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I've made a giant copy stand using an artist easel that slides up and down. It's working great, and after help on here I've decided to shoot my artwork archive project on manual (M) setting while having the light meter balance to a standard gray, that way dark subjects won't end up over exposed and light ones underexposed.

I'm able to use a lot of natural light, but the EOS utility will now show me the meter indicator where to set the exposure. I can only read that from the camera. I'm a bit surprised as it seems to show virtually everything else.

Sometimes though the camera is far above my head, and I'll want to check the setting multiple times. I figured out one way, to reach up, turn the switch to AV, check live view on the eos utility and it gives automated exposure time, then turn back to M, and use the EOS utility to set that same exposure. Even though I won't have to adjust it for every shot I will want to check it pretty often as the weather and light can change. This is all still quite tedious though.

Could I just get a simple light meter, put it on my test gray, and calculate the proper exposure from there? I'm not sure how light meters work but I'm hoping I can put in my same settings, like ISO 100 and Aperture 5.6, and it will then tell me how long to expose the shot the same way the Av setting would...

Also, if I balance to a gray, what gray should it be? Should it be a test pattern of grays, or one particular level of gray? Or will any old gray do as long as I stay with the same one? I'm using the spot meter by the way.

Lastly, I do see the histogram in the eos utility and, after reading some other posts on here, should I use this as my light meter instead?

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3 Answers

Any object under consistent lighting should be properly exposed under the same lighting conditions. If you get the exposure right once, then it should be fine for all subsequent shots. That said, you may want to vary the exposure based on what you find when you look at the histogram. As was stated previously, the histogram is your friend here.

Change the lighting or exposure until you don't have anything off the scale either in highlights or shadows. If you are copying flat sheets of paper like the page of a book, the histogram may be pretty spiky because you will have a lot of light tones not a lot of mids and some dark tones. Bear this in mind when analyzing the histogram. Just be certain not to let values of the histogram show blown out highlights or blocked up shadows.

If you really want to nail the exposure, get an incident light meter. It's the most reliable way to go. Much more reliable than your camera's, which is balanced to take things like horizons and common-use shooting habits into account. Your setup is not common and the meter can easily be fooled.

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Gray cards typically have 18% gray tone, because this is what the light meter is calibrated to prioritize in many cameras. You can get cheap, durable gray cards at pretty much any photo outlet. Some also feature a black card and a 90% white card, although I've never found use for those in practice.

The gray card will help you with the color balance, but I believe you should use the histogram function instead to ensure you get the proper exposure. The histogram will tell you if you're clipping the highlights or loosing too much information in the dark areas of the photo. Put another way, The light meter just provides a guess before you take the picture, whereas the histogram tells you exactly what you got.

Your light metering strategy depends on the subject and how the light varies across the subject. Again, you might find that you can ignore the meter altogether and rely on the histogram instead to adjust the exposure time, especially when shooting a static setup with the camera on a tripod.

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Gray cards usually have 18% gray tone, that has a 12% gray reflectance. Light meters tend to be tuned for 12% gray luminance, so when metering a scene off of an 18% gray card, everything should sync nicely. A gray card should never be marketed at 12% gray, since that would reflect something around 8%, and meter incorrectly. –  jrista Mar 20 '11 at 22:42
    
Thanks! I've updated my answer based on this information –  Kim Burgaard Mar 21 '11 at 3:56
    
12%??? It's the 18% reflectance that defines the photographic gray card. –  user2719 Jun 4 '11 at 13:26
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I am not sure if you are trying to meter exposure, or tune white balance. If you are shooting on manual, then the meter is not going to do much for you, really. Under a standard illuminant with fixed exposure settings, you should be able to photograph any of your pieces of art at the same settings, and have all of them expose properly. With a digital camera, it is easy enough to use a little bit of trial and error to get the exposure you want (vs. the exposure the camera may tell you is correct, which is based on a set of immutable rules that do not always reflect reality as closely as they should.)

If you are trying to tune white balance and/or check the color accuracy of the photographs once they are imported into your computer, then you can do one of two things. First, you could stick with your gray card. Include the gray card in one shot, then take a second shot without the gray card. During post processing in a tool like Lightroom, you can use the white balance picker tool to select the gray card, and the rest if the picture will be adjusted accordingly. Simply copy the white balance (temperture on the blue/yellow-orange scale AND tint on the green/magenta scale) to the second shot to get a properly color corrected image without the gray card in the scene.

If you want to not only calibrate white balance, but also check color reproduction, you can use a color checker card (originally GretaMacbeth, however it is now owned by X-Rite). A standard color checker card has a set of 24 standardized, specifically formulated colors that have a specific base tone and color under a specific illuminant (usually D50, I believe). To archive artwork with maximum color reproduction accuracy, you would want to set up your illuminant to the same color temperature as that used for the color checker card, which is going to be much more of a natural, normal white than the standard tungsten type of lighting. (I would avoid illuminating your scene with flourescent light, as there are inherent problems with such lighting.) A daylight balanced bulb (which is what a D50, D55, or D65 bulb would be) can be expensive...however its light is more pure and better for color accurate color reproduction.

When doing color checking, you will not only want to adjust your white balance on an 18% gray card, but you will also want to compare the colors of the swatches in your photograph with that of the real color checker card. Make sure you correct white balance first. Any perceptually significant deviation in any individual color on the card in the photograph will be fairly easy to spot. If there are any significant deviations, you can either try to correct them with some fancy curve work in Photoshop (or Gimp), or find a way to illuminate your scene with a purer white (i.e. switch to a daylight balanced bulb from a standard tungsten bulb.)

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