April 27 2011
Depth of field is how much of the image appears to be sharply focused and photographers use it as a creative compositional tool.
Portrait photographers seek to isolate their subject using a minimum or shallow Depth of Field while landscape photographers try to get as much of the scene in focus by maximising Depth Of Field.
Photographers control the Depth of Field using combinations of aperture, focal length, and the distance the subject is from camera. DOFMaster offer a range of solutions for calculating the effect on depth of field of different combinations of these factors for a range of digital cameras.
Portrait photographers often use the minimum depth of field to have the subject in sharp focus isolated against a blurry background. Commonly in portraits the subject is the main interest not the background.
This look isolating the subject is often created in the photography studio using backdrops and studio lighting. Using shallow depth of field professional photographers achieve this look for candid and location portraits.
To create the shallow depth of field portrait photographers set their lens apertures to a wide setting, that is a low f number. Then they choose a longer focal length lens as the longer the focal length gives an apparent shallower depth of field. Then they move in as close as they can until the subject fills the image fame in their viewfinder.
For landscape photography the opposite is true, and the settings and choices are reversed.
Here the normal aim is to have all the image appear in focus so there tends be greater distance to the subject and shorter focal length or wider angle of view lenses are chosen. Then instead of wide apertures the landscape photographer uses the smallest possible aperture setting, or larger f number.
There are some limits on how small the aperture can go. The first is shutter speed. Small apertures mean less light passing through the lens requiring a longer exposure time for correct exposure, That is a lower shutter speed. The problem here is any slight camera movement causes blurring so the whole picture looks out of focus. This is why tripods are an essential part of a landscape photographer’s kit.
The other problem is diffraction where the light bends around the lens aperture blades. This happens all the time to some of the light passing through a lens. At large apertures this percentage is so small it has no effect on the image. However, as the small aperture settings become smaller, the amount of diffracted or bent light becomes significant, resulting in distortion of the image.
Telephoto lenses exhibit an apparent shallower depth of field, so portrait photographers often use lenses with focal length of 50mm or longer. This is one of the attractions of 50mm prime lenses with maximum apertures in the f 1 to f 2 range.
The apparent shallow depth of field has more to do with the distance to the subject relative to the lens focal length. Even short focal length, or wide angle, lenses give shallow depths of field, but at much closer distance to the subject than for longer focal length lenses.
An example using the DOFMaster online calculator at the minimum focusing distanced s fro two lens at their widest aperture :
focal length = 135mm, aperture = f2.8 and distance to subject = 1metre
resulting depth of field = 29cm
focal length = 21mm, aperture = f3.1 and distance to subject = 0.2metre
resulting depth of field = 1.03cm
At its minimum focus distance the short focal length lens has very shallow depth of field. This becomes an issue for macro and close up photography even using a wide angle lens.
For portrait photography using longer focal length allows photographers a comfortable working distance from the subject. This is less intimidating than literally having the camera right in your face. This is important in creating a relaxed mood and getting good poses from the subject and capturing candid moments.
Only one distance from the camera is in sharp focus but there is an area of apparent focus around the focus point. Landscape photographers use focusing techniques to ensure they make maximum use of the area of focus they can obtain from the camera set up.
One standard technique is Hyperfocal focusing. Normally for landscape scenes Autofocus systems usually focus at infinity or maximum distance. This can result in a noticeable part of the foreground appearing out of focus. The Hyperfocal technique sets the primary point of focus closer to the photographer maximising the foreground area in sharp focus while still keeping the furthest point of the scene away from the photographer in focus.
Using DOFMaster for an APS-C DSLR with a 21mm lens set at f8 for a focus point 10 metres away, the total depth of field stretches to infinity behind this point of focus. The foreground is not in focus until 2.16 metres away. However, DOF master calculations suggest the hyperfocal distance is 2.78 metres. Focusing at this point, in front of the main point of interest, brings the foreground into focus at 1.39 metres.
DOFMaster is also available as an Iphone App so you can do the calculations on location.