To increase the depth of field , one need to decrease the aperture. This feature is useful when you take an landscape image, and you want to capture as much detail as possible. In contrast, to decrease the depth of field, then aperture should be increase. For instance, when taking a portrait image, you pay more attention in the expression in the face of the protagonist than the background.
For a portrait picture, it is usual to put the f-number as small as possible, well, depending in your lens (and its price). The idea is in such picture, you need to define a main protagonist, then the rest of the picture are forced to be blur. For instance, in the picture below of an anti-war activist in Washington DC, the f-number was chosen at 3.2; and the forcus point was into his eyes. I wanted to highlight his determination, his concentration, his “burn” while discussion with a young lady; it was clearly not the young lady that I am interested in.
For a lanscape picture, however, the f-number should be set as high as possible; since you want to capture as much detail as possible. In the picture of the mountain range in Chamonix, France, below, the f-number was set at 22. Note that there are two climbers on the right of the picture; it was intentional to have detail both on them, and on the mountain. I chose to name the picture: “La Contemplation” for this effect.
Take away message:
- higher the f-number , higher the depth of field.
- on a landscape picture, f-number is usually high
- on a portrait picture, f-number is usually low