Exposure with grey card in LR

[bcgal00]

If I take a shot of a grey card in the scene I can later in LR click on that to set the WB, right? My question is what do I cllck on in LR? The card has different rgb valves. Which do I click onto? For example, 48/46/45 or maybe 35/3'/28 or maybe 65/68/68.

Understand what I'm asking? Does it matter which one I click as long as all the boxes look grey in the sample area? This has me perplexed. I can't seem to find the answer in my LR book. Thx for any advice.

 

[Mark Sirota]

Click on anything that is supposed to be neutral. If it's a true color-balanced white balance target, click anywhere on it. If it's a grey card designed for exposure, it's probably not actually color-neutral and should not be used as a white balance target.

In general you'll get the best accuracy by clicking on a spot that's as bright as possible, so long as none of the channels are clipped.

 

[bcgal00]

I don't really understand what you're saying but it sounds like my grey card might not be the best thing to set wb with. I'll have to do some research on this.Thx for answering tho.

 

[Denis Pagé]

[quote author=bcgal'' link=topic=89'5.msg6''1'#msg6''1' date=126349'3'3]
I'll have to do some research on this.Thx for answering tho.
[/quote]

Maybe you can start here as there are good explanations on the difference with grey cards for exposure. There is a link to a good video too. Another more expensive pruduct may be of use too for white balance and more.

 

[Mark Sirota]

Sorry if that wasn't clear. To directly answer your question about what to click on, generally you want to click on something that's supposed to be neutral (white/gray/black). And the brighter the better, unless it's so bright that one of the channels (red/green/blue) is clipped (fully saturated).

In other words, pick the brightest neutral that doesn't show 1''% for any of the channels.

Now, the other issue is that most "gray cards" are designed to be used to determine exposure, not white balance. You point your meter at it, and set the exposure (shutter speed/aperture/ISO) so that it renders as middle gray (middle of the histogram).

That's a different thing than white balance. A gray card can be slightly green and it'll still work perfectly as an exposure target, but it'll work terribly as a white balance target.
For best results you need to use something designed for the purpose, such as the WhiBal that Denis suggested.

 

[tzalman]

[quote author=bcgal'' link=topic=89'5.msg59846#msg59846 date=1263322'35]
If I take a shot of a grey card in the scene I can later in LR click on that to set the WB, right? My question is what do I cllck on in LR? The card has different rgb valves. Which do I click onto? For example, 48/46/45 or maybe 35/3'/28 or maybe 65/68/68.

Understand what I'm asking? Does it matter which one I click as long as all the boxes look grey in the sample area? This has me perplexed. I can't seem to find the answer in my LR book. Thx for any advice.
[/quote]
This is an inevitable result of the nature of digital imaging. An area of the photo that seems to be a solid color when enlarged enough turns out to have small variations from pixel to pixel. This is because of the nature of Bayer demosaicing and of noise. Of course the noisier the image, the more the variation. OTOH the effect is counter-balanced by the fact that the probe averages together the values from a block of pixels.

Sample different spots on the card. The visible differences in WB will probably be small. Pick the one that tickles your aesthetic fancy.