Factors affecting Depth of Field
2. The Focal Length
The focal Length is the distance in millimetres from the optical centre of a lens to the camera film or sensor plane when the lens is focused at infinity.
It is measured in mm.
Example: A 50mm lens has a distance of 50mm from its optical centre to to the camera film plane. The 200mm lens has four times longer distance from its optical centre to to the same camera film plane. That mostly explains the physical length of the lens.
Field of view: It is the amount, measured in degrees, that a lens can view. It is related to the Focal Length of the lens. For example, a 14mm lens may show a wide diagonal angle of view of 114°. This is super wide.
A longer 400mm lens, on the other hand, will show a much narrower field of view of 6.2°. This allows you to fill the frame with a considerably smaller amount of the scene that you’re trying to photograph or in other words, a tighter crop.
From The above figure, fix the camera-to-subject distance, the aperture and change only the lens from a wide angle (Shorter focal length) to a Longer telephoto.
With the wide angle, light rays propagate through a longer path towards the camera filmplane, thus squishing our blue area and Depth of field becomes longer.
With the longer 200mm lens, light propagates through the shorter path to the film plane through the long lens, creating a shorter and a shallower depth of field.
Conclusion:
The longer the focal length (a telephoto), the shallower the depth of field.
The shorter the focal length (Wide angle lens), the longer the depth of field.