Alley in Pisa, Italy

by Lars Kotthoff

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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.

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4 Answers 4

up vote 9 down vote accepted

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.

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I have to agree with Guffa, you should really use manual mode to maintain consistent exposure. So long as the input lighting is consistent, manual exposure should do the trick. You might also want to try using a custom WB setting, by taking a wide shot of the gallery at large, and using that as your WB base. Might help with consistency. –  jrista Mar 10 '11 at 17:53
    
OK Thanks, This all makes sense to me now and means with the Raw images I'll be getting a very pure image capture all at a similar exposure. This may be for a different questions, but is there any particualar gray I should use? Or a combination of grays? I have some matte board at differentcolors. I'm shooting from the EOS utility to having a pattern I can auto focus on to start would help too, even thoug I'll fine tune for each subject from the computer. –  kellyllek Mar 13 '11 at 14:05
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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.

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Thanks Staale, makes sense and balancing all the images out to the same base will definitely be the way to go. –  kellyllek Mar 13 '11 at 14:07
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If you want to ensure that you don't have to retake all your pictures because you had your exposure off by 2/3 stop, you could take several pictures over a range of exposures for each piece of artwork. It would take a few extra seconds for each set of pictures, and you'd have a lot of additional files to manage, but when you're all done if you want to move your baseline by 1/3 stop or more, you would be able to do that without re-taking all the pictures (and I'm assuming that setting up for each shot will be the bulk of the work).

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Sadly we're talking upward of a 1000 subjects and I'm just not going to be able to take multiple images to choose from later. As suggested above though if I balance everything out to a standard gray it should work. I've only experimented with a couple so far but it looks good. I just need to find out if there is a 'particular' gray I should use or a pattern of grays, or if can just stick to one to have them even across the board... –  kellyllek Mar 13 '11 at 14:11
    
@kellyllek Just because you're taking that many pictures doesn't mean you can't take multiple images (I would think that with that many pictures, the chances you'll need to go back and do it again increases!), just that you'll want better tools if you do. For example, you could use exposure bracketing to take the pictures so you are only pressing the shutter once (although it does mean holding it down), and then use a script on import to look at the EXIF data and move everything that's not at the desired exposure to a backup folder so you never have to look at it until you need it. –  drewbenn Mar 13 '11 at 20:03
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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.

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Thanks ysap, that is what I'm going to try now.. –  kellyllek Mar 13 '11 at 14:11
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