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RifleScope School

Printed From: OpticsTalk by SWFA, Inc.
Category: Scopes
Forum Name: Rifle Scopes
Forum Description: Centerfire long gun scopes
Printed Date: August/13/2020 at 08:57
Software Version: Web Wiz Forums 12.01 -

Topic: RifleScope School
Posted By: Chris Farris
Subject: RifleScope School
Date Posted: March/15/2004 at 18:17


Rifle scope Terms



Whether you are a novice or seasoned riflescope user, these definitions will assist in understanding the different components and considerations that go into the engineering of riflescopes.



The most basic characteristic of a given scope, magnification is simply a measure of how many times better you can see an object than with your naked eye. It's typically the first number you see in a scope's name. For example, the a 6x42  has a six-power magnification: Six times better than the naked eye.

Variable power scopes allow a range of magnification for different hunting situations. In these scopes, the first two numbers, separated by a hyphen, indicate the range of magnification. For example, a 3-9x36 magnification can be adjusted as low as 3x, or as high as 9x.


Objective Lens Diameter 
The second figure in the specification, e.g. 6x42 on a fixed power scope have 6x magnification with a 42mm diameter objective lens.  A variable scope's specification would look like this, e.g. 3-9x36.  3x-9x with a 36mm diameter objective lens.  The number indicates the entrance aperture diameter or entrance pupil in mm.  It is a measure of the amount of light that can enter a scope.  Keep in mind that effective objective differs from outside diameter.


Ocular Lens
The Lens closest to your eye.


Field of View
This important term describes the actual width of your scope's sight picture at a specific distance.  Field-of-view is a function of magnification and the focal lengths of the objective and eyepiece lenses. But remember this: The higher the magnification, the narrower the field-of-view. Field of view is determined by the ocular lenses in the eye piece.  Different factories and brands will vary in design and stats.  Decreasing the eye relief in a scope will widen the field of view.  So its a trade off one way or the other.


Exit Pupil
The size of the column of light that leaves the eyepiece of a scope (usually measured in millimeters).  The larger the exit pupil, the brighter the image.  To determine the size of the exit pupil, divide the objective lens diameter by the power of the scope.  IE; a 4x32 scope would have a 8mm exit pupil.  32/4=8.


Twilight Performance 
Using the following formulas gives a basic evaluation of low light performance; however, one must keep in mind that they are mathematical formulas and do not take into effect some of the most critical features in optics: glass quality, number of lenses, precision of manufacturing and coatings.


Eye Relief 
This describes the distance between your shooting eye and eyepiece lens. It's an important safety consideration. Because if the eye relief is too short, there's an increased risk of dangerous contact between you and your scope under recoil.  Eye relief is determined by the field-of-view, and by the focal lengths of the objective lens and eyepiece lens. Generally, the higher the magnification and the larger the field-of-view, the shorter the eye relief.  Some scopes offer a soft neoprene eyepiece guard in case you get too close.  The Swarovski Professional Hunter line of scopes all have a recoiling eyepiece in addition to the soft neoprene rubber guard for the ultimate in "scope eye" protection.


Parallax is essentially an optical illusion.  Parallax presents itself as the apparent movement of the reticle, in relation to the target, when your eye moves off center of the sight picture (exit pupil) or in more extreme cases it appears as an out of focus image. It indicates that the scope is either out of focus or more specifically the image of the target is not occurring on the same focal plane as the reticle. Maximum parallax occurs when your eye is at the very edge of the sight picture (exit pupil). Even when parallax is adjusted for a designated distance, there is an inadvertent error at other distances.  Most brands of scopes that do not have a parallax adjustment are pre-set at the factory to be parallax free at or around 100 yards; rim fire and shotgun scopes are set at or around 50 yards.  Most scopes of 11x or more have a parallax adjustment because parallax worsens at higher magnifications.  Generally speaking parallax adjustment is not required for hunting situations and is primarily a feature used and desired by target shooters.  A 4x hunting scope focused for 150 yards has a maximum error of only 8/10ths of an inch at 500 yards.  At short distances, the parallax effect does not affect accuracy. Using the same 4x scope at 100 yards, the maximum error is less than 2/10ths of an inch. It is also good to remember that, as long you are sighting straight through the middle of the scope, or close to it, parallax will have virtually no effect on accuracy in a hunting situation.


Center Tube Diameter (1", 30mm, 26mm) 
The diameter of a scope's center tube (or main tube) impacts the overall strength and durability of the scope. And it obviously determines the size of bases and rings required for mounting. But beyond that, the center tube diameter must be adequate to allow a sufficient range of windage and elevation adjustment.


