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WB: grey card?
I can understand the use of a grey card to adjust White Balance to a neutral "color". But I see them packaged with black cards and white cards, too. When would you use black or white, instead of grey?.

Thanks..

Peter F...

Comments (12)

Grey cards were originally used for metering the correct exposure. This is because light meters are calibrated to average light out to the equivalent of 18% grey (technically it's a bit more complex than that and it's actually 12%, not 18% but for these purposes I feel the 'short' explanation will suffice).

In terms of setting the white balance - either a grey, black or white card will work equally well - as long as you know what colour it's supposed to be then you can adjust the white balance accordingly. Personally I always shoot RAW and so I just set the camera to Auto WB and make any adjustments in post processing.

Confused of Malvern.

'The greatest fool can ask more than the wisest man can answer'..

Comment #1

Photoshop's "curves" command has three eyedroppers to set white point, gray point and black point. If you have all three in a photo you can set curves using them and then apply that curve to other photos taken under the same conditions. These triple cards are sometimes used for that purpose..

Here's one example I just found:.

Http://freephotoshopguides.com/...cts/understanding-correcting-white-balance/..

Comment #2

Thanks for the help... but why would I buy a set of 3 cards (white, black, and grey) when only 1 card (grey) is needed to set the white balance?.

Peter F...

Comment #3

When you take a picture of the white/grey/black card (or three cards) you get a histogram with three peaks. There should be a peak in the middle and a peak near each edge. If your exposure level is off then the peaks will not be properly positioned. you can clearly see it because one of the edge peaks will be touching the edge. This means that your exposure will probably lose either shadow or highlight detail..

In any auto mode you simply use the EV control to increase or decrease your exposure. In manual mode you adjust the shutter or aperture to shift the peaks into the proper position...

Comment #4

So... it sounds like if I am only interested in WB adjustments, then all I need is the grey card.... and if I am intesested in adjusting my exposure then all three card are useful. RIght?.

Peter F...

Comment #5

All you really need is a white piece of paper to set WB...

Comment #6

How can it make no difference whether you use (1) white or (2) grey? I don't get it..

Peter F...

Comment #7

It makes no difference for white balance. That's because a white and gray reflect the same ratios of light across the spectrum. e.g. A grey card reflects the same ratio amounts of RGB as does a white card..

As another poster said, the grey card would be used when you want to obtain the average light level as well. But for white balance only, grey or white works fine..

And, yes, a white sheet of paper works fine, too. Even a square of white form core works well, and it'll last longer than a sheet of paper..

PeterNMIF wrote:.

How can it make no difference whether you use (1) white or (2) grey?I don't get it..

Peter F..

Http://www.pbase.com/digirob..

Comment #8

To set a neutral white balance you need something with neutral color. Both white and gray (and black) have neutral color because they have equal amounts of all colors. In other words, to determine color you need anything without color: white, gray (any shade of gray) or black..

Once the camera knows what the absence of color looks like, it can figure out what all the colors look like because it is starting from a clean slate: neutral. If it uses neutral as a starting point it can figure out all the colors from there based upon their offset from that point..

For example, under a particular red light both white and gray will be made the exact same amount more red than neutral. If you tell the camera that the white or gray it is seeing is actually neutral, it will subtract that exact amount of red from everything to neutralize all colors. The gray card will be darker than the white card but it will be just as red. Once that amount of red is subtracted out, the white will look white, the gray will look gray and all colors will look right..

I hope that makes sense..

PeterNMIF wrote:.

How can it make no difference whether you use (1) white or (2) grey?I don't get it..

Peter F...

Comment #9

Thanks for the clarifications. Now I see that what a white card and a grey card have in common is that they each have equal amounts of red, blue and green..

So that means I don't need to buy a grey card. I can use something white like the back of a piece of mat material or piece of foam board..

Thanks.

Peter F...

Comment #10

BendOR wrote:.

Photoshop's "curves" command has three eyedroppers to set whitepoint, gray point and black point. If you have all three in a photoyou can set curves using them and then apply that curve to otherphotos taken under the same conditions. These triple cards aresometimes used for that purpose..

I frequently have a picture with a white point or black point and sometimes both..

For setting white balance using PS curves is using one of these points better than using none?.

Jr..

Comment #11

It depends. The best scenario is that the image looks good and you don't have to do anything. The eyedroppers in Curves also set the brightness/contrast so if the whitest area isn't bright enough the white eyedropper will make it bright (as well as set the white balance). If the darkest area isn't dark enough the black eyedropper will make it dark (as well as set the white balance). You can do neither, one, the other or both as needed. There is a gray eyedropper as well..

Sometimes Levels is better for brightness/contrast (not white balance) because you don't have to make these points all the way white or black if you don't want to..

I'm sure others have other opinions so check it out and experiment. Good luck..

JohnRussell wrote:.

I frequently have a picture with a white point or black point andsometimes both.For setting white balance using PS curves is using one of thesepoints better than using none?..

Comment #12

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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