# Focal points ?

## Main Question or Discussion Point

Can you tell me the need & use of the pocal points on a lens , why do we need to have them , why do we measure them .

focal points can be used for various measurements but other than that ?

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Andy Resnick
A focal point is not much use at all, since it is not a constant point... maybe useful for burning ants, but that's about it. The location of the focal *planes*, however, are of use- they give you an idea of the magnification of an object, for example. They also help determine the numerical aperture, which sets the resolving power of the lens.

All lenses/lens systems have 6 planes of interest: 2 focal planes, 2 pupil planes, and 2 nodal planes. The optical field at the pupil and focal planes are related via a Fourier transform. Telecentric lenses place one or both of the pupil planes at infinity.

One common misconception I see is that the focal distance is confused with the working distance- that is, the assumption is made that a lens with a 200 mm front focal length needs to be 200 mm from an object to correctly focus it.

Would u please clear that misconception that u have stated , if the object is placed at the focal point of a convex lens then image is formed at infinity & if the object at infinity is focused then it will come to focus at focal point of a lens

Andy Resnick
Nope, that's correct.

Question related to combination of two convex lenses ?

The object distance (u) is measured from an imaginary point b/w the two lenses , why so , it does not exist in reality but we use it to find out (V) .

If the object distance is measured from the center of 1st lens or 2nd lens ,will it make any difference ?

I didnt get u ?

Can you tell me the need & use of the pocal points on a lens , why do we need to have them , why do we measure them .

focal points can be used for various measurements but other than that ?
One application is if you want to produce a "parallel beam" with a small bulb.
How do you do this? You put the bulb at the focal point of the lens. Have a look at http://kottan-labs.bgsu.edu/teaching/workshop2001/chapter2_files/image040.gif [Broken]. This is also called "collimating" the light from the bulb.

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