Depth of field
The depth of field is the distance between the nearest and the farthest subjects from the lens that will appear to look sharp. ‘Appear to’ is the most important part of that sentence, because in a technical sense only an image that is exactly at the distance the lens is focused at will be in focus at the film plane. But, when can the audience discern if an image is, to them, sharp or not?
When an image is shown on the screen, the near and far objects that appear to be in focus do so because the human eye has a limit to its resolving power.Any image we see at this resolution, or better, we will consider to be sharp. The extremes of the depth of field are the points at which the resolution of the eye has been reached.
Depth of ...