Focal Length ; how can I calculate the focal length of a lens ?

Question

How can I calculate the focal length of a lens ?
Thanks for answering

Answer 1

If you have a *real* lens......hold it a good distance away from a prominent light source. For instance, try to stand in a dark room with a shadeless lamp far away and hold the lens up in front of a piece of paper. Move the lens in toward and away from the paper until you see a clear image of the light source on the paper. Measure the distance from the paper to the lens and this will be your focal length (fl). It's tricky to measure this by yourself, but as long as you get the 'idea' you can probably find a creative way to do it.

If you need to do this mathematically the formula is:

1/f =1/d + 1/s

Where the "d" is the distance from the object to the lens, and "s" is the distance from the lens to the image. Of course "f" is the focal length.

Answer 2

f - focal length.
u - Location of object from pole of lens.
v - Location of image from pole of lens.

Use the equation:
1/f = 1/u + 1/v

Answer 3

Put an object, some distance, s, in front of the lens. Measure the distance, s', of image formed. Then use the relation,

(1/s) + (1/s') = 1/f.

Answer 4

1/f = 1/do + 1/di


where f is the focal lengh, do is object distance
and di is the image distance