When you photograph anything flat, like this ancient cave painting in Baja California, Mexico, make the back of your digital camera - i.e., the plane of the sensor - as parallel as possible to the subject. This helps maintain depth of field (DOF - the range of sharpness in a scene).
If the light level is low, as it was when I took this picture, and you can't use a tripod, a small lens aperture (high f/number) may not be possible because that will make the shutter speed too slow for hand-holding.
Therefore, the only way you can show all of the detail in the subject with tack-sharp clarity is to angle the camera such that the sensor is as parallel as possible. Then you'll have all the detail you want at any lens aperture.
More Notes: Jim Zuckerman is a widely published photographer who teaches at BetterPhoto.com's online digital photography school. His online photo classes include Perfect Digital Exposure and Eight Steps to More Dramatic Photography.