Quote Originally Posted by Shebang
I know how to get a specific aperture. But I don't know how to determine what the focal length is



How did you get 36.2 feet?
I used this site.

http://www.dofmaster.com/dofjs.html

For in the field - some lenses have a rough indicator right on the lens, you can get little charts to carry or I have seen a calculator that is an application in a cell phone.

TF