Depth of field is a measure of the amount of your image that appears to be in sharp focus, from near to far. Like any subject, a lot of nonsense is spoken about depth of field in photography, and I am going to cover some of it here.

1. Depth of field increases as the aperture decreases (f-number increases).

This is true. And, with shallow depths of field, it is roughly proportional too, i.e. going from f/2.8 to f/5.6 can approximately double the depth of field.

2. Depth of field increases as subject distance increases.

This is true. And, with shallow depths of field, it increases roughly proportional to the square of the increase, i.e. doubling the distance to the subject approximately quadruples the depth of field.

It follows that depth of field decreases with decreasing distance, so that halving the distance to a subject gives approximately a quarter of the depth of field — just ask any macro photographer.

3. Depth of field is greater with shorter focal lengths.

This is only true when you compare two focal lengths at the same subject distance. If you decrease your focal length by a factor of two, it will quadruple your depth of field — although your subject will be much smaller in the frame.

But what you may be surprised to know is that, if you adjust your distance to a subject and zoom to compensate (so that the subject stays the same size in the frame), you will get the same depth of field.

4. The Hyperfocal Distance is the optimum focusing distance for a given focal length and aperture which produces sharpness between the closest possible foreground point and infinity, and can be calculated using tables or a lens Depth of Field scale.

This old landscapers’ rule is only half-correct. There’s no such thing as sharp and not sharp – only degrees of sharpness – and the numbers used to calculate depth of field are based on outdated assumptions. The principle is correct, but the accepted methods of calculation are woefully inaccurate, and will produce poor sharpness by today’s standards. If you must use a depth of field calculator, consider dropping a stop or two of aperture to get a more realistic setting, or, preferrably, check your images for sharpness on your camera’s LCD.

5. To get the best depth of field in your photo between two distances, you should focus at a point one-third of the distance behind the nearest point.

This is sometimes true, but only applies in one situation (actually, it’s when the near point is a quarter of the Hyperfocal Distance). There is no simple rule, as front and rear depth of field are a function of distance, focal length and aperture, and the proportions vary between 1:1 and 1:infinity!

6. Small format cameras have more depth of field; large format cameras have less depth of field.

This is true, but not because of the sensor/film size. Smaller-format cameras use shorter focal lengths to achieve the same field of view, and it is for this reason that aperture decreases and depth of field increases. For example, going from full-frame to APS-C format cameras will effectively increase shallow depths of field by a factor of approximately 1.6.

Keith Nuttall, 2009