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Scopes - Eyebox Question

Discussion in 'Anything Airgun Related' started by JoeHeugh, Jan 13, 2021 at 9:14 PM.

  1. JoeHeugh

    JoeHeugh Beware the Springer

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    I've been spending some time checking on the setup of my rifles and started looking at the "eyebox". The effect of objective size is often discussed with regard to exit pupil and the ability of a scope in low light but I've not read anything about how it effects the positioning of the scope. So with a trusty piece of string I did some measurements and found this:

    eyeboxes-mini.jpg

    So at their highest magnification my eye has to be between 2.5 and 3.5 inches from the eyepiece with both scopes otherwise the image will be vignetted. If I dial down to each scopes lowest magnification I get a clear image between 2.5 to 5.5 inches with the 1.5-4 and 2.5 inch to a whopping 7.5 inches with the 3-9!

    Now when hunting my head position changes, close in when prone and further back when sitting but to use the scope effectively at the highest magnification my eye has to fall within that one inch band while maintaining a comfortable hold on the rifle. I then did a quick test on a 3-9x50 and found that it extended the usable zone to 1.5 inches.

    So I think I'm correct in saying that aperture has a role to play in the usable eye relief of a scope and there will be a mathematical relationship between them, i.e. a way of calculating the eyebox. Also I'm concerned that I've been singing the praises of small aperture scopes when they could lead to frustration for a beginner who doesn't know how to set up their scope and the leeway a bit more glass would give might be better for them. I realise if you only ever shoot from the same position this is not an issue but occasionally I see someone post a picture of their pride and joy and wonder whether they are some kind of contortionist!

    So what do you think?
     
    80yarder, Chouchin66 and dodgyeye like this.
  2. dodgyeye

    dodgyeye Donator

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    I'm what could be considered a beginner when it comes to scopes and think your explanation may explain why I felt I was having accuracy issues with a Hawke 3-7 X 32 on my '99 when BR and having now gone big (to big today lol) accuracy has improved :thumb:
     
    JoeHeugh likes this.
  3. Chouchin66

    Chouchin66 Pro Poster

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    Hi Joe, Leupolds , as a primarily big game scope, have a pretty " healthy" eye relief & recommend setting eye relief at highest magnification, so ? I believe their reccomendation is 3.7 - 4 ", don't quote me though...
     
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  4. JoeHeugh

    JoeHeugh Beware the Springer

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    Hi Chouchin

    Yes Leupold advise using the highest magnification but it applies to any scope because at the highest magnification you get the shortest usable zone (green above), reduce the magnification and the zone expands (into the blue). I'm sure you are fine mounting scopes so they fall nicely to your eye but it's probably much less intuitive for a beginner. It just got me thinking that if a 3-9x50 gives an extra half inch leeway over a 3-9x40 it might be something worth considering.

    I'll not be changing my scopes but I did find it awkward getting the position right when I started shooting prone, I needed a bit longer length of pull, I'm 14 1/4 inches but thats another story.

    Dodgyeye, I've enjoyed reading your journey with scopes, hope you enjoy the new one.
     
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  5. Chouchin66

    Chouchin66 Pro Poster

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    I was taught , ( right or wrong), when mounting a scope...close your eyes, bring rifle to shoulder, open your eyes...eye relief should be perfect, if not...wash , rinse, repeat, until satisfactory.

    .
     
  6. robs5230

    robs5230 Oversprung

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    Shooting prone offers other challenges, in that the butt section of a normal rifle is far from ideal in length. Its a similar situation when shooting from a bench though not as exaggerated.
    Body orientation determines where the scope needs to be positioned to get the correct eye relief.

    Basically its best to posiiton the scope in the position where you shoot most of the time.
    Any other position will be some sort of compromise
     
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  7. Spray1Mark

    Spray1Mark OFFICAL PELLET BILLIONAIRE!

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    Although once a big fan of Leupolds and I had many, except for some LPVO AR offerings I would never touch them again!

    The game changer for me was going to my first NF and how non critical the eye relief was vs Leupold, so for me you picked the worse brand as an example LOL!
     
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  8. dodgyeye

    dodgyeye Donator

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    Thanks Joe, looks like only rifle it will fit is the Hatsan PCP (boo, lol) but looking through it, it looks great and only £62 :thumb:. I'll take it out on Saturday and see what it can do at 55 yards.
     
    JoeHeugh likes this.
  9. JoeHeugh

    JoeHeugh Beware the Springer

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    Well I've delved a little deeper into this and it's seems there are no usable calculations and as is so often the case in the shooting world it's all voodoo magic! There is no proper definition of "eyebox" and the eye relief and exit pupil calculations are not accurate in the real world as there are unknowable constraints caused by the build of the scope. So for example a 3-9x40 gives the following exit pupil sizes:


    X9 = 40/9 = 4.4444 mm which is probably about right

    X3 = 40/3 = 13.3333 Nah, no where near, it's going to be somewhat less in the real world.


