mikezip007
Active member
Why do video cameras balance color using a white card and photography uses a gray card?
White balance question
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mikezip007
Active member
I know the difference in terms and in the photo studio I work at, we take a picture of a gray card to check exposure AND perform a custom white balance.
On the video shoots I work on, we use white cards to check exposure AND set custom white balance.
On the video shoots I work on, we use white cards to check exposure AND set custom white balance.
Ryan Patrick O'Hara
Veteran
As far as the gray card in film.... it is possible for a colorist to adjust colors based on making the gray card neutral or tinted. This is mainly done in uncontrollable situations, where the film is known to be wrong and you want it to be balanced out. IE A film student shoots under florescents but doesnt have the proper filters or gels to correct. They will shoot the gray card and say in the lab instructions.... 'balance for proper skin tones' or something like that.
So you have a point with the gray card in film. I personally think it's ridiculous to base video exposure readings off of a white card.... it tells you something, so it's not useless.... but I think it's incredibly unnecessary and ridiculous to base your exposure readings off of a white card.... unless you are trying to expose for something white in the frame as middle exposure... in any case, you're risking underexposing the entire scene. This can be done and work for special situations, but as an overall way to gauge exposure, it's a backwards approach.
So you have a point with the gray card in film. I personally think it's ridiculous to base video exposure readings off of a white card.... it tells you something, so it's not useless.... but I think it's incredibly unnecessary and ridiculous to base your exposure readings off of a white card.... unless you are trying to expose for something white in the frame as middle exposure... in any case, you're risking underexposing the entire scene. This can be done and work for special situations, but as an overall way to gauge exposure, it's a backwards approach.
mikezip007
Active member
Exposure set aside, why white balance in photography with a gray card and in video use a white card?
Ryan Patrick O'Hara
Veteran
They really don't use gray cards in the same way as white cards...
As stated the gray card isn't really used for color balance the way a white card is. It's mainly referenced for exposure information. They (colorists) could notice if the neutral gray card seemed warm, cool, green or magenta and perhaps correct it if it wasn't intentional.... but that is not the same as white balance. White balance on a video camera is uniquely a 'video' thing to do. Film stocks come in preset color temperatures and cannot be 'balanced' like video.... thus the gray card is not serving the same purpose as a white card in white balancing... because film cannot be balanced. The reason video needs white is most likely because all light wavelengths equal white light. So it's probably easier to tell what light needs to be compensated for by referencing something you are telling the camera is white. My guess, though. Film does not use gray cards for an all purpose 'white balance' like substitute. They use it for exposure information and occasionally could be referenced for color if something is amiss. Most colorists would probably rather reference skin tones versus then gray card anyhow, if that is what they are trying to correct for.
I hope that makes sense. Gray card is exposure and could be referenced by a human colorist for color information... white cards are for computer white balance in which it needs to detect what is white and what color light is missing and needs to be compensated for. The video white card is not for exposure nor can it be referenced in post quite like the gray card.
It's simply the difference in purpose and process. Both are different, although you would have more success white balancing to a gray card then basing exposure off of a white!
As stated the gray card isn't really used for color balance the way a white card is. It's mainly referenced for exposure information. They (colorists) could notice if the neutral gray card seemed warm, cool, green or magenta and perhaps correct it if it wasn't intentional.... but that is not the same as white balance. White balance on a video camera is uniquely a 'video' thing to do. Film stocks come in preset color temperatures and cannot be 'balanced' like video.... thus the gray card is not serving the same purpose as a white card in white balancing... because film cannot be balanced. The reason video needs white is most likely because all light wavelengths equal white light. So it's probably easier to tell what light needs to be compensated for by referencing something you are telling the camera is white. My guess, though. Film does not use gray cards for an all purpose 'white balance' like substitute. They use it for exposure information and occasionally could be referenced for color if something is amiss. Most colorists would probably rather reference skin tones versus then gray card anyhow, if that is what they are trying to correct for.
I hope that makes sense. Gray card is exposure and could be referenced by a human colorist for color information... white cards are for computer white balance in which it needs to detect what is white and what color light is missing and needs to be compensated for. The video white card is not for exposure nor can it be referenced in post quite like the gray card.
It's simply the difference in purpose and process. Both are different, although you would have more success white balancing to a gray card then basing exposure off of a white!
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j1clark@ucsd.edu
Veteran
Because we don't have video Shirley's to calibrate to when printing the video... wait... there is no printing in video...
And you would be 'grey' balancing in any case... The assumption is that the 'grey' card truely is color 'neutral', and so, a density reading of the film's response in the color negative, should yield 'equal' density of the 'color' (I'll not go in to CMY+base correction... but you should get the idea...), or, given the particular film stock, calibrated devations from neutral. The next assumption is that the film's response at higher densities, up to some max, is 'log-linear' so the light areas can be corrected, having only read the mid densities... but then if this worked with absolute accuracy, there wouldn't have been a need for Shirley anyway...