Depth of Focus
In photographic optics, the term Depth of Focus has two related but different meanings.
1. Depth of Focus (image-side version of depth-of-field) – The image-side representation of the depth of field as imaged on the sensor. The term ‘depth of focus’ in this context is often confused with ‘depth of field’. However, the depth of focus occurs exclusively inside the camera and depth of field occurs outside.
2. Depth of Focus (displacement) – The distance that the image sensor can be displaced (closer to or further from the lens) while the subject still remains in sharp focus. This displacement at the plane where the image is cast (the sensor face) can only have a tiny margin of error in its placement or the image will not focus properly on the sensor plane. Modern standards of construction are to a high degree of accuracy and quality control. As a result this concept is largely a notional one used to describe the issues around the design and placement of internal components. The term originated with film cameras where the placement of film had low degree of accuracy thus possibly affecting the sharpness of the internal focus.