Custom White Balance

[trustwoody]

I was wondering if someone could help me understand what happens differently for the camera when you are setting a custom white balance off of a white card or shooting a 18% grey card? It seems like there has been discussion where some of you have recommended the grey card and I am not sure what the camera is "seeing" differently between these two situations. Thanks in advance.

[jimsolt]

I was wondering if someone could help me understand what happens differently for the camera when you are setting a custom white balance off of a white card or shooting a 18% grey card? It seems like there has been discussion where some of you have recommended the grey card and I am not sure what the camera is "seeing" differently between these two situations. Thanks in advance.
I, too would like to hear a definitive answer. Even my camera manual is vague. EVERYTHING I had learned in the past says that when you white balance a camera (custom) the subject you frame is seen by the camera as white. Hence my belief that if you use a blue card the camera will see blue as white and a gray card used to white balance will be seen as white, not gray.
In addition to the EVERYTHING above I learned that EXPOSURE can be set accurately by using the 18% gray card. I think exposure is different than white balance, though certainly both must be accurate to get a good photo.
If I'm wrong, it would be nice to know it.
Jim

[sdommin]

I was wondering if someone could help me understand what happens differently for the camera when you are setting a custom white balance off of a white card or shooting a 18% grey card?

There should be no difference. You can use a gray card, a white card, or even a black card, I guess. All that matters is that the color of the card is truly neutral; in other words, the red, green, and blue components are equal. Your camera produces color by comparing the differences in the amount of light reported by the different red, green, and blue filtered pixels. If all the pixels are reporting equal values, then that object is rendered as some shade of gray (from black to white).

18% gray cards are nice because you can use them to gauge both color and exposure.

[jimsolt]

There should be no difference. You can use a gray card, a white card, or even a black card, I guess.
I think this statement is correct with respect to exposure, but incorrect with respect to white balance. My background is in color TV and in that medium you "white balance" using white. You then set exposure using a gray scale and in that gray scale the 18% gray would approximate skin tones which I think to be the basis for use of the gray card in still photography. Using other than a pure white card for white balance would skew the colors toward the color of the card used for balance.

[trustwoody]

Maybe what I was looking for when asking what is the camera seeing is better asked by what us the camera looking for? OR maybe another way is asking what does the camera find when it looks for something in a custom white balance photo? It just seems to me the camera is asking something and the photo is answering the question or questions and I wonder what that question is?

[kb244]

Black card might not work. But as long as its a nutral color, the light in the area will bounce off the white/gray card, and you gota correct the warmth/coolness caused by the light hitting the neutral card. Using a nuetral card allows for the camera to know whats the subject's color and what's the light's color, since subject color already determined ( a neutral color ), then its just a matter of adjusting the white balance for the light color reflected on the subject.

[CDickinson]

White balance is seen by the camera, I think, as color temp. So, when you custom white balance, the camera can adjust for whatever light you are in - and all lights have different "tones" based on color temp. When you shoot outside at night, the light is orange. When you shoot inside, you can get a different tint. The white balance adjusts for that.
I'm saying the same thing Jim said - digital cameras act like video with regard to light....so set your white balance by shooting a white card and then choosing the photo as the custom. Set exposure by shooting the grey card.
Then you're set for both.

C

[trustwoody]

C,

So shooting a grey card for custom white balance is going to give misleading information to the camera?

[CDickinson]

Yes. You need to read/shoot a white card for white balance. You shoot a grey card for exposure settings. I don't know what camera you are using so I can't help you out with where to find settings etc for white balance and grey card exposure.

C

[trustwoody]

Oh, sorry about that. I have a 20D and Dig Rebel. I have been shooting primarily in Auto White Balance and get pretty good results for the most part with a few clinkers now and then. I am trying to understand a bit more how this camera thinks. I was at a local photo store and the salesman was talking about the custom wb setting and had a flash on the camera and shot a white paper with the flash on to get his custom white balance. He was demonstrating some other brand of camera (Dig SLR) but I thought it was a bit curious that he shot with a flash because the flash has its own color. Or does the color of the light come from the space where the shot is being taken and the light source in not relevant? It seems to me that the camera needs a properly exposed, in focus shot of a white background in the environment that I am shooting for the custom white balance to work as it is designed to work. But, that is why I am seeking counsel here to help me understand this whole issue. I do appreciate your input.

