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Standardizing exposure across multiple images using a gray card

Update:11-30Source: network consolidation
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Hi,
I am studying chest feather coloration in Barn Swallows, and take pictures of swallow chests against a white background with gridlines, while always including a gray card in the image. In order to evaluate chest color between birds, the lighting, exposure and white balance for all images must be identical.
I place my tripod in the shade whenever possible and set exposure manually (using the spot meter on a D200) to be 2 stops underexposed, and use fill flash to provide the bulk of the light. This gives me even lighting, but because of the automative flash exposure adjustment, each photo has slightly different luminance.
I was under the impression that adjusting exposure would be a snap by clicking on the gray card in each image and telling my software to adjust it to 18% reflectance. However, Lightroom does not seem have such a feature. This seems very strange given how frequently one is instructed to standardize exposure with a gray card.
Adding to the problem is that my gray card (from Delta) has a lot of microscopic variation in its surface in brightness (and hue). While Lightroom gives me a white balance adjustment eye dropper, it seems to be limited to sampling one pixel, rather than averaging values over a number of pixels. Because of the limitations of my gray card, Lightroom's white balance eye dropper is of little use, as the white balance vary dramatically depending on which pixel on the gray card I sample. Is there a way to change the size of the area sampled for white balance purposes?
Thanks much,
Gernot

The Best Answer

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Gernot,
I think the thing to do is to use a consistent setup, which means no TTL on the flash and a set distance from the subject, and minimise ambient light effects (small aperture).
Alternatively you could photograph a "colourchecker" with each bird and use a calibration script on the raw files to sort out the settings each time - a bit lengthy though, expect about 20 minutes a time. However, it would be quite definitive.
As far as the "single point" problem goes, you could set all the noise sliders to the max to blur out the imperfections in the card, take a sample, then undo the noise corrections.
I can't help thinking that for reference work like this, Photoshop is the tool for the job and LR will help you get organised into PS, but it won't replace it.
Damian
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