How can I adjust for any inaccuracy in a homemade white card?

Question

Quite a number of years ago, I made my own white card. It is basically made up by two white access cards in plastic glued together and sanded down so that they have a matte finish.

The cards have been good enough for my meager use, I'm purely an amateur and hobby photographer, and though I knew that the card was almost certainly not pure white or a good balance, it has served me well.

However now I've finally purchased a white/gray card, and I'm wondering if I can still use the white card that I made, since it has a size that is a bit easier to carry around, and I have attached a strap to it so that I can carry it around my neck or wrist more easily.

The question is this:

If I take a photo, now, of my new real white card, alongside the one of my own creation, can I figure out the difference, so that if I keep using my old card, I can adjust white balance to it, and then apply an additional fixed adjustment to bring it into correct white?

Or, will the amount to adjust by not be fixed, but say vary depending on the light?

Or should I just throw it away altogether and forget I ever mentioned it? :)

Here's a Canon EOS 60D RAW file I just took. This is indoor lighting, so quite yellowish, but when I take a white balance reading from the small card and the large card, close together, they're off by about 50 give or take.

The homemade card is darker, but the difference which I originally took as a difference in tint, appears to be more about the brightness than about color.

IMG_0962.cr2 - About 21-22MB.

This tells me that it's not too shabby the card I made myself, so I can apparently keep using it.

To answer implied questions from the current answer:

• No, I'm not doing any product or production photography, this is purely for my own enjoyment
• Yes, I'm using RAW, and only RAW

Answer 1

unless it's extremely off, which probably isn't since you have been using it, and assuming that you're not doing product photography where accuracy in colour reproduction is more important and getting it right from start is a time saver, I'd say that you can continue to use it. this is especially true if shooting in raw where you'll be able to fine tune if the immediate result is not what you were expecting.

Answer 2

In the absence of a grey card, what do we do? Select something in the image we think should be neutral and let Photoshop or Lightroom use that to adjust the white balance. But that something we pick is unlikely to be perfectly neutral. That white wall is actually Antique White with a lot of pink in it, or the grey sky is actually quite blue. But you usually get a good result from picking neutral-looking items that are in fact much less neutral than your white card, so yes you should be fine.

The 50 degrees difference is encouraging. The way I think you should test it is to adjust your raw image until the grey card reads neutral, say the RGB values are 120,120,120. Then use the dropper tool on your white card, and the RGB values should be neutral within a couple of points. If the RGB values are 237, 235, 236, you're in good shape. If they were 235, 235, 247 then no.

You might want to try in daylight, fluorescent, and incandescent lighting and make sure the results are the same. I can't imagine why they would be, but one test in yellowish light might not be sufficient.