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Depth of focus often gets confused with depth of field. But whereas depth of field is concerned with making light from different subject distances all come to focus at one lens setting, depth of focus refers to how much you can change the lens-to-image distance without the focused image of any one subject growing noticeably unsharp. It is therefore concerned with tolerance in the lens-to-sensitive material distance in your camera or enlarger, and accuracy in focusing.
Depth of focus increases with small aperture and large permissible circle of confusion. However, depth of focus becomes greater the closer your subject and the longer the focal length of the lens. Both changes cause light to come to focus further from the lens, making the cones of light narrow.
These reversed features mean that in practice:
- A small-format camera needs its lens more precisely positioned relative to the film than a large camera. This is due to its shorter focal length normal lens, as well as smaller acceptable maximum circle of confusion. A large-format camera does not therefore need engineering with quite the same precision as a 35 mm camera, and its greater depth of focus also allows more use of 'camera movements'
- When you visually focus the image given by a camera or enlarging lens, always have the lens at widest aperture. This minimizes depth of focus, making it easier to see the true position of accurate focus. The picture is brighter too.
- Using your camera to photograph very close subjects (photomacrography) it is easiest to alter lens focus to get the image roughly correct size – then move the whole camera backwards or forwards to get the picture pin-sharp. You will be focusing by exploiting the shallow depth of field under these conditions, rather than struggling with deep depth of focus which could keep you finely adjusting lens focusing for a long time.
Fundamental of photography: picture structuring, lights, intensity, distance, focusing, exposure, printing
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