More about Depth of Field

Your only real control is aperture

It is often understood that using a shorter/wider lens gives more (depth of field) DOF, but that is only half of the story. That is true if the subject is smaller (such as taking both photos from the same camera position), but the tradeoff there is file size and resolution. The 35mm shot has to be heavily cropped to give the same subject size, so resolution (pixel count) is greatly reduced.

In practice, if you want to maintain the same subject size on the film/sensor your best control over DOF is to stop the lens down more. If you don’t need all the resolution of your sensor you can maintain the same camera to subject distance and use a shorter lens, then crop—giving you the same perspective. Or you can move back with the same lens and crop, but that will change perspective—the background will get larger (more compressed) and the subject will flatten out. 

The following animation shows 4 full frame images taken with a 200mm, a 135mm, an 85mm, and a 35mm lens all at f/2.8. The camera was moved in closer with each shorter lens to make the mannequin head approximately the same size in each image. The things to note are that while the shape of the face changes drastically and the size of the pattern in the background changes (both due to the changing camera to subject distance--refer to my last post, "The closer the camera to the subject, the smaller the background elements appear"), the depth of field remains almost identical. Pay close attention to the headpiece beads next to the ear. They look about the same in all four images...

dof-animation.gif

Below are two still images to make it easier to compare and see the similarity in depth of field for two very different focal lengths, but giving the same magnification at the same f/2.8 aperture. Each is shot full frame, moving the camera in closer for the 35mm image to maintain approximately the same size head in each photograph...

200mm-35mm-dof.jpg