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Verify Scope Power / Magnification

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/25/2016 at 21:44
JRSanDiego View Drop Down
Optics GrassHopper
Optics GrassHopper

Joined: May/25/2016
Location: San Diego
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Points: 3
Dear experienced spotting scope users. (applies to rifle scope power as well)

I have a couple of inexpensive scopes, several mid-range rifle scopes, and binoculars.  I noticed the other day that the target looked of different size thru the scopes on the same power.  It got me thinking..

I've read a couple of explanations for scope power:

1. If the scope is on 20x power, the image will look like its 20x closer than with the naked eye.  So for example, if the image is at 100 yards, divide by 20, and the image would look like its at 5 yards distance.  

>>This can't be right - when glassing an image at 100 yards, it may really only look like it is at about 50 yards.  On none of the scopes does this appear true.

#2.  A scopes power, for eg at 20x, makes the image look 20x bigger than it appears with the naked eye.  Ok, the target at 100 yards looks like its about 1 inch tall, and through the scope it appears to be about "20 inches tall".  

>>Hmmmm, maybe, but is there any way to measure or calibrate this?  Its a complete guess when you ask yourself "Does it really look about 20" tall?" Are there rulers or something that show what 12" or other increments looks like at 100 yards - 50 yards, etc.  I'm guessing you could look through the scope with both eyes open and see the target in one eye and ruler in the other and approximate if the image looks the right size.

Has anyone heard of how you "verify" a scopes' power rating is accurate?

This has really been bugging me lately how I could measure this.  To the point where I am thinking of going to the neighboring football field to test some things out!! LOL

Any insight much appreciated. :)   
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/26/2016 at 07:24
koshkin View Drop Down
Dark Lord of Optics

Joined: June/15/2004
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None of this can be easily done with the naked eye. In principle, you can set-up a camera to take a picture of an object of a particular size directly and then through an optical device.  If you keep everything else reasonably constant (camera focus, focal length, etc), you can calculate the actual magnification.  DO keep in mind that in order for any of this to work, the eyepiece of the scope has to be set-up correctly (if you are dealing with a riflescope), vibration has to be minimized, etc.

Simply eyeballing what it looks like is not going to work at all except at very low magnifications (near unity).

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/26/2016 at 09:31
JRSanDiego View Drop Down
Optics GrassHopper
Optics GrassHopper

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AHH - interesting idea.  I do actually have a camera system that attaches to the scopes.  So you are suggesting take a picture minimal magnification, then take a second picture at higher magnification.  Then compare the two pictures and one should appear larger by the power used.

Is it right to expect the image at 20x magnification to appear 20x larger than at zero magnification?

thank you
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/31/2016 at 23:29
admacdo View Drop Down
Optics GrassHopper
Optics GrassHopper

Joined: September/04/2014
Location: Canberra
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Points: 7

Sorry for the late addition.

It is indeed possible to check magnification accurately, however, it is dependant on your ability to force your eyes to do unnatural things. :)

You will need to be able to superimpose what you see through the scope over what you see through your other eye.

Magnification is indeed linear as you've described it.  A yardstick at 10 yards is 10 times smaller at 100 yards.

So if you look at a yardstick at 100 yards, with a 10x magnification, it should be exactly the same size as one at 10 yards to your eye.

If you'd like to check this relationship out easily, get yourself a 10x and 8x set of binoculars and look at ruler about 10 yards away with one eye looking through each at the same time.  Once you dick around for ages getting both focussed and pointing at the same thing, you'll be able to line up the bottom of the ruler.  Then take a look at where the measurements line up.  If you truly have a 10x and an 8x, they'll be overlaid at those measurements.

I've spent a lot of time doing this sort of thing with expensive binoculars to see if I can see any difference between different brands easily.  One day I picked up an 8 instead of a 10 and then realised how handy this can be.  This shows the magnification difference between two binos, but you CAN absolutely test magnification levels using one optic and two rulers at known distance relationships.

I recommend working out the math beforehand to make life easy though and using two identical rulers that aren't transparent plastic.  You will also need to have the ruler far enough away so that you can see it without obstructing it with the spotter lens. It takes a LOT of dicking around, but once you have the position of the rulers, distance factor between the two matching the magnification to be verified and your head and eyes in the right position, once the image from either eye overlays exactly, you know that the magnification is DEAD on. 

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