[Jesper]

C,

So shooting a grey card for custom white balance is going to give misleading information to the camera?
Yes. You need to read/shoot a white card for white balance. You shoot a grey card for exposure settings. I don't know what camera you are using so I can't help you out with where to find settings etc for white balance and grey card exposure.

C
No! For white balance, it doesn't matter if the card is grey or white, as long as it has a neutral colour. It's in the manual of your camera.

And do not use flash when you shoot an image for setting the white balance, because the flash light has a different colour temperature.

[trustwoody]

If you are shooting an event where you have two different rooms with two different types of light, there should be two different custom white balance shots used then, is this correct? And is the camera taking the available light and measuring it using the calvin scale?

Every once in a while I am shooting photos, I am amazed at what is happening in this little case with a lens on it. Geek heaven!

[CDickinson]

Correct. If the lighting in each room is mixed, ie, the sources are different temps you may want to test shoot with average white balance also. Your exposure times may also be different so check your grey card for each room also.

C

[Trident Photography]

I just bought a thing called an expo disc. It is a white translucent filter that snaps into the lens and it also sets exposure. This thing is amazing. You pop it on take a shot to use for your custom WB and 'pow' everything is perfect. The only down side is the cost. It was like $190 CDN for a 77mm size. But in the long run it is worth the time savings of color correction in post production.

Jeff

[Jesper]

I just bought a thing called an expo disc. It is a white translucent filter that snaps into the lens and it also sets exposure. This thing is amazing. You pop it on take a shot to use for your custom WB and 'pow' everything is perfect. The only down side is the cost. It was like $190 CDN for a 77mm size. But in the long run it is worth the time savings of color correction in post production.

JeffA white coffee filter costs almost nothing and also works really well. I haven't used an ExpoDisc, but I have used a coffee filter and I am convinced that the ExpoDisc isn't going to do it so much better that it's worth spending so much money on.

How the coffee filter trick works: Set your lens to manual focus, hold a white coffee filter in front of the lens, point camera and lens in the direction of the light source of the light you're photographing under, make a photo and use that to set custom white balance.

[trustwoody]

Jesper,

What is the advantage of doing this compared to shooting a white paper and filling the view with that page. The manual focus is set so that you can pull the trigger while the camera is out of focus? I am assuming that you are putting the filter directly in front of the lens. I understand Jeff's purchase. Being a complete junky for this stuff myself. I can see why that thing is a a viable option. It just plain looks cool! Anyway, my question that raised about what question the camera is asking perhaps is answered like this: "This is the color light reflected in this space off of a neutral subject. It is trying to tell me (the camera) that the subject of the shots taken in this room is blue (or amber, or whatever...) so I need to correct the misinformation that the light is trying to give by setting a white balance setting to ..."

[Jon]

OK - the camera doesn't have a colour temperature meter built in. What it does is take the area you've photographed as "neutral" and applies adjustments to the colour levels so that the red, green, and blue sensor cell values are the same. It then applies those adjustments to R, G, and B values throughout the rest of the photos you take with that CWB setting.

It matters not what neutral subject you choose, but it has to be neutral, and it has to be exposed as a mid-tone. If you meter for a white subject to render correctly tonally, you risk blowing out one or more of the channels, invalidating the CWB-setting process. If you meter to render a black as a black, the separation between the channels will be too low to get meaningful CWB adjustment factors, especially if by under-exposing the area one or more of the channels drop to 0 level. If you choose a mid-tone, you can be reasonably sure that the R, G, and B values will all be within the recordable values for the sensor, so a meaningful adjustment will be possible. That's why Canon is now suggesting in the manuals that you'll get better results photographing a photographic neutral grey card.

Please note that white paper, or white fabrics laundered in detergents, frequently contain brighteners which cause them to respond more strongly to blue than to other colours. That's one reason use of a true neutral subject, not just a handy sheet of paper, is vital.

AWB assumes a scene that will average out to a neutral colour; it doesn't rely on some magical colour sensor. For that reason, it may not be the best choice for photographing your white-balance standard. The defined light colour values, and CWB (even if you've previously set it), have known adjustments applied to the input (raw) values coming off the sensor.