Aperature/F-Stops...HELP!

Joined: 02 Apr 2006 Posts: 12 Location: Pahrump, NV

Photography has been a hobby of mine for about a year now and I enjoy it more than just about anything. I think the hardest part for me to comeprehend has been the setting of the aperture (f-stops).

I've read and seen diagrams that show how certain f-stops allow a subject to be in focus at a certain distance while another is not. This rule being called depth of field. It seems (after looking at sample pictures) that this rule is inconsistant. (Don't worry, I know the problem is me and not the rule)

Example: I've seen a landscape photo taken at an aperature of f-11 and than another portrait shot taken at the same f-stop of 11. If the depth of field rule applies, how can the same aperture setting work when in one photo the subject is 4 feet from the camera and the other photo the subject is 20 feet from the camera?

It's extremely hard to be in the moment of taking the shot and have to perform math equations of whip out a book to figure out what f-stop to use. Does this get any easier with experience or am I just impatient? I've got exposure down pat but this never seems to get any easier.

Posted: Sun Feb 04, 2007 2:56 am

Pokinatcha,

The DOF depends on two interrelated factors: the aperature and the focus distance.

Consider first keeping the aperature constant and varying the location of the subject relative to the camera: At a given aperature, the closer you focus, the shallower the DOF. Likewise, the further away you focus, the deeper the DOF. For a given lens, you might get a DOF of 2 feet at f/11 focusing on an object 5 feet away from you, whereas the DOF might be 2 feet to infinity at the same aperature when focusing on an object at a great distance.

Now, fix the subject's distance from the camera and vary the aperature: as you step down the aperature (go to smaller aperature/bigger f-stop number), you increase the DOF for a given focus point. Thus, for a subject 4 feet away, a given lens may have a DOF of 5 inches at f/2.8 and 2 feet at f/11.

To throw another variable, this effect varies by the lens involved, as well.

Here's a good link to info on DOF: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

Hope this helps - use your DOF preview button if you have one to figure this out on the fly. It does get better with experience.

Sigma147