Biggest photography myths busted

1 Focus one-third of the distance into the scene

This piece of advice pops up all the time, but it's an over-simplification. It assumes the depth of field (DoF) extends twice as far behind the focus point as it does in front, but this is only true in a narrow set of circumstances: for instance with a 50mm lens on a full-frame camera at f/8, with a subject three metres away. Change the lens, aperture, focus distance or even sensor size and the front-to-back DoF ratio also changes, often substantially. With close-ups, depth of field is almost equal in front of and behind the focus point, whereas with distant landscapes, it extends almost entirely behind.

2 Everything within DoF should look sharp at 100%

DoF calculations assume a specific ‘circle of confusion' to define what should look acceptably sharp in print. Conventionally they're not very stringent at all; for example with a typical 24MP sensor, the circle of confusion would be 5 pixels in diameter. If you want a finely detailed large print, you'll probably need to use a smaller aperture than that indicated by a lens's DoF scale.

3 Medium format gives the shallowest depth of field

The laws of optics mean that for any given angle of view and aperture setting, larger formats give shallower depth of field. So you might come across photographers waxing lyrical about the lovely blurred backgrounds that only medium format can provide. But the flaw with this argument is that much faster lenses are available for full-frame cameras, with only a handful of medium-format optics sporting apertures larger than f/2.8. So if ultra-thin DoF is your thing,