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martcol
19th of March 2003 (Wed), 02:38
I've seen several references on this forum to using a photo of a grey card as a reference to use in Photoshop instead of fiddling with WB settings. Anyone care to tell me how this is done both at camera end and computer end.

Many thanks

Martin

Roger_Cavanagh
19th of March 2003 (Wed), 04:23
Martin,

For completeness:

Method 1 in camera:

Take a picture of the gray card, so that the card fills, at least, the centre area of the frame - more is better. It doesn't matter what settings or that the card is in focus. I've seen some recommend that fuzzy is best because any minor variations in the colour are blurred.

Obviously, the card must be illuminated by the light that you will be taking pictures by.

Put the camera WB on custom and select the gray card picture.

The D30 manual suggests using a white card for this, but it is generally agreed that gray is better because there is less chance of blowing one of the colour channels.

Some organised photographers go to the trouble of having several gray cards shots taken under different lighting conditions on a CF, so they can just select the appropriate WB image. I believe (though not with 100% certainty) that the custom WB image does not have to be in the camera once it has been set.

Method 2 during conversion:

This can be used only when you are shooting raw format. For the first picture in a sequence under similar light, put the gray card in the picture. Shoot on AWB (although I guess it doesn't really matter).

When you are converting images from raw, use the eye dropper to set WB from the gray card. This setting can then be used to convert multiple images. The precise method will vary from converter to converter.

A change from using the gray card has come to light since the appearance of Adobe Camera Raw. Colour balance will be better in ACR, if a 3/4 tone white is used rather than the mid-tone gray card. This is something with RGB values of around 192. I've seen several people who say they always used 3/4 white anyway. I hypothesise that higher numbers means more light, finer variations and, therefore, better control.

This method can be used even if you haven't included a gray (or 3/4 white) card, if there is something in the picture that you know should be a neutral colour. The point about WB is not that you are specifiying what is white, but identifying a colour that is neutral and should have equal RGB values.

Method 3 in Photoshop:

This can be used whether you have shot raw or JPG and is basically the same as Method 3. Have something in the picture that should be a neutral gray and use the levels/curves middle eyedropper to set the graypoint.

If you have two open documents, you can use a neutral subject in one image to set the gray balance in the other. I have never tried, so I don't know whether you could record this setting and use it in a batch action. At least, I know you could record it, but I don't know whether it would work as desired, i.e., same gray balance of all images in the batch. I have just recorded these and the action does look like it would work OK on multiple images.

Does this clarify things?

A final comment: repeating something I read in a recent post by Andrew Rodney. Sometimes you might not want to set WB to produce perfectly neutral colours. For example, in a glorious sunset scene, whites do not look white. Your picture would look odd, if you corrected them to do so.

Regards,

Canon-1Ds
19th of March 2003 (Wed), 06:49
Here's a tip...

The Grey card or 18% tone can also be used as a good exposure average, if in doubt point your camera at a grey card or indeed a field of green grass (18% tone) and using the exposure settings given from the card or grass you will always get a perfect exposed picture.

Roger_Cavanagh
19th of March 2003 (Wed), 07:56
Canon-1Ds wrote:
Here's a tip...

The Grey card or 18% tone can also be used as a good exposure average, if in doubt point your camera at a grey card or indeed a field of green grass (18% tone) and using the exposure settings given from the card or grass you will always get a perfect exposed picture.

Actually, Canon meters are calibrated for 12.5% reflectance (according to the EOS Documentation Project http://eosdoc.com/manuals/notes/glossary - see entry under exposure value). So a reading from a gray card would give under-exposure by about half a stop. Mind you, if the camera is rounding to the nearest 1/2 or 1/3 stop, then a lot of the time this difference won't be noticed, plus with digicams under-exposure is to be preferred to over-exposure. But, I reckon, that every now and again, probability laws would give an under-exposure of 2/3 - 3/4 of a stop, which might give less than desirable results.

Regards,

martcol
22nd of March 2003 (Sat), 02:08
Roger

Thanks for that - sorry it took a while to get back. Anyway, I'll try it out. I always appreciate your contribution to the forum.

I find the colour stuff the most difficult part of digital photoraphy to understand. I use a G2 and am conastantly dissappointed with colourcasting. I often try the same shots using different WB settings including custom. But it is a distraction when you have to worry about the camera's settings rather than the shot.

I'm wiating for my 10D now and hope that life will get better.

Martin