Depth of field is the range of distances in your image which appears to be in sharp focus. 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 decreases as the aperture increases (f-number decreases).
This is true. And, with shallow depths of field, it is roughly proportional too. Going from f/2.8 to f/5.6 can approximately double the depth of field.
The parallax effect (from one side of a large aperture to the other) blurs objects outside the focal plane. The bigger the physical aperture, the bigger the parallax effect.
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 comparing 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 (and your subject will occupy a quarter of the number of pixels).
What you may be surprised to know is that, if you adjust your distance to a subject and zoom to compensate (keeping the subject 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 point and infinity, and can be predicted with a Depth of Field chart/calculator.
This old landscapers’ rule is only half-true. 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as sharp and not sharp – only degrees of sharpness – and the numbers used to calculate depth of field are based on outdated assumptions.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 old methods of calculation are inaccurate, and can produce poor sharpness by today’s standards. If you must use a depth of field calculator, take the reading with a pinch of salt, or consider adding a stop or two of aperture to get a more realistic setting, or (preferably) check your images for sharpness on your camera’s LCD.
5. To get the best depth of field in your photo between two points, you should focus one-third of the way between them.
This is rarely true, and actually only applies when the near point is a quarter of the Hyperfocal Distance [see 4 above]. Practically, there is no simple rule. Near and far points of sharp focus are a function of focal plane, focal length, aperture, and film/sensor design. The front-to-back ratio can vary anywhere 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 half-true, but not because of the sensor/film size. Smaller-format cameras use shorter focal length lenses with smaller physical apertures to achieve the same field of view, and it is for this reason that depth of field increases. For example, going from full-frame to APS-C format cameras, with proportionally smaller lenses, will effectively increase shallow depths of field by a factor of approximately 1.6.
Keith Nuttall, 2009 (revised 2011)