Tuesday, 23 February 2010

Depth of Field Confusion

When I first started getting serious about digital photography, I did quite a bit of reading. One of the things I read about was depth of field, in relation to aperture diameters. At that time (and since), I kept reading that crop cameras have a greater depth of field at a set aperture than full frame (i.e. sensors the same size as 35mm film). Due to the high pixel density of the 7D or more correctly, the small size of the pixels, diffraction becomes more of an issue, as the interference patterns start matching the pixels in size. For this reason, I started paying more attention to hyperfocal focusing, so that I could try to use wider apertures and still get sufficient depth of field for my purposes. This was where the confusion started, as all the hyperfocal distance charts suggested that full frame actually had greater depth of field.
I then started doing a bit more reading and it turns out, that the expressed wisdom misses out a very important point. The statement that full frame cameras have less depth of field is based around portrait (and to a degree, wildlife) photography, showing the same field of view. To get the same field of view, a full frame camera must be positioned closer to the subject than a crop camera. It is actually the distance between the camera and subject and the camera and background that makes the difference to the depth of field. The closer the camera is to a subject, the less depth of field there is, so it is another apparent effect and like the apparent magnification from the crop factor, it tends to be stated as an actual fact, when it isn't actually the case.
For portrait photography, where you are trying to frame in a certain way, whichever the camera, it is an important point to consider, as long as you realise it is a function of distance and not differences between the two formats, likewise for macro photography. However, for landscape photography, which was the reason I was looking at it, it is largely irrelevant. Technically, the depth of field for full frame cameras is larger, but in reality, provided the foreground is equally sharp, the slightly softer background is probably not very noticeable, as the differences are relatively small. Diffraction is probably a bigger issue than the narrower depth of field, as the larger pixels of the full frame sensor are less affected by the interference patterns, so the effects of diffraction are ameliorated and become noticeable at narrower apertures. This of course, then allows much more depth of field, which is where the real advantage of full frame cameras comes in for landscape photography.
It brings to mind the saying, "believe none of what you hear and only half of what you read". The more I learn about photography, the more I realise how many inaccuracies have crept into accepted wisdom.