Zebras and Gamma settings and Knee

ccroo

ccroo

Well-known member
Dear Folks:

Don't the gamma settings change the way the camera handles highlights? And doesn't the knee factor in here? It would seem like zebra settings would be useful to indicate where the camera cuts off all detail and goes to pure white. Does that point change depending on settings?

What are some good strategies for using zebras to determine exposure?

Raoul
 
Re: Zebras and Gamma settings and Knee

I second that!
I don't quite understand the zebra concept myself. What I have been doing using the zebra feature is this.

Set the Irus to auto for a reference exposer. then with zebra on I put the Irus into manual mode and adjust the exposer to taste.
Say theres a window in the background showing alot of stripes.
I just gently pull the exposer down a bit to soften it up.

It seems to work pretty well, but I don't know for sure that this is the correct way of using it.
There are different zebra zebra settings on the camera, but I don't understand there purpose. my guess is personnel taste as to the straingth of zebra???
 
Re: Zebras and Gamma settings and Knee

My understanding of it is that zebra 80% indicates all the areas in the scene that are 80% or more above the exposure latitude range. Zebra 100% indicates areas in the scene that are completely outside the exposure latitude range. And does it really matter if you get an area that is blown out? (real question here, not hypothetical) I would expose for areas in the scene that are important to me, and if something else is underexposed or overexposed, then that's just the way it is sometimes.
 
Re: Zebras and Gamma settings and Knee

The CINE GAMME allows whites to go past 100IRE and then cuts off abrubtly right around 110IRE. This is fine if you are planning a film out and want that extra detail in the highlights. But the hard cut off can be noticeable in video only viewing environments. The other modes will add a shoulder to the curve slowly cutting down the level to 100 IRE - usally this curve starts in around 90IRE or 95IRE.

Michael
 
Re: Zebras and Gamma settings and Knee

The % number shows basically the percentage of allowable brightness (basically, not exactly... the zebras actually are showing you the IRE value, and IRE ranges from 0 to 110). So at 100, you're almost at the limit of brightness. On the 100A, you can set the zebras to 105, which is even closer to the limit. Once you hit 110, there is no detail being recorded, just an ugly white blowout.

There are two zebra settings you can program into your camera, Zebra 1 and Zebra 2. By default I leave mine (on DVX100) to 80 and 100. If I had a 100A I'd change that to 80 and 105.

80 is commonly used for face exposures -- you want the specular highlights (the reflections of light off the shiny parts of the face, like the forehead) to just barely trigger the 80 zebras. That will give you a decent reference point for exposure on a face.

100/105 is used to monitor for any area of the screen that's blown out. If you have a lot of 100/105 zebras on your screen, you probably are shooting ugly video. Blown-out video is ugly ugly ugly, and you want to avoid it whenever possible.

So what do you do when you have 100/105 zebras on your screen? You can iris down until they go away, but that might ruin the exposure on the rest of your scene. If it does, you've got a contrast problem in your frame, and the way to solve that is to bring up the light level on your subject, so that you can iris down to eliminate the overexposure, while still maintaining proper exposure on your subject.

A little bit of 100/105 zebras isn't going to kill you, especially if they're showing up on something like a light bulb or some other intentionally overbright itme. But if you have a lot of 100/105 zebras, you have a problem in your scene that should probably be addressed.
 
Re: Zebras and Gamma settings and Knee

Barry, your comments on zebra is right on the button. I don't shoot without using zebra. I generally figure that faces (white) range between 70 & 80 naturally. Most of the time I use the 100% so I know my limits.

Thanks for your comments.

take care,

eric
 
Re: Zebras and Gamma settings and Knee

How 'bout a standard reference like a white card? As long as the white reference card is in the same key-light area as the main subject, you can get a real 100ish IRE reading that allows all the other infinitely variable skin tones to fall in appropriately along the straight-line portion of the exposure curve. Unless you're in a hurry, why try to use an 80ish IRE reference that attempts to find acceptable exposure on ambiguous skin tones that vary person to person. Isn't a true 100 IRE white-reference exposure able to to deliver a more accurate version of skin tone?

If a black man and a caucasian woman are standing side-by-side in a scene, what is the normal key exposure for that scene? How 'bout a white card right between their faces?
Assuming the subordination of creative exposure variation(Sigourney Weaver six stops overexposed during Close Encounters) to the necessity of normal exposure accuracy, the two faces--maybe 1 2/3 to two stops apart in a reflected light reading--will remain appropriately dark and light by respecting the known 100 IRE reference.

A creative exposure might be altogether different, depending on the story and scene...different discussion
 
Re: Zebras and Gamma settings and Knee

As usual, Barry Green was right on the money in his explanation of the use of zebras and IRE factors. And just for grins, Sigourney Weaver wasn't in Close Encounters. ;)
 
Re: Zebras and Gamma settings and Knee

[quote author=JBN link=board=news;num=1083121539;start=0#7 date=05/05/04 at 12:26:27]And just for grins, Sigourney Weaver wasn't in Close Encounters.   ;)[/quote]

I don't know... when the aliens came out of the ship they were so back lit and blown out, Sigourney could easily have been playing one of them. :D
 
Re: Zebras and Gamma settings and Knee

Not that I've had my camera too long, but on my first shoots I was afraid of Cine Gamma, thinking I would get that ugly knee if I overexposed. But now that I use zebras at 100% when using Cine Gamma, I'm not afraid anymore. The zebras let me know if anything (other than super bright items that should blow out) is blowing out in my shot. When I'm not using Cine Gamma, I usually have my zebras at 80% to give me more general exposure information -- skin tone, etc.
 
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