Thanks for the correction, I think I mispoke about at the light source. There is an ExpoDisk that is aimed at the light source.
Custom is also called Preset manual WB, The camera doing Custom is aimed at a white or gray card in the same light as the subject. Card should fill the frame, but good focus is not required, it's instead about the color.
I think the working definition of Custom White Balance is the system done in the camera, described in the camera manual, as Custom WB. WB of course can be done directly in photos too, I merely quibble about calling that photo method as Custom WB. Name is already taken.
First reviewing the most basic WB ideas...
If we have any photo object of color, lets say a white dish, it has a certain color, which is the color that we want to see it. But light is colored too, incandescent bulbs are orange, clear blue sky is blue, and direct sunlight is what we call white. All of these sources vary their color in some degree, flash too. A light that is say yellow can make the subject which is say white, appear to be more yellow, so we see a color tint, and maybe it looks dingy. But we can correct it.
A quirk of our human brain is that it tries to do automatic white balance for us, so that when we come in from outdoor sunlight into incandescent light, we simply do not notice that incandescent light is orange. We are not even aware of a problem. We see what we expect to see, what we imagine we see. But it definitely is orange, and the dumb camera will capture that orange color, unless we correct it.
White balance is correction to modify the photo color in a way that matches the color of the light, specifically to offset the lights tint, to make its color effect be zero. WB is about the light, not about the subject. And it is a very good thing when we can get a human brain to operate the camera too.
There are a few ways to do this WB correction, to make our photos better (more accurate color).
In all cases, the WB tool or method has to be in THE SAME LIGHT as our subject, in order to correct that subject. It has to involve the light we want to correct. It is only about the light.
Cameras have the few standard WB choices, Incandescent, Daylight, Fluorescent, Cloudy, Shade, etc.
These are just approximations, guesses about what the light color could be, expected to be, but rarely actually is. Ballpark, better than nothing.
Cameras have Auto WB, which tries to examine the photo result and make it look better color, but it is a dumb computer chip that cannot recognize the subject, and has no clue about the light source, and no brain to even think about it. The result can be good, or more likely ballpark, but it can also be dumb.
The camera
Custom White Balance is a special menu, where the camera is aimed at a white balance card in the same light as the subject.
The camera diffuses it and measures it in a way that it can make it be neutral, or colorless, or white. Then using that same Custom WB (in that specific scene and session in the same light), our photos come out in good color. It works because the card is actually neutral color (no color cast of its own).
Custom is a good system if you are shooting JPG (it corrects before the JPG step is done, i.e., has full working range), but it is maybe a bit fiddly to do.
However, IMO, Custom would seem a pointless system if shooting raw, because WB is not in raw yet. We have no place to go with it (there may be some ifs and buts, but there are also better ways to do it.)
I shoot raw, so I don't actually do Custom WB.
There is a too-expensive device sold called ExpoDisk, which is a filter put on the lens (for only this test shot) to diffuse the light, and it is aimed at the light source, which sort of creates equivalent of a gray card, and so basically does what Custom already does. Is for if the camera did not offer Custom maybe? Custom is a recent thing, Exodisk is older.
Another very common system is to simply place a
white balance card with the subject (in the SAME light), and take a test shot of it. Then software offers a tool to click on that white card in the photo, and it says to the computer "This spot is neutral, make it be neutral" (meaning no color cast), and it does. The human brain helps the computer to find the right solution, to choose the correct solution. Then the computer is fantastic about implementing it. This is a very good way, and I personally see no reason to consider anything else.
(speaking Adobe, others are mostly unknown to me). Raw editors offer a way to easily apply this WB correction to all couple of hundred session pictures in the same one click. And raw offers more range to do it. Some raw software does this same procedure also for JPG images (same lossless edits), but JPG has more limited range, and in extreme cases, raw can work better (and again, there is no WB correction in raw until we do this).
So I think raw and a white balance card is the best, fastest, and easiest way to correct our pictures. Seems flawless to me, a whole new world of photography.
Some use a 18% gray card for this same purpose (as a WB card), and it works, but it is pretty dark, and it is spec'd to be 18% reflection, but which says nothing about the neutral color. Most are pretty close to neutral, and it works pretty well, but I see no point of not using an actual WB card. The card I like is a Porta Brace White Balance card, $5 at B&H. I also have a couple of the more expensive WhiBal brand cards (light gray), and nothing at all wrong with them, but I tend to use the Porta Brace card.
Many of our scenes naturally contain some white object, actually white, intended to look white (instead of off-white). Maybe signs, or t-shirts, or polka dots on pajamas. Clicking those usually works pretty well. Like the 18% card, we may not be certain they are exactly neutral color, but they are white, and far better than nothing. If it works, it works. If not, we can Undo. Lots more at
Easy White Balance Correction, fix with or without Raw
