Depth of Field
Depth of Field (DoF) • The term depth of field refers to the depth of the zone that is visibly sharp in the picture, from the closest to the farthest parts of the scene. • Suppose you focus your lens on a person 10 feet away. Even though you focus precisely on the person, an area in front of and an area to the back of the person will usually be sharp. • The degree of that sharpness, from front to back, is the depth of field.
Depth of field zone • The depth of field covers a zone of one third in front of the focus point and two thirds behind the focus point. • In the figure subjects #2 and #4 fall inside the zone and will be in focus on the photograph. • Subjects #1 and #5 fall outside the zone and will be out of focus.
Factors that control DoF • Aperture • Distance to Subject • Lens Focal Length
Aperture • There's a simple, direct relationship between aperture and depth of field - the smaller the aperture, the more extensive the depth of field. • If you want to keep as much as possible sharp, you should set as small an aperture as possible - preferably f/16, or even f/22 if your lens offers it. • Depending on lighting conditions and your selected ISO, you may need to use a tripod or some other form of support at such small apertures, as the resulting long shutter speeds create a risk of camera shake.
Aperture • If, however, you want to concentrate attention on just one part of the scene, and throw the rest out of focus, you should select a large aperture, i.e. f/2.8 • For general picture taking, when you want most of the picture to be in focus, you might want to set a middling aperture of around f/8 to f/11. • This is what a program exposure mode sets when left to its own devices so whenever possible you should take control of aperture selection and use either an aperture priority or manual mode.
Distance to subject • The greater the focusing distance (from camera to subject), the greater the depth of field, assuming the lens aperture and focal length stay the same. • If you use a 90mm lens and focus on a subject 20 feet away with the lens aperture set at f/8, you will get much more depth of field than if you focus with the same lens at f/8 on a subject five feet away.
Distance to subject
(Examples)
Distance to subject • The closer you get to the subject the more limited the depth becomes. • In fact, when shooting close up subjects it can extend to just a few millimeters in front of and behind the subject.
Lens focal length • The shorter the focal length of the lens, the greater the depth of field. • If you use a 28mm wide angle lens, you will get far more depth of field than if you use a 200mm telephoto lens set at the same lens aperture and focused at the same distance. • Example, a 24mm lens set at f/8 and focused 10 feet from the subject has greater depth of field than a 200mm lens that is also set at f/8 and focused at 10 feet. • The longer the focal length, the more restricted the depth of field is.
Lens focal length
(Example)
Lens focal length • The ability to render your subject uniformly sharp is one of photography’s great strengths, so most times you will want as much depth of field as the situation allows. • However, there are times when you will want to have the subject (or another part of the image) sharp and the background or foreground blurred, such as when you focus on a portrait subject and let the background go out of focus.
Four common techniques using depth of field • Everything Sharp • Main subject sharp with background completely out • Main subject sharp • Zone of sharpness deliberately limited
Everything sharp • With subjects such as landscapes, you will usually want to keep everything sharp. Using a wide angle lens set to a small aperture (e.g. f/16) will give you extensive depth of field, perhaps from around one meter through to infinity. • But there will be times when foreground interest is closer than that. Here you need to resort to a neat little technique called Hyperfocal focusing that allows you to increase the depth of field.
Everything sharp • As a rule of thumb, there's twice as much depth of field behind the subject than in front of it. • If you photograph a distant subject such as a landscape and focus on infinity you waste lots of depth of field. • By focusing a little closer, you will extend the depth of field at the front so it comes nearer to the camera, while still making sure that infinity falls within the depth of field behind the actual point of focus.
Everything sharp
(Examples)
Main subject sharp with background completely out • There are some subjects where you want the main subject to stand out strongly from an out of focus background. Portraiture, where the emphasis is on the person, rather than the location, is probably the most popular area. • What you need here is to use a telephoto lens at its widest aperture. It's also worth moving the subject as far away as possible from the background - in cramped studio situations this is often impossible. • Take care, though, that you focus accurately, as the limited depth of field will be unforgiving of any focusing errors. For portraits focus on the eye for the best results.
Main subject sharp with background completely out • Plants often have distracting backgrounds so focusing on the flower head and throwing the background out is a common technique used to isolate the subject.
Main subject sharp • Sometimes throwing the background completely out of focus is going too far. • You want to show the subject in its natural environment, but with the background toned down to it doesn't compete for attention. • A person on the beach, for instance, an animal in the zoo, or a flower in a garden. • Here a standard to short telephoto lens, somewhere from 50mm to 135mm, is ideal - especially if it's coupled with a middle range aperture of around f/8
Main subject sharp
(Example)
In this shot of hay the background has been deliberately pushed out of focus but not by too much so that it is still obvious what it is.
Zone of sharpness deliberately limited • Occasionally you may want to limit the depth of field to a very specific zone. • Maybe in a portrait you want just the eye in focus, and not even the ears or the tip of the nose.
Zone of sharpness deliberately limited • Here just the crown and stone in the Cladagh ring are sharp.
Summary • Depth of Field is not a real control, but more of a result of how you used the aperture control. • Overall, the practical use of depth of field can be summed up as follows: • To maximise depth of field and have as much sharp as possible use a wide angle lens, set a small aperture and stand back from your subject • To minimise depth of field with only a small zone of the scene sharp use a telephoto lens, set a large aperture, and get closer to your subject.
Useful Links • • •
Video tutorials on the subject How to blur the background http://link.brightcove.com/services/link/bcpid1155328764/bctid1270735525
• •
Understanding depth of Field http://www.ephotozine.tv/video/Understanding-Depth-of-Field-728
• •
Why you should use Aperture priority http://link.brightcove.com/services/link/bcpid1443726324/bctid1568089439
• •
Depth of Field Tutorial http://www.ephotozine.tv/video/DepthofField-Tutorial-642
• •
How to create shallow Depth of Field in any portrait (Photoshop) http://www.ephotozine.tv/video/How-to-create-a-shallow-depth-of-field-to-any-portrait-1109