A number of budding photographers often confuse depth of field with depth of focus. A number of cheap photography manuals also use the terms synonymously, but this is not correct. Depth of field and depth of focus are two completely separate concepts, with one occurring in front of the leans and the other behind the lens. Depth of focus is the distance behind the lens where the image plane will be in maximum focus (see the picture), whereas depth of field is the range of acceptable focus of the subject in front of the camera.

Both Depth of field and Depth of focus primarily depend on the same factors, but these factors have a different effect on each term. Aperture has the same effect on both Depth of field and Depth of focus; both of them increase with the increase in the f number, or decrease in aperture. Depth of focus increases with the increase in focal length, whereas depth of field decreases with the increase in focal length. In order to use high-end lenses, like the Nikon 500 F4 VR, correctly and to their maximum potential, it is important that you fully understand these concepts. Object distance is another factor that has the same impact on depth of field and depth of focus; both of them increase with the increase in object distance.

Depth of field is a photography concept that happens in front of the camera and is measured in macroscopic distances, such as metres and kilometres. Depth of focus, on the other hand, happens between the lens and film plane; hence, it is measured in microscopic distances such as fractions of a millimeter or thousandths of an inch.