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Question on mag power and exit pupil/eye relief

This is a discussion on Question on mag power and exit pupil/eye relief within the Optics forums, part of the Gun Forum category; I need some input to see if I understand something correctly. I am debating between the XTR2 in either 3-15 or 4-20 power. As a ...


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Old April 8th, 2017, 06:40 AM   #1
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Question on mag power and exit pupil/eye relief

I need some input to see if I understand something correctly. I am debating between the XTR2 in either 3-15 or 4-20 power. As a total noob I'll never need more than 15x, and as a target only shooter would probably never go as low as 3x. So, if I understand things correctly, the exit pupil will be larger at less than a scopes maximum magnification? If this is true, how would the "eye box" compare on the 3-15 set at 15x compare to the 4-20 set at 15x? Would the 4-20 scope have a more forgiving eye relief/exit pupil, a larger eye box? If so that might be a reason to go for the 4-20. Hoping for some clarification to see what the real world differences would be, thanks!

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Old April 8th, 2017, 07:11 AM   #2
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Far be it for me to explain optics, but from a practical viewpoint, if shooting prone/sling as in long range matches, slow fire, etc. the so called "eye box" is important. Reason being in such a position your eye is closer to the scope than other positions and if you have to keep repositioning your self to stay in that "eye box" your scores will not be good. Be it bolt gun or an M14 find that the eyepiece will need to be about even with the bolt shroud or forward of the rear of the heel of the M14 in order to attain the comfortable "eye box." Obviously if all your shooting is done from bench or other positions it is not as critical. Some years ago shooting at 600yd. prone match a young shooter in position beside of me had his new M1A with some humongous scope on it and the rear of that scope was at least couple inches behind the heel of the action and prior to the match mentioned to the line officer that he may want to advise the fellow of a problem with his set up. Heard the officer politely warn the fellow of the possible problem and response was "I know what I am doing..." and officer merely walked away shaking his head. First shot, heard a loud "ouch" and blood dripping onto the rifle and mat. Education often requires "blood , sweat, and tears" they say.

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Old April 8th, 2017, 07:23 AM   #3
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Divide objective lens size by magnification and you get exit pupil.

50mm objective / 15x magnification = 3.3mm exit pupil.

A lot of it depends on your eyes, I'm photosensitive with large pupils so I like a larger exit pupil. Most people are in the 2mm to 2.5mm pupil size fully constricted in the light. A larger exit pupil will make the scope a little more forgiving if you're not perfectly aligned behind it.

Hope that helps a little.

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Old April 8th, 2017, 08:21 AM   #4
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Thanks for the input, it all helps and my eyes are pretty bad too which is why I started to think about this.

Also just ran a little excel math on those two scopes. Using the posted max and min exit pupil specs for each, and making a big assumption that the exit pupil size changes linearly across the scopes mag range, I found these numbers. At 15x the 1-15x50 scope exit pupil is 3.3mm, and the 4-20x50 scope exit pupil is 5.2mm. The 4-20x50 shrinks down to 2.5mm at 20x, but again I don't need that range. At 15x it really seems like the 4-20 scope would be easier to see, no? I know not all else is equal between these two scopes, but I am willing to give up a little image quality to get an easier to use eye relief if neccessary. So if this is all nonsense let me know!

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Old April 8th, 2017, 08:55 AM   #5
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You're doing some weird math...

50/15=3.3 and 50/3=16

50/20=2.5 and 50/4=12.5

All you have to do is divide the objective size by any given power and you have exit pupil. So if you set either the 3-15x50 or 4-20x50 at 15x, you still have a 3.3mm exit pupil. There's not much to interpret there. Set either scope at 10x and you have a 5mm exit pupil.

It comes down to usable magnification, can you use 20x if the exit pupil is too small for your eye? I had a 6.5-20x50 and couldn't go much beyond 16x without the exit pupil being too small. This told me I needed at least a 3.1mm exit pupil and I better be right behind it. This is what made me go with a lower magnification scope with smaller lens, my 1.8-10x37 is just right for my eyes.

Looking at the specs it seems like they both have a max exit pupil of 11mm, likely due to lens or mechanical design. This doesn't change the above math though.

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Old April 8th, 2017, 09:10 AM   #6
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Quote:
Originally Posted by NoExpert View Post
You're doing some weird math...

50/15=3.3 and 50/3=16

50/20=2.5 and 50/4=12.5

All you have to do is divide the objective size by any given power and you have exit pupil. So if you set either the 3-15x50 or 4-20x50 at 15x, you still have a 3.3mm exit pupil. There's not much to interpret there. Set either scope at 10x and you have a 5mm exit pupil.

It comes down to usable magnification, can you use 20x if the exit pupil is too small for your eye? I had a 6.5-20x50 and couldn't go much beyond 16x without the exit pupil being too small. This told me I needed at least a 3.1mm exit pupil and I better be right behind it. This is what made me go with a lower magnification scope with smaller lens, my 1.8-10x37 is just right for my eyes.
That doesn't work on the minimum mag end though, both list 11mm with different magnification. The smaller obj. lens is an interesting idea and one I hadn't thought of, something to look at, thanks!

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Old April 8th, 2017, 09:25 AM   #7
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Quote:
Originally Posted by L1329 View Post
That doesn't work on the minimum mag end though, both list 11mm with different magnification. The smaller obj. lens is an interesting idea and one I hadn't thought of, something to look at, thanks!
Yeah, I made an edit to my post. Looks like there's a lens or mechanical limit keeping it at a max of 11mm for both scopes.

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