    Eye relief and exit pupil are important when it comes to head position but because of the unknowns one scope may be more "comfortable" than another even though the specs are the same. Nevertheless objective size does play its part and a larger aperture should give a longer eyebox for the same magnification which may make shooting more comfortable if your gun fit is not ideal, you are using multiple stances or you are not quite sure what you're doing.


    What I'm personally coming away with from this is, if you're going to shoot in all stances including prone, then set the scope up at it's highest magnification in the prone position. If the scope is then too far forward in another stance simply reduce the magnification to get a clear view.


    Robs5230, yes I completely agree. I didn't start shooting prone until I got the rimfire and then general gun fit took on a whole new meaning - now theres a topic! I've cracked it now but it got me thinking…


    Spray1Mark. Budget Leupolds are all I have to go on at the moment. I think when I buy another scope I'll be paying particular attention to "eyebox" and when I can I'll have a look through some higher end optics. (What chance a Northern Shooting Show this year?). Might just have a look at NF, after all when I do get that new slot, I'll need some glass to put on it :)


    Dodgyeye. Let us know how you get on, hopefully with a photo of your setup!
     
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  10. dodgyeye

    dodgyeye Donator

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    Here you go Joe, as you can see it is long! I like the turrets really easy to use. Hopefully will post pics n the field Saturday and also do a review.

    IMG_20210114_161610_427~2.jpg IMG_20210114_161527_243~2.jpg
     
  11. Chouchin66

    Chouchin66 Pro Poster

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    Joe, interesting you mention gun fit, HW , "long stocked" as they are, I being 5'- 7" & short coupled, still have a proper trigger finger placement. I tend though, to have/ use a rather rearward cheek weld...kind of benchresty? I think the fit thing will be as subjective as one's favorite color

    .
     
  12. 80yarder

    80yarder Engaging Member

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    As far as I know there is no general calculation that will determine how 'deep' the eye box is. Although there must be limits, and it might be possible to calculate for any given scope if you had all the data, in practice it depends almost entirely on the design of the scope. You can - and have - calculated the maximum diameter of the 'box', ie the exit pupil, but again, in practice, scope design has a big effect and the usable pupil may be considerably smaller than the theoretical maximum pupil.

    It is true to say that, in general, bigger objectives and lower magnifications tend to give a wider, deeper eye box (eye relief is totally down to design and bears no relation to either) but in the end it's the usual case of try before you buy. And remember that more expensive does not always mean bigger, and bigger is not always better: some top-end scopes aimed at target shooting and/or extreme long range use deliberately reduce the size of the eye box to minimise parallax error and force you to use a consistent head position.

    Alan
     
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  13. BallisticBill

    BallisticBill Too much keyboard, not enough action!

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    I don't think there's a simple way of predicting how much eye relief you will get from a particular scope. Magnification of a twin-lens system (objective and ocular, no magnification) equals focal length of objective / focal length of eyepiece. The factor that gives more depth of field is the diameter of the lens relative to its focal length (f number). Relative diameter of objective and eyepiece will affect how you see the objective through the eyepiece and to what extent the light entering the objective is cut off by the eyepiece, as in the diagram:

    vignetting.png

    You can see, if the objective diameter is reduced with no changes to focal lengths or diameter of the ocular, then the vignetting is reduced.

    Additional elements (reticle and zoom) introduce complications. Internal mountings may restrict the light path causing additional vignetting. The zoom will also modify either the focal length of the objective or ocular (depending on whether FFP or SFP) which changes the light path reaching the eyepiece. As a general principle, widening the angle of the light path must increase vignetting and therefore reduce the range of acceptable eye relief... that's zooming in with SFP. (I'll have to think about FFP because zooming also changes the effective f number of the objective.)
     
    JoeHeugh likes this.
  14. JoeHeugh

    JoeHeugh Beware the Springer

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    Alan and Bill, thanks for your comprehensive replies. I'ts becoming clear there are a few variables and no easy answers, and I'm going to have to have a think on this and brush up on optics. I think I can see whats going on in the chart Bill, I'd made the assumption that a larger objective would make for a better "eyebox" but you say "if the objective diameter is reduced with no changes to focal lengths or diameter of the ocular, then the vignetting is reduced" which is the opposite. Of course my assumption was based on a rough comparison of one 3-9x40 against one 3-9x50 and therefore pretty meaningless.

    Apologies in advance if my replies are slow, a couple of things going on here.
     
  15. BallisticBill

    BallisticBill Too much keyboard, not enough action!

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    An interesting question, nonetheless.
    I have two pairs of scopes that share common elements so I'll try to do some comparisons.
    The scopes are 3-9x40 and 3-9x50 Nikko Panamax... only difference is the objective diameter (I think... need to check).
    And 3-12x44 and 4-16x44 Optisan EVX... the only difference is the focal length of the objective lens (I'm sure)... so lower f number on the objective.
    Don't expect any quick answers... not sure when, but you've got me thinking too! :facepalm:
     

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