You can use these calculators to calculate focal length, focusing distance, native extension, and more..
Focussing distance between marks
When a focussing scale is intermediate between two marks, how do you calculate the proportional value?
The lens calculator helps you calculate the focal length of the lens you will need in order to capture a specific scene at a certain distance.
Macro Camera Lenses
Magnification is controlled by just two lens properties: the focal length and the focusing distance.
Macro Extension Tubes & Close-up Lenses
All camera lenses have some amount of native extension, which is used to focus on everyday objects. Lenses with closer focusing distances usually have more native extension, but these will also benefit less from extension tubes. Try experimenting with different lenses in the calculator to see how extension affects magnification.
Understanding Camera Lenses
The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position.
What is a close-up filter?
Includes the basic stuff about a lens (1/u + 1/v = 1/f), and calculators for this and lens combination.