How would you find the focal length of lens?

Answer 1

Have light from a very distant object directed through the lens onto an opaque, white surface. Change the distance from the lens to the projection surface until the distant object shows a sharp image on the surface. The distance from the lens to the surface at that point is the focal length.

Answer 2

For most purchased telescopes, the focal length of the objective lens will be printed on

the lens' mounting ring, and it will definitely be stated somewhere on the packing box

and in the literature that comes with it.

The focal length of any unknown lens can be measured by using the sun, the moon, or a

distant street-light at night.

Focus an image of the distant light, through the lens, and onto a piece of white paper or tissue.

The distance between the lens and the sharpest image is the focal length of the lens.

Technically, the object must be infinitely far away. So the farther the light source is, the more

accurate the measurement is. That's why it's good to use the sun or the moon as the source.

Answer 3

The focal length of a lens can be found by either locating the point at which a parallel beam of light converges using the lens (for converging lens) or by locating the point from which a parallel beam of light appears to diverge after passing through the lens (for diverging lens). The distance between this point and the lens is the focal length.

Answer 4

For a lens ...

1/image distance + 1/object distance = 1/focal length of the lens

Answer 5

To get the focal length when the magnification is given, simply divide it by the focal length of the eyepiece.

Answer 6

you could use a lensometer.