Check that the item you are photographing is uniformly lit, without shadow or fall-off of light (in an ideal situation, 4 identical light sources should be used, each pointed towards the opposite corner, and positioned twice as far away as the camera). A large matte neutral white sheet can help to level out the light, as a reflector. Take care with the surface texture : the light coming from a different direction will change the finish. Using only the built-in flash on a camera is “lighting at its worst” : too close to the subject matter, and coming from the same position as the lens.
The best solution is to attach the camera to a tripod. The film surface or sensor should be perfectly parallel to the surface of the art. The subject should be perfectly centred in the frame. These ideal conditions are difficult to recreate, and great care is needed to avoid reflections (especially if the surface is shiny).
Choice of lens
Avoid using a wide-angle lens, which may cause distortion, except if you are using top-of-the-range equipment or if distancing yourself from the work of art is not possible. If possible, use a fixed focal lens of around 50mm with a full frame sensor. For small objects which need a closer approach, a macro lens can be used.
Barrel distortion and pincushion effect can be visible particularly at the edges of the frame. To lessen the effect, you can take your photo with a slightly wider composition, leaving space around the object in question (choose the highest definition possible).
The diaphragm aperture is also responsible for image quality. In general it is better to avoid the widest and narrowest apertures and instead choose a medium aperture, with which most lenses give the best results. A narrower aperture is only needed for a large depth of field, for example when photographing a sculpture.
Colours are reproduced better if you calibrate the white balance (in the settings on your digital camera) on a sheet of white paper beside the object you are photographing, or on a neutral grey area. Neutral grey can be achieved with a cardboard box that uniformly reflects all light rays. It is also as important to choose a good colour temperature, in the case of digital cameras, or to use daylight balanced lights with a film camera.
Using software to correct distortion
Distortion can be minimised by using a telephoto lens (the best option of all) from a further distance away. If you are working with the built-in flash as the sole source of lighting, it is better not to be directly in front of a painting, to avoid reflections. Any distortions can be reduced to an almost imperceptible level with post processing software.
Adjusting colour and contrast
Before starting to retouch, don’t forget calibration : what you can see on screen might not necessarily be what will be printed, or what you might see on another screen. To get it right, everything in the image production line needs to be calibrated to get a print exactly as seen on screen.
Contrast and colour adjustment of a digital photo, or a scan of a negative or a slide, is a delicate operation which can be done using software. This is when having a colour chart comes in handy : usually the work of art is not at hand when you are adjusting the image. The “color balance” tool will help correct any dominating colours. There is a “brightness/contrast” tool which, in my view, needs to be used with moderation, and only if a satisfactory result has not been achieved with the “levels” tool.