There are many Depth of Field apps available for the iphone but this one is unique. Phocus has the ability to calculate depth of field based on selecting your optimum aperture or minimum and maximum focus distance. These options are useful for composing images where focus is critical between two key areas or where you need to know what will be in acceptably sharp focus at a given aperture.

The ability to calculate this used to be available with most manuel lenses. Now with autofocus lenses the depth of field calculator has gone and you only have the distance scale available, at least on Nikon lenses this is the case. I don’t tend to need this kind of tool
for most of my work. However occasionally its useful for critical work or when working close to the subject.

http://japko.net/phocus/