Oblique angles

When you are shooting a flat subject and the back of the camera can't be parallel with the surface, you must use a small lens aperture for complete depth of field.  You need to have the entire surface sharp, and if the subject and digital sensor (i.e. the back of the camera) are oblique relative to each other, you won't be able to create as much depth of field as you need.  In this picture of Newspaper Rock in southern Utah showing ancient petroglyps, some of which are 2000 years old, the face of the rock leaned back away from my camera position.  I couldn't raise the camera up 10 or 15 feet to shoot downward a bit, so the only option I had was to use a tripod, aperture priority, and f/22.  Because this was lit by diffused light (fortunately), I didn't have enough light for both a fast enough shutter plus f/22.  Hence, the tripod.