What is shallow depth of field?

It is depth of field (the part of the overall image that appears acceptably sharp) that doesn't extend very far throughout any portion of the distance from the near and far points in the image. By this definition, the limited depth of field can be in the foreground, middle ground or background, but it is a small portion of the image overall. The average person would find their depth of field to be in the near 1/3 of the distance from the near and far points.

Distance to the subject, lens focal length an aperture are the main controlling factors, but the size of the reproduced image and the distance between the viewer and the image contribute also. Your perception of the depth of field can change as you approach a large image such as a poster in a mall.