Stray Light
When light entering the scope reflects off of air-to-glass surfaces, the reflected light eventually exits in the scope in the form of stray light. This unfocused light typically diminishes the image quality of the sight picture.  To limit the detrimental effects of stray light, manufacturers employ proprietary lens coatings. Additionally, all interior surfaces can be anodized in a matte black finish to prevent reflection of the metal. Also, some higher end scopes do not contain any lubricants, such as oil, that may leak inside and reflect light. The net result of these manufacturing techniques create an image that is crisp and true to color.


Lens Coatings
As light strikes an air-to-glass surface, a small

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Posted By: Lewis
Date Posted: April/02/2004 at 22:00
 I would just like to thank you for the article it help me a lot.  I been shooting for a while but did not know much about scopes. Thanks

Posted By: md10/22
Date Posted: April/03/2004 at 08:01
Great addition to the forum!  I listed a link to this from RFC I hope it will draw some participation to this forum.  I think this forum is a great supplement to the RFC forums!

Posted By: dgolfers
Date Posted: April/07/2004 at 20:33

Posted By: jbyrd
Date Posted: April/09/2004 at 18:41

Posted By: Afrikaander
Date Posted: April/21/2004 at 15:12

Most understandable and helpfull


Thanks !!

Posted By: ConradCA
Date Posted: July/02/2004 at 00:38



Everyone who hunts with a scope should read "Optics for the Hunter". It does a great job of explaining this stuff. You should consider carrying it for your customers.


You forgot to include the fact that there is a maximum usable exit pupil. The eye can only open so wide to except light. A young persons eyes can make use of at most 7mm. Middle aged eyes are about 5mm. This means that the effective exit pupil = min( max usable exit pupil,calculated exit pupil).



Posted By: Chris Farris
Date Posted: July/02/2004 at 11:03
You are correct.  The school is in no way a complete lesson yet as it is a work in progress.  It gets more added to it as we get time.  Mr. Barsness' book relies too heavily on the old DEVA test.  If you read the book and just ignore any sentence that has the word DEVA in it then its a wonderful read.  We sell - "Sporting Optics" by Wayne Van Zwoll. 

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Posted By: kwoytek
Date Posted: September/06/2004 at 14:25

Chris, and Fans:


Kudos for your basic treatment of optical characteristics for riflescopes !! Please consider the text addition of typical eye pupil dilation that refers to hunting conditions (dawn to dusk in USA) that is nowhere near the pitch black (cloudy, moonless, starless, rural, and/or darkroom) conditions under which a young eye can open to 7mm. diameter and middle-age eyes can admit a maximum of 5mm. diameter shaft of light.  Therefore, working scopes need only provide an exit pupil size of 4-5mm. diameter under normal conditions, and under 2mm. in bright noonday-sun target shooting instruments, unless a larger pupil is desired for jump-shooting, quick access sight pictures.


The objective clear aperture must also be chosen to avoid diffraction limited resolution constraints, wherein high magnification scopes suffer "empty maginifcation" when squeezed through a smaller than ideal objective lens designed to maintain a low profile on the rifle for stance comfort.


A short treatment of boresighting theories would be of benefit to most marksmen.  Coarse alignment of scope to bore with mount adjustments is invaluable to conserving the internal adjustments for fine-tuning, and wind or range compensations, only.  In this way, the common problem of "running out of adjustment" is minimized, and the mechanical axiom is preserved (that everybody forgets); when decentering any optical components in the path degrades the image quality, and sometimes the adjustments at the end of their travel have "dead" clicks, or may have a windage implication for extreme elevation corrections.


Any questions, please sound off?




Posted By: Chris Farris
Date Posted: September/08/2004 at 12:53



Very good information, thanks for it.  I like to have a larger than necessary exit pupil for the exact reason you cite.


I have heard that as long as you are in the center 20% of the scope's adjustment range you should be OK.   What do consider an acceptable amount for adjustment?  The erector tubes desire to follow a curving path when adjustments are bottomed out can be solved by using a square saddle assembly but you will still have problems if you have adjusted your scope this far as spring tension is not equal on all sides.  What do you know about the advantages of a coil spring vs. a leaf spring to help prevent this problem?

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Posted By: kwoytek
Date Posted: September/08/2004 at 18:01


There is no concrete rule, when it comes to acceptable deviation from the optical axis, since every optical design is different, even when the power ranges are identical in two different models.  The amount of aberrations in each design dictates how much off-axis image can be used, and the rule of thumb is to minimize the offset in every way possible, then live with the consequences.  In some scopes the astigmatism, color, Petzval curvature (cup-shaped image plane,) distortion, etc. will be apparent at 20% of full travel; in others (costly) there may be no noticeable degradation.

