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Pondering Exit Pupil, lowlight, parallax...discuss

Printed From: OpticsTalk by SWFA, Inc.
Category: Scopes
Forum Name: Rifle Scopes
Forum Description: Centerfire long gun scopes
Printed Date: January/23/2019 at 18:27

Topic: Pondering Exit Pupil, lowlight, parallax...discuss
Posted By: Parallaxed
Subject: Pondering Exit Pupil, lowlight, parallax...discuss
Date Posted: March/05/2013 at 03:05
Hey all, 
This is my first post here, I've been lurking for a couple of years and am sifting through scope options for a few different rifles. This got me wondering as to the usefulness of certain scope configurations for various roles. I'm looking for one, a lightweight AR "recce" type rifle for 1-500 yard effectiveness, and two, a lightweight .300win mag for moose hunting out to 300-400 yards, and also the ability to stretch the legs of the .300 tactically, out to 900-1000 yards. 
It seems that both of these rifles are somewhat hybridized, in that it can ask a lot of an optic to excel at multiple things well. The AR, being a tactical type rifle, that could be used under stress in a non-bench rest setting (i.e. running, rollover prone, slung etc.) the most important thing is shouldering the rifle and instantly acquiring a proper sight picture (which of course is the sole purpose of CQB sights). And the .300 win, at dusk, 400 yards away a dark colored moose moving into a tree line, low light performance is critical, if you can even see the reticle against the shadows. 
Am I right in assuming that a scope's ability to work well in challenging environments like these is directly related to Exit Pupil size? I've heard mention of scopes like the 1.5-6x42 config being "easy to get behind" and having forgiving "eye boxes". 
So, for the AR, engaging a man-sized target at 300 yards would be most comfortable at lets say around 4x-9x. So, with an objective of 42mm, at 6x we'd have our full 7mm exit pupil. My question is then, at 3x would we have twice the room for error when shouldering the rifle, or half the room for error at 12x? (and also much less image brightness) I read Chuck Hawks say once, when referring to the now discontinued Sightron SIII 1.5-6x50, that the 50mm objective was wasted because even at full 6x, only 42mm was needed to reach the magic 7mm human pupil size. Also, i began wondering, if you have a 7mm human pupil being bathed by an image with a 33mm exit pupil,such as on the Sightron at 1.5x, if the image isn't centered up, such our pupil being clear to the edge of the image, does this induce parallax error? Or is this the"very forgiving eye box " I've heard about? Also, how does eye relief "range" come into play? So, I guess in essence I'd like to know more about the physics involved, and what would make a tactical carbine/recce rifle easy to get behind under stress, and a longer range scope cranked up to 9x or 14x and have good lowlight performance and eye placement forgive-ability without having a larger than a 50 or 56mm objective. I've got to place some kind of weight restriction for both of these scopes and their roles, and I think 24 oz is about max. Is this a boring and useless topic or thought provoking? I love this forum! Thanks guys! Curt

Posted By: MZ5
Date Posted: March/20/2013 at 18:55
I think they're good and interesting questions.

Unfortunately, I can offer no answers.  ;)

Posted By: koshkin
Date Posted: March/20/2013 at 19:20
A couple of questions and observations:

1) First of all, anything Chuck Hawks says about scopes should be taken with a couple of tons of salt.  If he says something worthwhile about optics, it is a pure accident.

2) What price range are you looking at?

3) Ease of getting behind a scope is a function of exit pupil and a variety of other factors primarily due to eyepiece design differences.  They are not easy to glean from specs.

4) Sightron 1.5-6x50 does not have a 33mm exit pupil at 1.5x. For most variable scopes out there, at the lowest magnification the objective lens is not the limiting factor.  Another stop inside the scope limits the amount of light; i.e. at the lowest magnification the scope does not use all of the objective lens. 

For a general purpose scope intended for a Recce-style AR, a 1.5-6x42 is actually a pretty good option.  There are not all that many of these scopes around since it is not a "fashionable" configuration, but I know at least one gentleman who got a very high end 1.5-6x42 for his AR recently (he is a forum poster here) and seems very happy.

I could make a pretty good case that for your rifle something more compact (though not necessarily lighter) like a 1-6x24 or 1-7x24 would make a very good option.

For the 300WM, if I understand this correctly, you really need a jack-of-all-trades scope with good low light performance.  If you are willing to spend the money, consider Premier LT 3-15x50.  That should do everything you need.


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Posted By: ccoker
Date Posted: March/20/2013 at 20:02
I do not profess to have 1/10 the knowledge of optics that Koshkin has.

I do have a lot of experience with a lot of scopes, especially geared towards low light hunting, 3gun and long range.  I know what I like and the pros/cons between varying units but certainly don't have the optical engineering education and professional experience that he has.

Having said that:

Agree on the 1.5-6x42 as being a good setup..
I run one on my 16" 6.8
I am running an S&B but if not and I was to go buy one today I would probably get the Meopta in the same config.  I haven't tried that one but run a 3-12x56 RD and have ran several other scopes of theirs, binos, spotter, all GREAT stuff. 


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Posted By: Gerry Atric
Date Posted: March/21/2013 at 00:51
For low-light hunting (we´re allowed to hunt moose 1 hour before sunup and to sundown, other deer-species 1 hour before sunup to 1 hour after sundown) I´m a firm believer in:
Good glass ( I´ll go for used Euro scopes)
Ample front-lens diameter (there´s a trade-off between mobility and light-gathering components in this equation)
I have a S&B 1,25-4x20 that I use until it´s way too late....Cool
An old Pecar 4-10x48
A Kaps 3-12x56 that will replace a Leupold VX-III (illuminated)
I´m looking for a 1,5-6x42 again, they are not that scarse here in Sweden, they are still popular though so it´s hard to find a cheap one.
Gerry Atric

Posted By: Oldtrader3
Date Posted: March/25/2013 at 00:20
The thing that I pick up on here is shooting a lightweight .300 Magnum in competition.  During my 70 years, I have owed a lot of rifles up to and including the .375 H&H and the 9.3x74R but only one lightweight .300 Mag and it was the .300 WSM.  I kicked much worse than my .340 Weatherby ever did.  That was why I go rid of it.  You might consider that when you start buying a .300 mag.
Most people over forty years old only have an eye exit pupil of 5mm or less.  Therefore we do not need scopes that go up to 7 mm at 6X.  All of my scopes and I have (12) go to between 4 and 5 mm at 10X.  I do not want a scope over 50mm eyepieces for cheek weld issues just to get more exit pupil that I personally do not need.  The Europeans shoot head up and don't worry about cheek weld.  Plus, they hunt at night and need the 56mm scopes for light gathering.


Posted By: ccoker
Date Posted: March/25/2013 at 09:00
I will be 48 next month
I can definitely see a difference between a 40 and 50mm objective at last light with the same scope level.
At 10-12x magnification and same scope level I feel I get and exta 5 min of light and a few more with a 56

For me, at 10x or over I want a 50 if used for hunting.
I have also found that with a 50mm if it has higher magnification about 12 is max useable in very very low light.
That is even with top end glass.


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