eye relief margin

[Natty Bumpo]

Eye relief for scopes is generally just given as a single, static figure, such as 3.5 inches. In reality, there is a window around 3.5 inches where you'll be able to get the full field of view through the scope; say, from 3.3 - 3.7 inches, for example.

My question is, why are some scopes more forgiving in this respect than others? With variable power scopes, as you crank up the power does this window shrink? Does objective size, tube diameter, number of lenses, or any other easily quantifiable parameter affect this?

 

[LouBoyd]

Do a little experiment. Aim your riflescope at a target at normal distances ( 50 to 500 yards with the rifle held so the crosshairs are centered on a small target. Now move your eye from very close to the eyepiece to around twice the normal eye relief from the eyepeice. You'll see that tthe ON AXIS image changes very little. All that changes is that the field of view shrinks on either side of the "optimum" eye relief.
Another experiment. Bring your scope indoors and aim it out a window into bright sky outddoors. Put a white card at the location where you would put your eye for "normal" eye relef. You should see a bright spot on the card. Move the card a little closer or further from the eyepiece and at some distance the spot should be uniformly illuminated with sharp edges. That distance from the card to eyepiece is the optimum eye relief for that scope setting. If you have a variable power scope cnage it from minimum magnification to maximum magnification. The spot will be largest at low magnification. For any scope the diameter of the spot on the card will be equal to the entrance pupil diameter (usually the clear aperture of the objective lens) divided by the magnification. The only light from the scope which goes into your eye is in that spot.. If the spot is large than the iris of your eye some light will be lost. If you move the card closer to the eyepiece or further away you'll see that the spot gets bigger. Obviously if you have your eye in a place where your iris is smaller than the spot, you loose some light. To understand what information is in what parts of the lgiht cone you need to see a ray tracing of the light going between the eyepeice and the eye. not all locations contain the same information.. However, on either side fo the optimum eye relief the outside edges of the cone only contain light which entered the scope off axis. That's why you get the tunnel effect if you're outside of the "optimum eye relief range.

The design of the eyepiece determines the eye relief. There is always a tradeoff of apparent field of view vs eye relief for a given eyepiece diameter. You can get longer eye relief or wider field of view (or some of both) with a physically larger eyepiece.

Eye relief and eye relief tolerance have little to do with the quality of the optics. It's just a natural result of any eyepiece design You get the same reduced field effect if you move a binocular or spotting scope eyepiece away from your eye..

You can however use any riflescope beyond it's "normal" eye relief. It just gives a tunnel effect, but it doesn't affect aiming accuracy or image brightness. It can even help accuracy as any lateral offset of your eye will be obvious. Keeping your eye centered will eliminate parallax error if the reticle is a little out of focus relative to the target.

 

[Chas1]

LouBoyd, now that's putting a subject that's not easy to understand let alone write about into an easy to understand explanation. Well written, very informative and I learned something. Thank you.

 

[LouBoyd]

Re: eye relief margin (more for night vision scopes)

My previous explanation of eye relief behavior does not apply to night vision scopes which use image intensifiers or CRT or LCD screens. On those there is no tunnel effect with eye relief other than seeing the full area of the display with the eye close and at long eye relief only seeing a portion of the display though the eyepiece unless the eyepiece is huge. This is because the light exiting the eyepiece has no angular relationship to the hight which entered the objective. Night vision scopes have no distinct exit pupil. The light exiting the eyepiece does not converge to a spot as with a conventional scope and instead just expands. Night vision scopes always waste a lot of light from their output screen. That's why night vision scopes need a flexible eye shield to keep from being visible by someone else with a night vision scope or even a conventional scope.

The lack of angular information also the reason that any night vision scope with a front projection reticle (or a software generated reticle in a video scope) may have fuzzy focus if the objective is out of focus but it cannot have any parallax error resulting from eye position offset as an optical scope does. Night vision scopes with a separate reticle after the intensifier (or video screen) can have parallax error, but if it does it can't be removed by adjusting the objective lens focus or the eyepiece focus. Some NV scopes have a third (internal) reticle focus. (AN/PVS-12 for example). Some import NV scopes have a reticle affixed to the glass of the phosphor screen which introduces parallax error which cannot be removed.

 

[NJS]

The new Leica ER riflescopes have 4" of eye relief through the entire power range - fantastic for magnum calibers. NJS

 

[Chas1]

NJS, have you used the Leica scopes and how do you like them and how do they perform?

 

[Natty Bumpo]

The new Leica ER riflescopes have 4" of eye relief through the entire power range

My post isn't about the distance of the eye relief; whether it's 3.5 or 4". It's about the "window" of eye relief. How close does your eye have to be to 4" from the objective to get the full image. Is it a narrow margin, or a wide margin? Wide is better.

Personally, I cringe when I see 4" of eye relief on a scope. I don't shoot big magnums and am not too concerned about recoil and getting a cut above my eye. But I like to shoot with a sling and when I get prone with the rifle in tight, I can't see the the full image with if there's 4" of eye relief (I can only bend my neck back so far).

I've started putting extended scope bases on my rifles so I can get the scopes further forward to compensate for this.

There's a lot of talk about light transmission, clarity, sharpness, and how many MOA of adjustment is built into the scope. Nobody talks about how forgiving the eye relief is. It seems to me to be an important but overlooked issue.

 

[Chas1]

You raise a fair point anbout how forgiving eye relief is...I too like fuller image and do concern myself with that. Make no doubt about it, that while it may not be discussed as much most of these folks are well aware of it and could probably write volumes on it. But, it is a good point especially for folks like myself who still have alot to learn about optics...well for that matter most everything else too.

 

[LouBoyd]

You raise a fair point anbout how forgiving eye relief is...I too like fuller image and do concern myself with that. Make no doubt about it, that while it may not be discussed as much most of these folks are well aware of it and could probably write volumes on it. But, it is a good point especially for folks like myself who still have alot to learn about optics...well for that matter most everything else too.

One way around the eye relief problem is to mount the scope on a cross roller slide. When the gun recoils the scope does not. The mount require a light spring and damper to return it to battery and a dust shield. http://www.techno-isel.com/lmc/Products/Images/StandardSlides.jpg

The problem is that there are no commercial rifle scopes with short enough eye relief to make it worth the bother.

 

[Chas1]

Cross roller slide. Thanks. What will they think of next.

 

[NJS]

NJS, have you used the Leica scopes and how do you like them and how do they perform?

Mrs. Santa was good to me, gave me the ok to use the ole Master Card and order the 2.5-10... I'll let you know how it performs when it arrives after the first of the year. :>) NJS

P.S. I think she just wanted to shut me up I have been telling her about them for a while now... <wink>