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Focus and Depth of field

 

This is a preliminary introduction to the concepts of focus and depth of field. For a more advanced and technical approach, please refer to the article Depth of Field.

 

Everyone surely knows how a simple lens, such as a magnifying glass, can focus the rays of the sun to a tiny but intensely bright spot, and even set things on fire, if it is held a certain precise distance away. The sun's distance from the lens is enormous, virtually infinite, and the distance of the lens from its point of focus is then known as its focal length.

 

If you try the same experiment with a light-bulb just a few feet away and project its image onto a sheet of paper, you will find that the point of sharpest focus is now a little further from the lens than it was before. There is a scientific formula that relates this image distance, from the point of best focus to the lens, to the subject distance, from the lens to the source of light. For details, refer to the article Depth of Field.

 

When composing a photograph, you generally want the main point of interest in your picture to be sharply in focus. To achieve this, you use your camera's focusing mechanism (as described in the Start here article) to move your lens closer to or further from the image plane. Depending on the type of camera you are using, you can see which part of your image is in best focus. Check the article on Camera types.

 

The point to remember here is that there is only one subject distance at which exact focus is achieved with any given image distance (or focus setting). Everything that is closer to or further from the camera than this will be more or less blurred, or out of focus. Objects that are near to the exact focus distance will be only a little blurred, but objects that are much closer or further away may be very fuzzy indeed. The issue of how much fuzziness we can accept brings us to the question of depth of field (DOF).

 

Depth of field is the range of subject distances between which the image of an object looks sharp. This range of subject distances extends from a point somewhere in front of the point of exact focus (i.e. closer to the camera) to another point somewhere behind, further away from the camera. When we say 'looks sharp', we mean that by looking at the photograph, we cannot detect any fuzziness. The degree of sharpness needed for this depends on many factors: chiefly the resolving power of human eyesight and the size of the photograph - how much it has been enlarged.

 

Depth of field depends on how quickly fuzziness increases as an object is moved closer to or further from the camera than the point of exact focus. This increase is always faster when the movement is towards the camera: there is always more depth of field behind the point of focus, away from the camera, than there is in front of the point of focus, closer to the camera. There will be less depth of field when the fuzziness increases quickly, as an object is moved away from the point of exact focus. The rate of increase of this fuzziness depends on subject distance and the aperture setting of the lens.

 

Fuzziness increases faster when the subject at the point of exact focus is closer to the camera, so that depth of field is often extremely limited in close-up photography. Fuzziness increases relatively slowly when the point of focus is at a great distance from the camera.

 

Fuzziness increases faster when using a wide lens aperture (see the Glossary if you are unfamiliar with any of these terms). If you want to throw the background out of focus in order to have your main subject stand out in clear focus, then use a fast lens with the aperture wide open. Fuzziness increases relatively slowly when you stop down and use a small aperture. This is frequently what you want in landscape shots. Of course, using a small aperture requires longer shutter speeds, which is why serious landscape photographers so often use tripods.

 

Other things being equal, depth of field is greater when using lenses with shorter focal length, and more limited when using longer lenses.

 

Once again, please refer to the article on Depth of Field for additional information.