Consistent image exposure using Canon AV (aperture priority) setting for artwork

Question

Sorry this isn't a clearcut question.

I've been setting up to shoot artwork. I'm using the AV setting on my cannon which allows me to set a higher aperture (5.6) and set the ISO (100). The problem I'm worried about is the automatic exposure with artwork subjects that are of different brightness.

The camera, (on tripod) automatically exposes long enough to capture the image. But for a brighter images with lighter colors won't the exposure take less time, and for darker images won't it take more ? (I think this was happening in my first experimental shots, but it could have been other variables). The results seem OK but I want to make sure.

I want to use an automatically exposure if I can. With the higher aperture and low ISO, even with studio lighting, an exposure can take a while. But I also want all the images to be consistent, and not have the darker ones over exposed.

And I also don't want to set different settings every time I photograph an image.

Should I maybe use a test pattern, or neutral gray to estimate the exposure length, then set the exposure manually for all the images no matter how light or dark they are?

Or perhaps does the camera automatically know there really are just darker colors and will provide a consistent image no matter the subject?

Or further should I just worry about this during the RAW image processing. (I read somewhere that RAW captures images at a different gamma level than your eyes naturally see).

NOTE, I'm using a Canon T2i, shooting RAW, with a 35mm Macro lens on a sturdy tripod, using the EOS utility to operate the camera from the computer. I have a decent softbox light setup at 45 degree angles, though I may have to sometimes adjust them depending on the finish sheen and texture of the paintings. The experimental shots I've done using the automatic exposure seem pretty good so far. I just want to make sure I get a consistent shot for all the images.

Answer 1

If you want consistent exposures you should use manual mode. If you let the camera measure the light, it will compensate for lighter or darger colors and try to make the average of the scene standard gray.

Alternatively you could use spot metering, and use a gray card in front of the artwork, so that the camera will only measure the light that falls on the subject, not how much light it reflects.

In the RAW processing you have some margin to adjust the exposure, but you should try to make the exposure as correct as possible from start. If you have to compensate too much in the post processing, you will get a noticable loss in image quality.

Answer 2

The light-meter in the camera has no way of knowing how bright or dark your subject is in absolute terms. It only sees what the camera lens happens to capture, and works under the happy assumption that what the light-meter sees will have the luminance of 18% grey (or perhaps 12%). Which is why relying on the automatic modes in a case like yours is perhaps not so great an idea if you want "correct", consistent exposures.

Answer 3

According to what you describe in your question, then the only way to get consistent exposures on all artworks (that is, make lighter works look brighter than darker works) is to use manual mode. Set it to some average settings to make sure you are not saturating (over exposing) or clipping all of the artworks.