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How to calculate magnification?

Printed From: OpticsTalk by SWFA, Inc.
Category: Other Optics
Forum Name: Spotting Scopes
Forum Description: Big Eye Optics
Printed Date: July/05/2020 at 20:39
Software Version: Web Wiz Forums 12.01 -

Topic: How to calculate magnification?
Posted By: Sparky
Subject: How to calculate magnification?
Date Posted: January/09/2019 at 22:50
How does one calculate the magnification using a fixed eye piece? I was under the impression that the magnification of a spotting scope is the ratio of it's focal length to the focal length of the eyepiece you are using. But some info on magnification I see doesn't match this formula. So either the formula is incorrect or the info is.

Also can you calculate the focal length of your spotting scope or do you have to get it from the manufacture? The spotting scope I have is a Pentax 80EDA.

I am thinking of using the Pentax SMC XW Wide Angle Eyepiece. What magnifications would you use? I shoot mainly 100 and 200yds cast bullet shoots. But the range does go out to 600yds.

Posted By: koshkin
Date Posted: January/10/2019 at 02:50
What info are you seeing that contradicts that?

The magnification of a spotting scope is indeed the ratio of the focal lengths of the objective/body and the eyepiece.

Choice of eyepiece/magnification is pretty personal.  For general purpose use looking at things on the range and otherwise, I mostly end up in the 20x to 30x range, so I would definitely get XW20 eyepiece which is 26x on the PF-80 body (520mm fl).

If you routinely use more magnificaiton, XW14 gets you into mid-30x. 

Back when I used a Pentax spotter, I ended up with XW20 being used 90% of the time.  When I really needed to look at something up close and personal, XW10 seemed to fill that niche.


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Posted By: Sparky
Date Posted: January/10/2019 at 10:07
That explains it, the focal length I found was 500mm for my PF 80EDA(On a competitor's website.) So my magnification numbers were different from those I finally found. Initially I didn't find any info on magnification, just focal length. So that is what got me started on this.

ILya is the fixed XW eyepiece a step up from the SMC zoom eyepiece in your opinion? I currently have the 20-60x SMC zoom. Thought it might be a less expensive way to upgrade my Pentax.

Posted By: Rancid Coolaid
Date Posted: January/10/2019 at 11:06
 The fixed wides tend to be brighter, which may or may not be of value.  I have a Vortex gen2 HD with wide angle and that is the one that stays on the scope.  I'm not sure where the original eyepiece is, don't need it.

Whether it is an upgrade depends - like Koshkin said, on how you use the scope.

Freedom is something you take.
Respect is something you earn.
Equality is something you whine about not being given.

Posted By: koshkin
Date Posted: January/10/2019 at 11:14
SMC zoom is a very good, but aging eyepiece design, so XW eyepieces is a noticeable step up, although not as big of a step as from XF zoom.

There is a big difference in usability because the apparent FOV on fixed eyepieces is usually wider and on XW it is really impressive.  It is a much more relaxing view, aside from the general image quality improvement.


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Posted By: Sparky
Date Posted: January/10/2019 at 14:10
Thanks ILya and RC. I will probably start with the XW20 since it is very close to the same magnification as the older Kowa 27x that started me thinking of going with a fixed eyepiece. And possibly a XW10 for stargazing.

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