this is the "burning ants" distance

Measure your magnifying glass focal length by focusing a distant light source on any surface and measuring that distance.

Lens Focal Length - _____________________mm ______________________ inches

The f/stop or aperture is determined by the size of the lens opening in relation to its focal length.

First measure the focal length of your lens._________________mm ______________inches

The size of your lens diameter: __________________mm ___________________inches

The f/stop is the focal length divided by the diameter of the lens (or lens opening).
Example 1 - if your lens focal length is 100mm and your lens diameter is 25mm... your f/stop is f/4
Ezample 2 - if your focal length is 10 inches and your lens diameter is 2 inches your f/stop is f/5

Angle of view:
The angle of view is often referred to as normal, wide angle and telephoto. Modern zoom lenses can cover all of these.
Large format cameras don't have zoom lenses.

The angle of view is determined by the focal length AND the size of the film format.
The chart on the right is for 35mm film.

The "Normal" lens with about a 45 degree angle of view has a focal length equal or close to the diagonal measurement of the film. Normal for 35mm is 50mm, normal for 4x5 is 150mm

A lens that is less than the "normal" focal length will give you a wide angle view.
A lens that is more than the "normal" focal length will give you a telephoto view.