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Depth of Field and Open Diaphragm

The variation of the depth of field with aperture is induced by the variation that occurs in the depth of focus. In the diagram annexed a situation can be observed for diaphragm open at the top. This causes the existence of a certain depth of focus formed by all image planes located immediately before and after the focal plane (sensor position) in which the cone angle for the solid image whose vertex is P 'detemine cuts smaller than the circle of confusion and therefore indistinguishable from a dimensionless point for the purpose of human vision.
If we proceed to close the diaphragm, as shown in the bottom, the angle is narrower and therefore the depth of focus increases. This is because the size of the circle of confusion is exactly the same on both occasions. Depth of Field
is linked to the depth of focus in the sense that the planes A 'and B' are respectively the image planes corresponding to planes A and B approach and if it widens the gap between A 'and B 'also widens the gap between A and B, ie the depth of field increases.
So you can set the following rule:
a) If the aperture opening decreases, increasing the depth of field.
b) If the aperture opening increases, decreases the depth of field.