by Mike McNamee Published 01/08/2007
Depth of field in practice
Older lens for 35mm and medium-format cameras used to carry depth of field markings which enable the photographer to both assess the real depth in a situation and also, if preferred, set the focus manually to the hyperfocal distance. Partly because the lenses today are fitted to cameras with a variety of chip sizes and partly because everything these days is dumbed down, the markings are not always provided. It is thus useful to have a working idea of your depth of field and so we show here a trio of scenarios. It is one of life's puzzles that most photographers will grab two stops of speed, when offered (ie a possibility to move from 400ISO to 100ISO). Those same photographers often fail to realise that they can have a couple of stops extra 'speed' in the form of a wider aperture if they understand depth of field and exploit it.
Consider then the three scenarios using a small chip DSLR
(eg Nikon D200, Canon 350):
· A tightly cropped facial shot.
· A head and shoulders portrait of a single person.
· A group shot of about 30 people arranged in three rows.
Write down the aperture you would expect to need to keep both ears and nose sharp in the first two and the whole group in the third scenario. Go on write it down! Imagine you are standing in front of a wedding, what aperture are you going to use? If you ask "what focal length?" go to jail, you have not been listening - go back and read point three above! Pay your £200 to get out of jail, then look at the bottom of the next page for the answers. As a professional photographer, hired by your client to do a job for them, it is your duty to know the answer to such questions. Most photographers underestimate the first, get the second close and over-estimate the requirements of the group shot. If you were badly out then do some tests and stick a small label on the back of your camera reminding you of what aperture you need. If you can capture the group on the altar at f2 imagine how much more scope you have to bounce a flash off a high church ceiling and get away with it. If you have the camera on 'Program mode' you are totally out of control of the situation.
The fact that you can get sufficient depth of field at f1.8 does not mean you are recommended to use such an aperture on an outdoor, sunshine, group shot - this would require a shutter speed of 1/32,000! However, you can make the best attempt at blurring down that advertising hoarding across the road from the registry office! Your fill flash will be the determining factor in setting your shutter speed. If it is an oldfashioned unit with a synchronisation requirement of 1/250 you are going to be stuck with quite a small aperture and a lot of background clutter. Conversely the loss of effective flash power from a multi-burst modern unit may not matter if you can grab the group at f2 or f2.8.
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