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right exposure for shooting with cine gammas


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#1 Andy Lehmann

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Posted 03 September 2007 - 07:40 PM

When i use the normal gamma, 18% grey will be at 53 IRE, when i use CINE 1 it will be at 47 IRE and useing CINE 2 it will be at 40IRE. The lowest zebra setting is at 70 %. This works quite well for the normal gamma. When you use a 70% equivalent zebra for cine2 it must be at 52%. So, if you want to use the 70 %zebra with CINE 2 gamma to set up the "right" exposure for highlights of caucasian faces, it will end up in overexposure. Is this conclusion right or am I mistaken? If this is true and you have to underexpose while useing the cine 2 gamma, how do you control exposure e.g. when shooting for a documentary while not having a monitor and /or a waveformer?
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#2 Michael Nash

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Posted 04 September 2007 - 04:36 AM

How are you deciding these "normal" exposures? A "normal" exposure is usually referenced by the midtones; in other other words an 18% reflectance gray card should come up close to 50 IRE. If a certain gamma setting pushes a graycard higher or lower on the scale, then try adjusting your exposure until it falls closer to 50%. Then observe how the camera handles highlights and shadows relative to that exposure. Don't use auto-exposure to tell you what IRE proper midtone should be!

Gamma settings are just a tool for redistributing luminance across the tonal scale. If you're going to do that (manipulate the tonal scale), you need some point of reference to compare them, AND by which to judge exposure. Usually that reference is midtone, as it is with film. I've seen too much confusion and botched exposures when people try to use white as a reference -- thinking that whites are somehow sacred in video -- and let their midtones wander all over the place as a result...

Pick a gamma setting that gives you the picture quality (i.e. tonal reproduction) you like. Test this gamma setting with a properly lit Caucasian face (and a proper monitor) and see where the various zebra levels fall. Pick a zebra reference that's easy for you to find while shooting. Practice and commit this to memory before shooting your doccu footage.

Without a solid reference for what the gamma controls are doing (testing), the exposure looks like (a proper monitor), or what luminances the camera is recording (waveform monitor), you have no reference for exposure. At some point you have to review your footage and then decide what combination of gamma and exposure gives you the look you want.
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#3 Andy Lehmann

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Posted 04 September 2007 - 06:22 AM

Normal tv-gamma in a videocamera is around 55 IRE and not 50 IRE, because it´s a gamma for tv and not theatre. A TV gamma has to been watched in a lit envoirenment, so the midtones must be brighter. The electronically gamma sets up the midtones - neither the black- nor the whitepoint. If you change your gamma in cc or in camera your midtones get brighter or darker. All the film-, cinelike or whatever they call gammas normally have a minus gamma (that means that the gamma value is a minus one): the picture looks darker, the curve gets flatter from black to 18% grey and above 18% it is steeper (hope that this is the right engl. term). If you do this in camera with a fixed gamma curve you are not sure, if they only change the gamma value or other things as well. So, sometimes you can expose your filmgamma the same way you do with your normal gamma, but sometimes you have to underexpose it a bit for best results (e.g. panasonic cinelike gamma)
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#4 Andy Lehmann

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Posted 04 September 2007 - 10:42 AM

Today I checked the camera with a software waveform monitor. When i shoot a greyscale in cine 2 the picture looks darker and the curve is flatter than with the other two gammas. When I shoot with normal gamma and underexpose by one stop the curve looks quite the same like the shot with cine2 gamma. Only the colors look a bit different. Perhaps the different gamma settings are fake. Tomorrow i can check this hopefully on a real hd monitor and waveform monitor.
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#5 Michael Nash

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Posted 13 September 2007 - 05:57 PM

Normal tv-gamma in a videocamera is around 55 IRE and not 50 IRE, because it´s a gamma for tv and not theatre. A TV gamma has to been watched in a lit envoirenment, so the midtones must be brighter.


It's true that the reference for medium gray in standard def TV has always been 55 IRE, not 50. I said "close to 50" because that number is not carved in stone, especially since the same image these days may be displayed in different ways (LCD screens, video projection, or filmout). Many people use 50 or even 45 IRE as a reference even when the image is destined only for TV display. Keep in mind that CRT displays already have a higher gamma than LCD screens or film projection, so you don't have to push your midtone reference too bright just because it will be viewed in a brightly-lit room. But my point was that the midtone reference should always be the same between gamma settings, whatever number you chose.

I don't know how Canon has programmed their gamma settings, but if they are like Panasonic then the gamma presets affect the entire gamma curve, not just the midtones. You might find the same midtone exposure between two different gamma presets (and the same absolute black and white), yet the shadow or highlight curve might be a little different.

Also keep in mind that with higher gamma (brighter midtones) color will appear less saturated and vice versa. Gamma curves that are overall flatter usually need an andjustment to the color saturation to appear "normal." Panasonic's "Cinelike" color matrix is designed to take care of this; I'm not sure how Canon deals with it.
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