NF 2.5-10 on a .308?

[C-grunt]

Is there a reason you dont see this scope on a .308 bolt gun? Is the objective to small? Or is it just that people want more than the 10x upper range? I see them for sale for around 1k and think it would work well with my LTR.

[Belmont31R]

You can get the same magnification range with a 40-50MM front bell which would greatly improve exit pupil and such.



When you have a 10X top end and 32MM front objective the eye box is going to be very small at 10X.


1-4X works pretty well with a 20 or 24MM front objective. Even 20MM on 4X would give you a 5MM exit pupil or 36% increase over the 3.2MM exit pupil on 10X of a 2.5-10X32.


Exit pupil has two main effects on a scope. Eye box which is how much side to side movement before the image is distorted and how much light is transmitted. If you go to the Nikon website they have a write up on exit pupil and basically at night a human eye opens up to about 7MM in a healthy adult. So if your exit pupil is only 3.2MM you're eye is only seeing an image slightly more than half as bright through the optic as you would with the naked eye.



FWIW I actually did own a NF 2.5-10X32 at one point, and have done side by side tests with it with another member in low light conditions. The eye box was one thing that bugged me and is there no matter what light conditions you face. On a rifle with more recoil like a 308 its harder to get back on target as you momemtarily lose the sight picture. Basically the image coming through the scope would only be 3.2MM wide so your eye has to be within that 3.2MM circle to get an image, and even then it has to be closer to the center of that circle to get a clear sight picture. When I sold that NF I bought a SB 3-12x50 PMII, and on 10X it would have have a 5MM exit pupil. It was much easier to get back a useable sight picture and I noticed under recoil the image didn't go black for the split second or so it takes your body to come back to rest.


Ive posted in other threads that I would rather see SB turn their 1.5-6X42 Zenith hunting scope in a PMII model with externally adjusted turrets and some sort of mil dot reticle. The exit pupil on that scope at 6X is 7MM which is the max the human eye expands at night so the image, negating loss through the lenses, would be just as bright as the naked eye and would have a huge eye box. Their upcoming 1.1-8X has a small eyebox like the NF 2.5-10X32.

[C-grunt]

Well thanks for the reply and good explanation. I think Ill cross that off my list.

[broylz]

vortex has a viper pst thats a 2.5-10x which ive been looking at. it gets very good reviews.

[Belmont31R]

Well thanks for the reply and good explanation. I think Ill cross that off my list.




Its still a pretty good optic and much better quality than most. I just wasnt a fan of the specs after using it and testing it against other optics we had at the time. I probably would have been a lot happier with a slightly better build quality and 42MM lens.



Plus with the cost I would rather bump up to a used Swaro, SB, Zeiss, ect.


Here are some optics you can look at from Eurooptics (no affiliation other than being a customer).. Basically demo scopes at great prices. The 2nd one is the Z6i which Alaskapopo on here has done some nice posts on, and can give you some better specifics. Just stuff to think about, and I check their used section about once every 1-2 weeks. Some awesome deals on there, and they are good people over there.


http://www.eurooptic.com/swarovski-z6-17-10x42-br-ballistic-reticle-excellent-condition-with-light-ringmarks-item-ub167.aspx


http://www.eurooptic.com/swarovski-z6-1-6x26-cd-i-illum-circle-dot-reticle-excellent-conditiion-with-ext-faint-ringmarks.aspx


http://www.eurooptic.com/swarovski-z6-1-6x24-4-matte-black-like-new-ua301.aspx



Just some different options to look at.

[bp7178]

The following was from a post on optics talk and explains the technical aspects of how the exit pupil is effected by the scope's design.


The little tidbit you are missing is that exit pupil is not objective diameter divided by magnification, but effective entrance pupil divided by magnification. At high magnification, entrance pupil is usually identical with objective diameter, but that is not the case at lower powers in scopes with a high zoom ratio. Some designs with a more conservative 4x zoom ratio actually use the full objektive size even et low power, but none of the 5x or higher zoom scopes do as far as I know.

You can see what "entrance pupil" (or effective objective diameter) means if you look into the objective of one of our scopes at high and low power from a distance of let's say 20 feet. The bright circle inside the objective bell will appear smaller than the objective size at low power. This is even more pronounced in the 1-8x, which has an effective entrance pupil of ~10mm at 1x.

The reason for this effect is space constraints inside the tube, the lens diameters in the erector don't allow the full cone of light that would create a 24mm exit pupil at 1x to pass through. This is one of the tradeoffs in a design with such a high zoom factor, but we're still getting a larger exit pupil than the competition in the 1-8x, and in the 3-15x and 5-25x the exit pupil at low power is still plenty big also.

http://www.opticstalk.com/new-18x24-premier-heritage_topic30115_page2.html

When comparing exit pupil specs, watch out for calculated and measured specs, the latter being ideal.

The one scope I recently started looking at was the Swarovski Z6 1.7-10x42. The Swarovski ballistic turret can be bought with the scope and is a pretty intresting peice of hardware.