Before we discuss the factors that govern the depth of field, I would like to demonstrate a little example which is related to the propagation of light.
From Fig. 2 above, the depth of field is represented by the blue triangles with their vertices (tips) being the area of sharp focus or our focal plane on the subject. This is where we focus our camera to.
Focus is sharpest at the point of intersection of the light rays.
As the rays of light spread apart, subjects go out of focus. That is before and beyond the subject.
Now, I will represent the depth of field area (The blue area) by tooth paste and we will imagine squeezing those red light rays around the tooth paste in the directions indicated by the 4 black arrows around each of illustrations a, b & c in Fig. 2 above.
In illustration (a), if we push out in the directions of the 4 arrows, the paste is sucked in, giving a “Shallow” depth of field – Less paste area.
In illustration (b), we push inwards. Paste is squeezed and we have a “Longer” depth of field.
If we push further in, illustration (c), the paste is really spread out giving longest depth of field.
We will use this demonstration to further enhance the factors that control the depth of field.
Please remember to go back to this page if you need to look again at the illustration.