# How to get the magnification?

1. Jan 15, 2008

### josue2503

How to get the magnification of my telescope if my objective lens (double convex) has a focal length of 500mm, and an eyepiece w/ 4 lens (double convex) that is close together (touching each other) and has a focal length of 500mm each lens?

2. Jan 15, 2008

### pixel01

The magnification equals F/f where F is the focal point of object lens, f is the focal point of eyepiece. You can calculate the f of eyepiece or even measure it (approx.).

3. Jan 15, 2008

### josue2503

how to find the focal length of my eyepiece w/ 4 lens touching each other having 500mm focal length each lens??

4. Jan 15, 2008

### pixel01

A typos? the focal point of each eyepiece lens is 500mm, isn't it?
You can measure roughly the focal point of the whole eyepiece by putting it into the sun and get the image of the sun on a piece of paper. Adjusting the distance between the lenses and the paper so that the sun's image is smallest (the paper may burn). That is the eyepiece's focal point. Probably it is less than 100mm.

Edit: this method is not used for concave lens

Last edited: Jan 15, 2008
5. Jan 15, 2008

### josue2503

thanks...so..when the sun rays converges to the paper, the distance between the paper and the lens is the focal length?

6. Jan 15, 2008

### pixel01

Exactly, that is what the theory says.

7. Jan 15, 2008

### josue2503

thanks very much so my telescope magnification is 7x and if i used an objective having 500mm focal length and an eyepiece having 10mm focal length.. my telescope magnification is 50x, right?

8. Jan 15, 2008

### pixel01

Yes. But remember the magnifition must go along with the resolution of the telescope.