Depth of Field: explained

A colleague of mine recently asked how to obtain shallow depth of field (DOF) and so for her and others, here are the 4 factors that affect DOF, in order of greater impact to lesser impact on DOF.

1. Lens aperture, or lens f-stop #, e.g., f1.8, f2, f3.5, f4, f5.6, etc. 

A smaller f-stop # = wider opening = shallower DOF. A lens with a smaller f-stop # is said to be a faster lens (lets in more light) and is always more expensive than a similar focal length with larger f-stop #. It will also be bigger and heavier heavier since it will incorporate more glass elements and more exotic coatings, etc. All things being equal (same sensor size, same lens focal length, same camera-to-subject distance), a smaller f-stop # will produce a shallower DOF.

2. Camera sensor size. Cameras come in different sensor format (from largest to smallest) such as Full-Frame (Canon 5DIII, Nikon D800, Sony a99), APS-c or cropped sensor (Canon 7D, Nikon D300, Sony A77II), micro 4/3rds (Olympus EM-D, Panasonic GH4), 1" (Lumix L100, Sony RX10), 1/1.7" (Panasonic LX3), 1/2.3" (all point-and-shoots). All things being equal (same f-stop #, same lens focal length, same camera-subject distance), the bigger sensor will produce a shallower DOF than a smaller sensor. For example, a 50mm lens on Full Frame will have significantly shallower DOF than the same lens on a micro 4/3rds.

3. Lens focal length. All things being equal (same f-stop #, same camera, same camera-to-subject distance), a longer lens, e.g., 100mm will have a shallower DOF than a shorter lens, e.g., 50mm.

4. Camera-to-subject distance. All things being equal (same f-stop #, same camera, same lens focal length), the closer the camera is to the subject, the shallower the DOF.

If it sounds confusing, it shouldn't be. Thankfully, some advanced cameras have a stop-down button, that when pressed, "closes down" the lens to the desired taking aperture (or f-stop) so that one can have an approximate visual idea as to the DOF. (All cameras always have the aperture wide-open until the moment  the shutter is pressed, at which time the aperture "closes down" to the desired aperture.)

There are a lot of samples on the web to illustrate what I have just explained.