Calculating the distance between the optical center and focal length

[stevepham]

-Hello everyone. At school, I studied about convex lense and concave lense. They are very interesting, we learned about how lights would be after going through the lense. But what I want to ask is how we can calculate the distance between optical center and focal (I means focal length). Focal length is a constant, but is there any formula which can give us the value of focal length .

I would be grateful it someone could help :)

 

[Drakkith]

Per wiki:http://en.wikipedia.org/wiki/Focal_length#Thin_lens_approximation

The focal length of a thin lens can be determined by using it to form an image of a distant light source on a screen. The lens is moved until a sharp image is formed on the screen. The focal length f is then given by

where u is the distance between the light source and the lens, and v is the distance between the lens and the screen.

Real lenses aren't thin lenses, but this is a good approximation to use in many cases.