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I know 18% grey cards are supposed to provide a neutral color to set custom white balance. But why are grey cards used instead of pure white ones?

My assumptions: Is a specific greytone easier to produce than pure white? Does white stain so much easier? (or Does grey just doesn't show stains that much?)

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6 Answers 6

up vote 15 down vote accepted

When you have already set your exposure parameters, white could be clipped in some single color channel (but not all, so your camera won't show it as blown), therefore not being very good basis for color balance adjustment.

Also, paper will turn yellow during time. And just looking at the sheets currently on my desk, there's three different tones of white papers already.

That said, I've used white paper many times and it will get you close enough in all but very critical studio shoots. Very often you'll have multiple different light sources, reflections from colored surfaces and/or shadows vs. lit areas, all having much more impact on colors than slight imperfection in metering the white balance. Just make sure you don't clip any color channels (e.g. by spot metering without compensation from the paper, or verifying exposure parameters using color channel histograms).

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2  
Doesn't matter whether you use black, some variant of gray, or white. What matters is that the color is neutral and doesn't have any of the channels clipped. That makes white and black poor choices under some lighting conditions. –  Eric Jan 20 '12 at 23:42
    
Black is also harder to judge whether it's really a neutral black or some kind of dark blue/brown. –  Imre Jan 21 '12 at 8:19

Grey and white are the same color to the camera. You can consider grey as dark white or white as light grey. So it is very much the same and you can take a white-balance measurement from any shade of white as long as it reflects light uniformly. As you read from the answers you already got, white paper is not necesarily white.

Depending on your camera, the custom white-balance function either uses a set or an automatic exposure. In the former case, using a bright object may cause it to be over-exposed while will either give you an error message or an incorrect white-balance reading. In the latter case, the camera would be able to meter on any shade of white and, if you will the frame, even a white-paper will appear roughly 18% grey.

If you buy an exposure card in a photo store, they will sell you something 18% grey. If you buy a white-balance card, it will be closer to 80% grey, so almost white. You can buy combined products, like the one I use which has one dark side and one bright side. The difference exists because a digital camera is able to make a more accurate measurement with brighter values. Dark surfaces are more susceptible to image noise which often introduces a color-shift because you may have more noise on one channel.

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I assumed the reason for 18% grey as a white balance target was because the incident light meter in cameras is measuring for 18% grey. Therefore, by presenting a target that the light meter is most sensitive to, and one that exposure is being calibrated against, gives the best result.

I also think this is related to consistency: use a grey card in all your images, and when white balancing, all your images will be based on the same reference. Presumably you could use a piece of printer paper, as long as you used the same piece of printer paper (and the paper didnt yellow over time).

I always thought it odd that Canon recommends using a 'white' target for manual whitebalance. My suspicions are that they do this so as not to confuse most photographers, forcing them to get a 18% grey target.

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Cameras don't have incident light meters, they have reflected light meters. That is, they measure the amount of light being reflected by the subject/scene. An incident light meter measures the light directly as it is emitted from the light source while being held in the location of the intended subject. –  Michael Clark Oct 22 '13 at 5:14

Maybe because you already have the 18% gray card with you for setting exposure so why don't you use the same card for white balance?

And as a bonus, because it's in the exact middle of the exposure it's the least likely color to clip.

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As far as I know white paper is not necessarily white in the spectral view (more a little blueish) hence not neutral. A special made grey card should be 100% neutral.

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Most white paper containers "brighteners" -- basically a fluorescent dye that glows (slightly) when exposed to the small amount of UV in normal daylight. As a result, the spectrum you get from the paper does not exactly match the spectrum of the light falling on it. Worse, the spectral deviation is somewhat unpredictable, often differing between one brand and type of paper to the next, and sometimes even varying between one lot and the next of paper that carries identical labeling.

I should add, however, that many grey cards aren't much better. Just for example, the classic Kodak grey cards have a bit of a magenta tint.

I only know of one grey card that really maintains a neutral color under essentially any kind of illumination, so the light it reflects consistently has the same spectrum as what falls on it. Unfortunately, I don't know of any distributors outside the UK, so about all you can do is pay too much, and wait too long to get what should be a fairly basic piece of equipment.

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