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# Calculating magnification of compound lenses

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### #1 klebics

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Posted 15 April 2019 - 01:36 PM

Sorry to join a forum just to ask a question, but I hope you will believe that I did my best to research this subject before asking for your help.  I have ground/polished working glass lenses before, and this time I want to make a perhaps stupidly large rifle scope that has acrylic lenses of THREE inch objective and a TWO inch eyepiece with 8X power (in case you wonder why so big, it's because i want to see whether large eyepieces really do help you sight your target better)

I know that magnification power is simply objective's focal length divided by eyepiece's focal length.  But what if you have something like this?

I would need to "stretch" and/or "compress" the diverging/converging rays by adding additional lenses because, obviously, the 2:3 ratio of eyepiece:objective I want to (perhaps stupidly) design cannot produce an 8X magnification without it.

I apologize that, in some ways, I kind of know that maybe the answer might simply be to keep grinding and checking...by projecting an image into the objective and see if it comes out 8x smaller on the other end...until I get it right.  But if anyone can give me any help, I would greatly, gratefully appreciate it.

Thanks,
Caleb

### #2 MitchAlsup

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Posted 15 April 2019 - 02:54 PM

https://www.telescope-optics.net/

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### #3 BGRE

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Posted 15 April 2019 - 03:38 PM

Telephoto style lens objective.

at 8x the exit pupil will be around 3/8 inch somewhat larger than the typical daytime eye pupil of around 1/5 inch or so  at most.

The effective entrance pupil will be around 1.5 inches if the eye pupil diameter is 1/5 inch.

### #4 Oregon-raybender

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Posted 15 April 2019 - 04:39 PM

Look for Edmund Sam Brown books, Fun with Optics or All About Telescopes. They have simple methods to calculate.

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### #5 Vla

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Posted 15 April 2019 - 06:25 PM

Effective f.l. is a product of the effective focal ratio and the entrance aperture. Using your illustration:

### #6 Jon Isaacs

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Posted 15 April 2019 - 07:47 PM

I have ground/polished working glass lenses before, and this time I want to make a perhaps stupidly large rifle scope that has acrylic lenses of THREE inch objective and a TWO inch eyepiece with 8X power (in case you wonder why so big, it's because i want to see whether large eyepieces really do help you sight your target better)

That looks basically like a Barlow lens.

It seems to me your acrylic lens are going to exhibit severe chromatic aberration, the image will be of very poor quality. I think the objective needs to be an achromatic double, I'd just buy one. A vendor in the Classifieds sells 70mm F/4.5 air spaced achromats in a cell for \$26.

I don't know what your planning for an eyepiece, it needs multiple elements, and the mounting of the sight reticule..

Jon

### #7 klebics

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Posted 16 April 2019 - 12:52 AM

Many thanks everyone!

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