I have been asked many times lately to explain the technique I use to photograph paintings. What follows is an explanation of the setup and use of a system which works pretty well- I hope this is useful to anyone who needs to photograph art that is wet, shiny, too large to scan, or which just doesn't "scan well".
There are three components of this process to take into account: the camera, the lighting and the art itself. Let's start with the camera:
A digital SLR camera is the obvious default camera- it should have the ability to shoot in "manual" mode". Every camera will vary a little but the settings that I find work best:
ASA: 400 (Higher may be grainy/noisy)
Exposure: aperture f8,this is a good aperture because it will allow for a little depth of field while avoiding a very long exposure. The corresponding shutter speed 1/20th. I might vary this slightly for very dark or very light paintings but this is the default for me, using two 500 watt lights polarized (see below). You will need to test your own exposure but once you get the right numbers you should have consistently useable exposures. You can objectively test your exposure value by photographing an 18% Grey Card (which is the value your camera is calibrated to render as the default average exposure).
Shooting Mode : RAW . This allows you to correct color temperature with photoshop while keeping the original RAW file untouched.
Lights: 3200 k photo lights are best-I use Lowell Tota-lights with 500 watt bulbs. I like the lowell lights because they are portable and the have a built in gel-holder socket. You need to suspend a sheet of polaroid filter in front of each light close enough to filter the light but far enough not to burn. Without filtering the lights AND the lens, you can't cancel out glare, which is the whole point of this excercise.
Polarizing filter in front of light. It has to be "on axis" as there is an invisible "grain" to the filter which must be perpendicular to the grain of the filter on the lens.
the lights should be at a 45 degree angle to the art. a good rule of thumb is to put them about 1.5 x the diagonal of the art distance from the artwork. So if the art is 20" wide diagonally, each light should be 30 inches away.
Make sure that the art is level and square to the film plane of the camera.
I shoot everything horizontally- the camera is more stable on the tripod that way.
if you have a light meter its a good idea to check the light falling one each side of the art- obviously it should be the same. If there's a hot spot, adjust the position of the light accordingly.
kodak color bars are taped to the edge as a color reference for later correction.
Focus the camera and rotate the polaroid filter on the lens until the glare is eliminated- then take the picture. Without doing so you get a picture like this: Lots of glare, shine and reflections:
With the filter rotated onto it's axis, you eliminate the glare.
Drag the RAW files directly into Photoshop, the Camera Raw window will open, allowing you to tweak the color temperature if needed. You shouldn't need to do much.
In the end you should have a color corrected, glare-free repro-quality image.
If possible, use a remote release to avoid camera shake. If you don't have one, use the timer to release the shutter- camera shake is the enemy.