Depth of Field (Continued)
As you can see, the f# we choose will have a great impact in the the final result. It is you, the photographer, who has to decide on which diaphragm to use, depending on the type of picture you want to shoot. If I'm shooting a landscape, I would probably do better with a closed diaphragm (f/22) for all the landscape to be in focus. On the other hand, if I'm shooting a portrait, I would be better off with an open diaphragm (f/2.8) to achieve a better Depth of Field. This not only focuses the attention on the person I'm photographing but also makes background elements less of a distraction.
Of course, this is not a rule we have to follow, it is simply a suggestion or guideline for you to begin using different diaphragms.
Besides the f#, there are other 2 factors that influence the Depth of Field.
The first factor that affects the Depth of Field is the lens focal length. The less focal length in a lens, the more Depth of Field I'm going to get. For example, if I take 2 pictures using the same diaphragm and the same distance, but in the first picture I use a 35mm lens and in the second picture I use a 200mm lens, the first one (35mm) will have a greater Depth of Field than the second one (200mm).