Spotlights of a lens

Main focuses

A lens has a pair of main focuses: main focus object (F) and main focus image (F '). Both are located on the main axis and are symmetrical with respect to the lens, ie the distance OF equals distance OF '.

Focus Image (F ')

It is the point occupied by the image focus, can be real or virtual.

Focus object (F)

It is the point occupied by the focus object, can be real or virtual.

Focal distance

It is the measure of the distance between one of the main foci and the optical center, this measure is characterized by the letter f.

Anti-main points

They are points located at a distance equal to 2f of the optical center (O), ie at a distance f of one of the main foci (F or F '). This measure is characterized by THE (for the anti-main point object) and THE' (for the anti-main dot image).


Given a spherical lens in a given medium, we call lens bias (V) the unit characterized as the inverse of the focal length, ie:

The unit used to characterize vergence in the International Measurement System is the diopter, symbolized by di.

a diopter is the inverse of one meter, ie:

A diopter equivalent unit, well known to those who wear glasses, is the "Grade".

1di = 1 degree

When the lens is convergent positive focal length (f> 0) and for a lens divergent negative focal length (f <0).

For example:

1) Consider a focal length converging lens 25cm = 0.25m.

In this case, it can be said that the lens has + 4di vergence or that it has convergence of 4di.

2) Consider a diverging lens of focal length 50cm = 0.5m.

In this case, it can be said that the lens has a -2di bend or that it has Divergence of 2di.