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Depth of field
Depth of field (DOF) refers to what is and isn't sharp in a picture. Images with great depth of field have a large zone of sharpness, so that everything from just a few feet in front of the camera to infinity will be sharp. Depth of field at a given focusing distance extends 1/3 in front of the point of focus and 2/3 behind it. The three key factors to depth of field are aperture, lens type or focal length and subject distance.
Aperture
By closing the aperture, you are not only limiting the amount of light that reaches the CCD but you also create a greater area of sharp focus. The smaller the aperture you choose (say 16 or 22) the larger the depth of field.
Lens type
At any given aperture, a wide-angle lens or the wide-angle setting of a zoom lens yields far more depth of field than telephotos or tele settings. A 28-mm lens set to f8 gives a larger area of sharpness than a 300-mm lens at the same aperture.
Subject distance
The closer your subject is to the camera the less depth of field you get for any given lens type or aperture. This is the reason there is so little depth of field in macro photography where the subject is often only inches away from your camera.
Distance of Subject - the pitfalls
Due to the way different lenses or zoom settings affect perspective, it pays to know what focal length to use for
certain subjects.
In landscape photography for instance the idea mostly is to show a lot of the scene in front of you.
This would best be captured with a wide-angle lens since this is able to show the wide open space before you.
Because wide-angles open up perspective, some foreground interest will be necessary to avoid creating an "empty" photograph.
For the same reason a wide-angle should not be used for close-up portraits, as the facial features of a person will
be distorted beyond recognition. If you definitely want to use a wide-angle to show a person in its surroundings,
make sure he or she is not situated directly in the foreground or at the extreme borders of your image, as this
is where perspective distortion will manifest itself most strongly. Keep the person away from the extremes of your
image and a natural looking portrait will be the result.
When using long tele lenses be aware that with distant subjects, haze and mist can affect the apparent sharpness
of an image. A skylight filter will lessen this effect, or shoot early in the morning before the sun has warmed the ground.
Take great care when focusing with a long tele as depth of field will be very shallow. A long telephoto at maximum aperture
and focused on a subject 5 meters away will only give a few inches depth of field.
Distance of Subject
The way we see objects in an image is dependent on the distance it is removed from us and the type of lens used.
If a subject is captured with a 50mm lens (35mm equivalent) we experience a more natural perspective than if it is
captured with a wide-angle or tele lens. With a 50mm lens or at a standard zoom setting we can guess the difference
between objects according to our real life experience.
Wide-angles open up perspective, so it is much more difficult to guess the actual distance between subjects. Our view
will be lead astray by the way these lenses open up perspective, and objects nearby appear to be much wider apart than
those farther away. On top of this the background seems to move far away into the distance.
Tele lenses compress perspective and objects appear to be closer together than in real life. The background is drawn
towards the foreground and seems to be a lot closer to the main subject than we actually see it with our own eyes.
The longer the focal length the more obvious this effect is.
Since tele lenses have shallow depth of field even at smaller apertures, the background may be blurry
( which can be effectively used to separate subjects from their background) but it seems to be relatively close.
In this case we speak of "flat" perspective.
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