First, what youll need:
-35mm camera (with Tungsten-balanced slide film), with manual exposure option. Or, multi-megapixel digital camera (Nikons give good color); someday we'll have affordable desktop digital-to-slide printers, or one would hope. If you can, get a cable release, one with a braided metal cable, not a plastic one. Your lens should be long enough to avoid distortion (80mm+ or so), but not so long you have to back up into the next room.
-18% grey card, a standard grey used to set exposure
-two 500 watt Photoflood (color-balanced) lightbulbs, and fixtures and reflectors with ceramic sockets (or you’ll have two fires), stands to raise them to camera height, and an extension cord if needed (minimum 500W rated, most aren't).
-Black flock-paper, like a fake velvet, for backing.The Set Shop in NYC has it. Otherwise, some kind of black velvet that wont reflect any light into the lens.
-optional: polarized gels/clips for the lights and a polarizing filter for the camera lens.
Its best to work at night, or in an area where no daylight will be getting to the set.
This is all about precision; if you check everything right, you’ll get what you expect, an evenly-lit rectangular image. Miss anything, and it will certainly show up in the final result. Definitely don’t think because it looks okay in the viewfinder it’ll come out good; your eye can easily fool you. Imagine a line going perpendicular out from the center of each piece you want to photograph. If the center axis of your lens is precisely on this unique line, then the film plane of your camera will match your piece's position perfectly, and you’ll get a rectangle. Anywhere else at all, period, and it’ll be skewed.
Light placement is very important; dont cheat on it. The lights go at 45 degrees from the center of the piece, and about as far from it as the camera. If you see hot spots, move the lights further back along the 45-degree line.They should also be exactly as high as the center of the piece. Any deviation from this, and you will certainly see uneven illumination in the result, even if it looks okay in the viewfinder.
For a lot of work, direct lighting will work okay (be sure to test before you blow a lot of film and processing though), but if you’re getting a lot of glare (are the darks throwing off shiny highlights?), you might have to adapt. Better lighting stands have umbrella reflectors, so that you point the light away, reflecting it from a much larger area. Again, be sure these are aimed right at and set at the same height as the center of the piece. Alternatively, if there are walls to either side, you can center the piece between them and turn the lights to bounce off the walls, arranged so the bounced light leaves the wall at 45 degrees. If the walls aren’t white or equal-value greys, hang some white paper over the reflecting area or whatever color is there will affect the result. Be sure the shadows from the edges of the reflectors are outside the piece.
Once you have the lights in place, bring your camera and tripod right up to the painting. Set the height of the camera so the center of the lens is even with the center of the painting. Hold the 18% grey card at the front of the painting, parallel to it, facing the camera, and without any part of your body casting shadows (or reflecting significantly), and set your camera’s exposure. You can use a fairly open aperture (i.e. low number), as you won’t need any depth of field. Unless you move the lights closer to or further from the center of the set (up and down don’t count), this is the exposure setting you will be using for every piece.
Now, keeping the camera on the line extending perpendicular from the center (you can mark this line on the floor with tape if that’s easier), move it back until the piece is the right size in the viewer. Leave a little black around the edges, but not too much. If you wear glasses, put them on; it’s an easy mistake to get a sharp image for bad eyes, with blurry results (using a tighter aperture is good insurance if you’re not sure). Manually releasing the shutter can introduce vibration, which is why the cable release is preferred. If you don’t have a cable release, you can use the self-timer button most cameras have; only cock it a little ways so you don’t have to wait five seconds every time.
To be safe, take additional shots at one stop over and under; this is called bracketing. Not all pieces look right under by-the-numbers exposure. If you need to take many copies of each piece, then do a test session first (recording where the lights were, etc.), determining which works best for each, then just shoot as many as you need after.
For paintings in which shiny surfaces are too evident, you can place polarizing gels in front of the lights (not too close or theyll burn). Orient them so that the polarity is vertical, then turn the filter on your camera so the polarity is horizontal, and voila, a pretty hefty reduction in glare. Of course youll need to set your exposure again if you do this.
For pieces that you might want to get accurate reproductions made from, you’ll need calibrated color bars; just tack these near any edge. Otherwise leave them out. If you need to mask the bars later, or any light leaks, you'll need special reflective mylar tape.
Start with the smallest pieces, and work your way to the larger ones; that way any scuffing on the velvet will be covered up by a larger piece. Every single time you put up something of a different size, you have to raise/lower the lights and camera accordingly. Again, do it exactly right, be precise, or do it again until.
I've recently had pretty good luck with an online digital-to-slide outfit called gammatech.com. as of this writing they were $2 for the first slide of any image, and $1 for copies at the same time, and they ship them out very quickly, often same day. The advantage of working from digital files is that you can crop and resize, and remove distortion, retouch out glare, etc., as well as color correct. If you get back a set of slides and the color seems to be off, you can make a copy of your file, try to correct it in the direction the slide needs to go, which is not to say what will look good onscreen. Then if you need a set of dupes you can just fire off an email and there they are. But I'm still hoping for that affordable desktop film recorder someday, that'll be neat.