peak white (plus a few percent) is hard clipped (you run out of digits or modulator range in analogue), there's no detail to recover. Pro and broadcast cameras have a "knee" function, which dramatically reduces the gain in the presence of over-white signals, compressing them in amplitude. It doesn't affect most of the contrast range, only that part near peak white. Properly set, this can let you record at least two stops of overexposure and get away with it in NLE by tweaking the contrast curve, tedious but possible. This also has the merit of mimicing the contrast performance of film, but that's another story. Anyway, if your camera has a knee, you can safely overexpose by a little and get away with it, the highlights will be compresed rather than clipped. I have a graphic example of this in my latest project, my DX100 footage, and that from a DS11 I later borrowed, both clearly exhibit the characteristics of a knee (but don't allow setting the control of it), detail isn't lost on highlights, but footage from an 8mm camera just hard clips at 100% and looks awful. My old SVHSC Panasonic did the same, that's one reason for going up-market.

Use your eyes, play with the camera, get to know what it can do. Then go and shoot masterpieces.

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tom hardwick
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Joined: Apr 8 1999

Wonderfully detailed reply from Alan (this is turning into a genuine mutual admiration society). However (and there's always a however) Alan talks in terms of still photography, i.e. move the actor, equalise the lighting and so on to avoid contrast ranges that will always be outside the limited range that video can tolerate.

We tend to think the human eye has a great exposure latitude and poo-poo video for it's restrictions but in fact that's not the case. Your eye is always in the auto exposure mode (unless you've had pupil dilating drops applied) and as such whatever you look at in whatever light (within limits) will be "correctly exposed" on the retina. This is God's way of ensuring that you have a retina to use tomorrow.

So Alan's correct when he says that he leaves the 110 in the auto mode and it never falters, always delivering him near perfect exposures. He can lift any frame from the timeline and print it out happy in the knowledge that it fits happily within the straight portion of the graph.

But my contention is that life's not like that, and it certainly isn't when you shoot movies. The whole point about using locked exposure on the talent is simply to stop the auto exposure flicking this way and that in an effort to reduce everything to a neutral 18% reflectance.

Let's take a very simple example. Your imaginative DOP wants a high viewpoint shot of the talent as he crosses the street on a zebra (that link word again) crossing. The man leaves the curb at f5,6 and steps onto the first white rectangle of paint. The camera thinks oh,oh - it's got brighter, and immediately stops down to f9,5. The next step takes our character onto the matt black rectangle and the camera immediately reacts to this darkness (remember the lightmeter is only seeing a blurred integrated image) by opening up to f3,5.

So what have we got? A mess, that's what. Every step taken varies the exposure of the talent on film, yet he's being illuninated by a nuclear explosion 93 million miles away that shines with unfaltering brightness. As he crosses the road he'll be varying in brightness by 3 stops with every step he takes, and this will be most obvious and very unwelcome.

So lock the exposure good people. Auto focus is an obiedient servant alongside auto exposure, and causes far less technical errors in films that I see. I'd rather you burn out the unimportant highlights and block up the shadows than have the exposure flicking about willi-nilly.

Your go, Alan.

tom.

Alan Roberts at work
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Joined: May 6 1999

No contest Tom. Every word correct. I don't do that sort of shooting.

The eye responds only to about 100 different grey levels in an image, so we only need 7 bits, so, theoretically we've got a stop in hand with the 219 levels we use in digits before we see quantising levels. So a bit of NLE tweaking is OK. While the eye has a huge response range in terms of levels, it always comes down to about 100 levels that are relevant in any image, so we can get away with it.

Similarly, the eye sees all parts of every real-world scene in sharp focus, because it habitually refocuses on the detail in the fovea. In practice, the whole image is soft apart from the foveal part, the eye creates the impression of a sharp image by scanning it randomly. Clever post-processing in the brain. The art of the video/film Director is to stop the eye doing this in, and making it look at the important parts of the picture using lighting, depth of field, sound, movement, and this is why the small formats of consumer camcorders are unsuited to big scale drama (conventional thinking) apart from fly-on-the-wal stuff, where it's ok to swing the camera about (Blair Witch, lots of TV stuff etc).

Have we buried this one yet Tom?

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Julian
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Joined: Aug 24 2000

WOW! You guys really know your stuff (maybe you need to get out more).
Although, I have learned alot (I think!)

Julian.

tom hardwick
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Joined: Apr 8 1999

Julian, we "learned our stuff" precisely by getting out more. You too will learn that you will learn little by staying in and reading this; you'll have to get out there to learn. Experience is a hard and somewhat expensively won comodity you'll find.

tom.

Alan Roberts at work
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Joined: May 6 1999

Correct again, Tom. I keep on saying this, but experiment is the best way to learn. Ask for advice and take it from us, your very welcome to it. But get out there and make movies, and USE YOUR EYES to work out what's going on, then you'll be more expert than me before long. When that happens I'll sit back and enjoy reading your advice to the next generation (I retire in 4 years 5 months, don't ask what I've been doing all my career, I'll let that out after I retire).

Apart from other things, I'm a bellringer, and teach at my local tower. It's wonderful seeing the penny drop occasionally, usually after making a casual remark off-topic, but I can see that a learner's just spotted something and is now really the best person to teach that bit, because it's fresh to him and he's still keen.

Keep making mistakes, but learn from them, and above all, watch your own footage as though it's the first time, query every decision, ask your self how every shot could have been better, whether camera angles, camera setup or editing. Only when you can do that will you really be on top of it (and before you start critising me for this, I'm still learning as well). I've watched some rewally good professionals at work, and it's a revelation, watching the artistic process in full flow, often taking up to a dozen passes through a mixer to get a cunning effect, but the results can be mind-blowing.

Go forth and use the kit, enjoy yourself, but keep learning.

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Julian
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Joined: Aug 24 2000

I wasn't critising, I was only joking.
I've learnt a hell of a lot from "getting out there" and from reading the posts from Tom and Alan and others.

much respect.

Julian

Alan Roberts at work
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Joined: May 6 1999

Julian, don't read our comments as criticism, take them as encouragement. We all learn most by doing, having first spent some time thinking. The discoveries you make yourself will always be more vivid than those you're told about. You just need 35 years doing it

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tom hardwick
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Joined: Apr 8 1999

Same goes for me. Just as Alan says you need 35 years and in the old days a lot of that time was spent waiting for the film to be processed and sent back to you, so you could test out your theories. And you can bet your wellies that the more experimental effort you put into the test footage, the more likely were the Post Office to loose it.

But with video it really is instant teaching Julian; wonderful! I gave a talk to Wanstead Video Club last Friday and part of the TRH lecture was showing how well your camera can teach you. As you film you talk into the mics, telling the camera what you're doing, with what lens at what aperture with what filter and so on. Although DV enables the technical data to be displayed on screen for every frame shot I still like the talk-to-the-mic technique.

tom.