SURFACE APPEARANCES
Obviously, a second light on the other side of the subject would help provide more even illumination. (This is exactly why most copy setups do, indeed, use two lights.) With extremely shallow lighting angles, however, the second light still does not provide uniform exposure. We simply get two overexposed areas instead of one, with a dark area in the center. One solution to this problem is to move the light closer to the camera. (An extreme example of this is a flash mounted directly on the camera.) Then the light is roughly the same distance from all points on the surface, and the illumination is more even. But this solution is also likely to place the light in the family of angles that cause direct reflection, which is a worse problem. The only solution to this problem that always works is to move the light farther away from the subject. In theory, a light that is an infinite distance away will produce exactly equally bright diffuse reflections at all points on the surface, even at the most shallow angle. Unfortunately, a light an infinite distance away is also likely to be infinitely dim. (We will not even begin to deal with the problems of finding a light stand that high.) In practice, we do not usually need to get the light quite that far away to obtain satisfactory results. We just need to get the light far enough from the subject to produce acceptably even illumination, but we need to keep it close enough for acceptably short exposure times. We could offer you mathematical formulas to calculate an acceptable distance between the light and the subject at any given angle (and for any given acceptable side-to-side exposure error), but you would not use the formulas because you do not need them. The human eye is good at judging the acceptable compromise distance, provided the photographer is aware of the potential problem from the start. Place the lights so that the illumination looks reasonably even; then double-check that judgment by measuring various points on the surface with a light meter. DOING THE IMPOSSIBLE The preceding examples tell us that even illumination and glare-free illumination can be mutually exclusive goals. The closer the light source is to the camera, the more directly it lights the subject and the more even the illumination becomes. However, the farther the light is to the side, the less likely it is to be within the family of angles that causes direct reflection.
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