At one-level it’s any easy concept to get your head around; but the more you think about it, the more complicated and confusing it can become. Let’s start with depth of field – how much of your image you want to get in focus. The inter-play between aperture, where you’re standing with your camera and the focal length of the lens you’re using is well described in this article. You can also visualise how the three elements work together in these three graphics …
Watching this video about Depth of Field, should make everything much clearer, and this graphic might help you make more detailed decisions on how to set up your camera in different situations …
So what is Focal Length and how is it defined? It is the distance, in millimeters, between the “nodal point” of the lens and the camera’s sensor …
… this article is then worth reading.
It just remains to consider Hyperfocal Distance, and then you should have all the knowledge you need to take really sharp images – front-to-back. This article explains it very well, and there’s an excellent video embedded in it as well, which is well worthwhile viewing.
In summary (and simplifying everything), to get everything you want to get in focus, you should consider using manual focus and focussing on a point twice the distance from the minimum point you want to have in focus, as in this graphic …