Adjustment springs are best when not compressed enough to take a permanent set (plastic deformation.)  Some beryllium copper and exotic alloy leaf springs are designed well enough to maintain elasticity, but it's generally easier and cheaper to find coil springs that will not go solid under full compression, expecially if they have a conical helix that collapses completely into itself.

Hope that helps.


Posted By: David Wood
Date Posted: October/18/2005 at 07:48

 Sorry for being new to your forum I wish I had known about it before I spent money unwisely. I would like to make a request of you that may not be doable but here goes.With all the scope choices Chinese, Japanese, European, USA, could you make a general comparison of manufacturing techniques and differences and what well known MFR's contract out scopes to other countries.The reason I am so perplexed is, I have:

 2 BSA 36X (Japan) excellent but ordered  2 more same but China (junk)

 2 BSA 6X24 (China)  OK

 3 Redfield 2ea 3X9 1ea 4X12  OK to pretty good

 2 Leupold 1ea VX2 and 1ea VX3   that I consider Gold standard


This may be a Honda, Volkswagen, Cheverolet,comparison but you can buy any of these and expect to get 100,000 miles from each. Thanks David Wood

David Wood

Posted By: catt_tracker
Date Posted: January/16/2006 at 19:19
Thank you that helped clear things up.

Posted By: giuseppe
Date Posted: May/20/2006 at 06:06

Dear Chris,

Can I have your permission to translate this interesting article in the Italian Language and to put this article on two italian airgun forums?

Thare are many italian airgun enthusiastics that cannot read the english language.

Obviously I will write your name and the link to your forum on the article. And I will send you the link in the italian forums.

Thank you

Giuseppe Stama

From Italy


P.S.: if you give me your permission, can you send me the pictures that I cannot see on this article? (magnification and reticles)

Thanks again

Posted By: Chris Farris
Date Posted: May/26/2006 at 14:42

Sure, use anything you want on O.T.  I'll try to find those images and get them restored.



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Posted By: medic52
Date Posted: October/20/2006 at 14:09
EXCEPTIONAL writing, Thank you for the time and effort,  as being new to this board I have been brought from dark to light concerning scopes. It has help so much that I have purchased two new scopes from the recommendations and articles on this board.

"The true soldier fights not because he hates what is in front of him, but because he loves what is behind him." G.K. Chesterton

Posted By: William Ronald
Date Posted: October/25/2006 at 21:05
I gotta say that I am very happy to have found this site, after reading the post I have a feeling I will learn alot about scopes and exactly how to use them correctly! Thanks for taking the time to post this!

Posted By: sebastian
Date Posted: May/03/2007 at 10:19


Just new to this forum, and find this thread very informative. ...Thanks so much for the poster for the great informations!


To understand more about the Leupold competition series, do you have a sectional drawing/photo just like the pic in the first page?


Many thanks in advance,

sebastian in Indonesia.




Posted By: Chris Farris
Date Posted: May/03/2007 at 11:04
Sorry we do not have any cut away type pictures of the Leupold.

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Posted By: RifleDude
Date Posted: May/03/2007 at 19:46
Originally posted by sebastian sebastian wrote:


Just new to this forum, and find this thread very informative. ...Thanks so much for the poster for the great informations!


To understand more about the Leupold competition series, do you have a sectional drawing/photo just like the pic in the first page?


Many thanks in advance,

sebastian in Indonesia.


Hi, Sebastian,  welcome to OT!  Based on the fact you live in Indonesia and the fact you're inquiring about a Leupold comp scope, indicating you're probably a benchrest shooter, you wouldn't happen to be the same Sebastian who manufactures the excellent SEB coaxial front rest would you?  If so, your rest is very nicely designed and built, my friend!  If I didn't already have a good front rest I'm pleased with, I would have bought yours.


For those who are interested in a great coaxial (joystick) style front rifle rest for benchrest shooting, you owe it to yourself to check out the SEB rest: -



Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle.

Posted By: sebastian
Date Posted: May/03/2007 at 23:51

Mr. Farris, thank you for your response.


RifleDude, thanks for the plug. Yes, I'm seb.


Posted By: RifleDude
Date Posted: May/04/2007 at 08:08
Originally posted by sebastian sebastian wrote:

Mr. Farris, thank you for your response.


RifleDude, thanks for the plug. Yes, I'm seb.



No problem; I am happy to pass along information about excellent products!


Again, welcome!


Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle.

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