I've been doing professional work in oils for 10 years, as of this writing. For 9 of them, the step where I had to photograph the piece made me consider going back to digital painting full-time. I like making traditional art though, and I like selling it even more. After buying some proper equipment and taking the time to learn what Photoshop is actually for (i.e. editing photos. Who knew?), I have a setup that's reliable and leads to a minimum of screaming. What follows is a simple list of the gear and steps I use to photograph art. I haven't included much in the way of explanations - if you're interested in the 'why's' of all this, these articles by Dave Palumbo, Dan Dos Santos, and in particular Robert Hunt are very thorough.
All in all, the setup cost about €1000. You can use a cheaper one, and I have - you can also use canvas panels, old brushes, and Georgian paint. Another option is to have your work professionally photographed, but I assume that you, like me, aren't making ads for plastic surgeons.
Equipment:
Camera: Canon EOS 5D Mk2. I got an old one used for around €400, which is a steal. What you want to look for is a camera with a full size sensor. I used a Nikon D3200 for years, and that had a cropped sensor, which means that the farther out of the middle of the shot you get, the blurrier the image. I used to have to take around 24 different shots of a painting to stitch together, since only the very center was usable. With a full sensor, the image is sharp pretty much edge to edge. Generally, the lower the number in the camera name, the better.
Camera settings: Aperture priority mode, f-stop f10, ISO 100, file type RAW, manual focus, shoot on a 2 second timer.
Lens: Canon EF 50 mm f/1.8 STM SLR Telephoto Lens. This is a prime lens, which means the zoom is fixed.
Lights: GVM 800D Video LED Lights
Circular polarizer (attaches to lens): Hoya SMC CIR-PL, 49mm. After attaching this to the lens, while looking through the viewfinder, turn it until the glare spots disappear. Like magic.
Linear polarizer film. I bought a few sheets and cut them to fit in the lights' diffuser slots. To make sure they’re oriented properly before mounting them on the lights, hold a sheet in front of a screen and rotate it until the screen goes dark.
Tripod: Anything that isn’t wobbly will do.
Shooting:
The lights should be set up to either side of the painting, at an angle of around 10-20 degrees to the surface, and much closer to it than you think. The polarizers will take care of almost all the glare. Make sure that the area behind the camera is dark, otherwise you’ll see reflections of it in the painting.
Make sure the camera is at a right angle to the painting surface. Since I use a prime lens, the camera and tripod position will depend on how much of the painting you're trying to fit into the frame. I usually have it close enough to photograph in 6-8 sections.
Editing:
Each RAW image is opened with Photoshop, which takes you to 'Camera RAW.' Here I tweak the following:
Exposure: Usually over the whole image, sometimes specifically on the highlights. If the white balance is off, you can tweak it here.
Optics: Use profile corrections
Geometry - Guided: Drag vertical and horizontal lines to correct any perspective distortion.
Crop: To slightly larger than the area I want to use.
Save the file as a TIFF, and repeat for the other sections.
Once that's done, go to Automate - Photomerge and select all your TIFF files. In a few minutes, the whole image will be stitched together. At this point, I usually need to go to Filter - Camera Raw Filter, and do the Guided Geometry correction one more time over the whole image. Once that's done, flatten the layers and crop to size.
Now and then, especially for a dark image, further digital work is needed to correct color, or to iron out artifacts with the healing brush or clone stamp. But 90% of the time, the image I get from this is already good to go, and the whole thing takes about half an hour.