What is the Depth of Field

Shelton Muller explains in this video what the depth of field is and how it can be used.

First of all we make an assumption that we expect our lens to be sharp.

There is a difference in what photographers want to achieve in landscape and portrait shooting. In landscape we want everything to be in focus and the whole image to be sharp. To get this result, you need to use the apeture of 22 or larger. Of course since the hole is tiny, the amount of light coming to the sensor or film is minimal and we have to set longer shuter speed (and therefore having to use a tripod). Landscape photographers usually prefer wide angle lens.

With portraits we try to highlight the subject, and especially the model's eyes. So we want a very narror length to be in focus and the rest blured. It enables us to remove the subject from noisy background which otherwise would distract. Smaller apetures (1.4, 1.8 or 2.8) and longer lens (85mm, 100mm or 200mm) are good for achieving this effect. Shelton shows sample photos of the same model in the same background, shot with different lenses and different apetures.