White balance - white vs grey

When setting custom white balance we are told that we can use either a white card or a grey card. Similarly, if post processing RAW images, one of the ways of setting white balance is to click on either a white or on a grey part of the image to set the white balance.

I assume that must be correct since it seems to be generally accepted. But what I don't understand is the following. I can understand that clicking on white somehow tells the camera or DPP that that is the colour that we want to look like white. But white and grey are different colours (or shades or whatever). We don't really want the grey areas to look like white.

Could somebody please explain why white and grey can be used interchangeaby.

wallace99 said:

Could somebody please explain why white and grey can be used interchangeaby.

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white and neutral gray cards can both be used for white/color balance because they are both neutral with no other colors. if a white/gray card appears yellowish/bluish/reddish in your photos, you can adjust the white balance on your camera or computer until it is white/gray again.

a neutral gray card can perform an additional function in that it can be used to set exposure. in any particular lighting condition, digital cameras generally adjust exposure such that the subject appears about 18% gray ... the problem lies in the black suit/white dress situation ... if determining an exposure using black suits, the camera will believe the suit to be 18% gray and over-expose faces ... alternatively, if exposing for a white dress, the dress becomes 18% gray and faces could end up being underexposed and dark suits could lose detail ...

why not use the neutral gray card to make sure? if set using gray card, light colors should be appropriately light and dark colors should be appropriately dark. the gray card could be re-used if lighting conditions change ...

The reason is that neutrality is the important quality...not the brightness. Both white and gray have equal proportions of red, green, and blue, and that is what white balance is all about.

It's called white balance because you are balancing the color from the light so that white (or gray) objects look white (or gray). Consider the light from a household lamp. It has a lot of red in it and gives all objects a red cast. When you take a picture of an object that's supposed to be white, it has a red cast. By selecting that object when performing a white balance fix, you are saying to the software, "adjust this color until it has equal amounts of red, green, and blue." That adjustment is also applied to the rest of the photograph, and all colors look accurate (although that might not always look best...for example, you probably don't want to white-balance a sunset scene.)

It's not good to use an actual white object for your white balance correction. The reason is that few objects are truly white. If you put 10 white objects side by side, you'll likely see 10 different "whites". It's best to use a white balance reference. These are usually a light gray. Their defining quality is that they're spectrally neutral. That is to say, these WB references will reflect all light equally.

You definitely don't want to use a white piece of paper. White paper usually contains optical brightening agents, which convert invisible ultraviolet light into visible blue light. That actually makes the paper look "whiter". The reason is that we usually view white paper under artificial lighting, and such lighting usually has excess red. The blue will counter the red and makes the paper look whiter. It's like having a white balancer built into the paper. Unfortunately this confuses the camera, and a white balance setting taken from such paper will remove too much blue and give the image a yellow cast.

Good white balance references are The Digital Graycard, WhiBal, and Novoflex Zebra Check Card. The Novoflex is also an 18% gray card for exposure.

http://store.rmimaging.com/digitalgraycard-100.aspx

http://www.amazon.com/GENUINE-WhiBal-Certified-Neutral-Balance/dp/B004G3M36Q/ref=cm_cr_pr_pb_t

http://www.bhphotovideo.com/c/product/379038-REG/Novoflex_ZEBRA_1_Zebra_Check_Card.html

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Color space has three axes. You can read about one way to assign axes here:

http://en.wikipedia.org/wiki/Lab_color_space

Exposure is concerned with the axis commonly called lightness, value, or brightness.

White balance is concerned with the other two axes. Both true white and any shade of true gray will be the same in all except lightness.

To put a different way, if you're in a room lit by blue lightbulbs, adding more lightbulbs won't change white balance -- just exposure. Only changing to e.g. red or white lightbulbs would change white balance.

How do those differ from the $2 Kodak gray card I bought at my local camera store?

Alphoid said:

How do those differ from the $2 Kodak gray card I bought at my local camera store?

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First, they're plastic so they won't rip apart like the thin-cardboard Kodak cards. They can all be washed when dirty. The color of the The Digital Gray Card and the Whibal (and also the Digital Grey Kard) go through the plastic, allowing you to sand the surface to refresh it should it get too much embedded grime or get too reflective. They will basically last for your lifetime.

Second, they're designed to be spectrally neutral. The Kodak card is designed to reflect 18% of the light that falls on it. Spectral neutrality isn't required for that task. That's why we can set exposure using a clear blue sky about 45 degrees up, or the sunny side of healthy green grass. But setting white balance requires spectral neutrality. A scene with neutral objects will easily show the difference between setting white balance with a piece of copy paper and setting WB with a proper neutral reference. The copy paper will likely give the image a yellowish cast. That's because the copy paper is not spectrally neutral (quite intentionally so.)

Will a Kodak card work? Very likely. But a thin piece of plastic of around 4x6 is very easy to carry in a back pocket or in the booklet pocket of a camera bag. You can whip it out to set white balance in the rain or any other condition, and drop it in the mud and not give it a second throught...just rinse it and put it away. It's the convenience of the material that increases its usability. And something you pay 30 dollars for and use, is a better value than something you pay 2 dollars for and don't use.

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wallace99 said:

Could somebody please explain why white and grey can be used interchangeaby.

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Very, very good question. I have often wondered about this myself. I have some ideas, based on an primitive hunch that a computer can distinguish (and ignore) either "bright" or "dark", but cannot differentiate ""white" from "bluish white" or "greenish white", as they are not luminosity levels per se.

If anybody can answer, or suggest a google search, I also would be interested in the answer.

Bueller... Bueller....

Mike

Thanks for all the replies.

It looks like I have been partly confusing white balance and exposure and that, in simple terms:

• both white and grey have the same white balance ie neutral
• in exposure terms, white is simply a brighter version of grey with grey generally being in the middle of the brightness range (and normally suitable for setting exposure) and white at the top of the range (and usually not suitable for setting exposure)

I think I am now fairly happy that I understand why we can use either for white balance.

If I understand this correctly, the Kodak gray card (the one used for determining exposure, not setting white balance) does not need to be gray at all. It could be red, blue or green. It merely must be designed to reflect 18% of the light that strikes it and it's color doesn't really matter. If this is not the case, there is still an element of this discussion that needs to be explored in order to complete my understanding.

Doug Haag said:

If I understand this correctly, the Kodak gray card (the one used for determining exposure, not setting white balance) does not need to be gray at all. It could be red, blue or green. It merely must be designed to reflect 18% of the light that strikes it and it's color doesn't really matter. If this is not the case, there is still an element of this discussion that needs to be explored in order to complete my understanding.

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Yes, no, maybe.

Using a neutral grey target for setting exposure is recommended.

The problems with using for example a red target are twofold.

Firstly, being red, it will reflect 0% of the green and 0% of the blue light which hits it. That means it will reflect about one-third of the light at best, but possibly less depending on the exact shade of red.

Secondly, depending upon the colour of the light source and the white balance setting, the red (for example) card may appear either darker or lighter. As such, results will be unpredictable.

Certainly it is possible to meter from green grass, or from the palm of one's hand, but these are not necessarily ideal targets.

Regards,
Peter