Relation between eye relief and rear aperture size

[Humpers]

Hello all, first post on this excellent forum. I've only been doing smallbore prone target shooting for a few months, and am trying to learn as much as possible in order to improve my scores, so please excuse a simple question.

At the moment I have a 1.2mm fixed iris on my Anschutz rear sight, and have been experimenting by altering the rear sight position on the mounting rail to adjust the apparent (to the eye) aperture size. Obviously moving the sight also affects eye relief. Part of the reason for this experimentation is to try different amounts of "space" in the sight picture between the outside of the foresight tunnel and the inside edge of the rear aperture.

Is moving the rear sight forward, to make the rear aperture appear smaller to the eye, different in any way compared to just fitting a smaller aperture (with the sight closer to your eye)? I realise that moving the sight to change the apparent size affects the amount of light coming through, and have used this to cope with changes in light conditions. But does it have any other effects because of, for example, the difference in eye relief, particularly on the focus of the front sight or target?

At some point I'll buy a variable rear iris, but pending that (and potentially after that) I'd like to understand the implications of moving the rear sight and thereby altering eye relief.

[KennyB]

Is moving the rear sight forward, to make the rear aperture appear smaller to the eye, different in any way compared to just fitting a smaller aperture (with the sight closer to your eye)? I realise that moving the sight to change the apparent size affects the amount of light coming through, and have used this to cope with changes in light conditions. But does it have any other effects because of, for example, the difference in eye relief, particularly on the focus of the front sight or target?

I'm going to say that changing the apparent size of the peephole by altering eye relief is different from just opening up the iris (which doesn't, in my experience, alter the apparent size of the peephole).

What I subscribe to at the moment is: altering eye relief to get a good compromise between ease of centering the foresight in the peephole (further) and not making the "sweet spot" in the centre of the peephole too small so the foresight ring is sharp and round (nearer).
Opening the rear iris will let more light in, make the "sweet spot" larger but at the expense of depth of field - so the sight picture is a bit less sharp. If you have the right lens this is less of an issue (do you use a lens?).
Somewhere in between will be the optimum for your eyes under those lighting conditions.

Apparent size of the rear peephole is also dependent on lighting i.e. how dilated your pupil is. You may experience this outdoors when the light level goes from very bright to very dull or vice versa.

K.

[Humpers]

Thanks Kenny, really helpful.

When you talk about the sweet spot, am I right in thinking this is where gradually closing down the rear iris you get to a point where the centre of the aperture starts to turn black, and that the sweet spot is just before that happens? I've heard about it but haven't been able to experiment as I haven't got an adjustable iris (yet!).

I don't use any lens at the moment, other than my usual glasses (or are you refering to a diopter in the rear sight?). Actually I use a spare pair of glasses which I changed the legs on so that I can wear them upside down! It looks strange, but it brings the lenses of the glasses up higher towards my eyebrows, and tilts the lenses back slightly so that the right lens is more perpendicular to and closer to the centre of my line of sight when shooting prone. Definitely better than my usual glasses. I suspect that if my enthusiasm for target shooting continues as it is at the moment then I'll get some shooting glasses from the chap at Bisley sometime.

Just remembered, the only lens I've tried is a 0.3 eagle eye which was fitted to the front sight when I bought the rifle. But I've recently taken that off because I've been trying different front aperture sizes (I've got an adjustable front iris), and I found the eagle eye seemed to make the target too large on anything but the largest setting of the front iris (4.2mm). Basically I've just been experimenting to see what works best.

[Tim S]

Humpers,

Ken meant a lens worn as glasses, or attached to the rearsight, one that was prescribed specifically for shooting to take into account the correct focal distance slight ahead of the foresight, as well as your normal vision correction. With the right lens having perfect depth of focus through eye relief isn't quite so critical, since the eye is focused at the right point.

An eagle eye is just a simple magnifying lens. It makes the target look larger, which can make it easier to see if it's normally fuzzier than you would like, but it won't change the focus.

[KennyB]

Hi Humpers, I was wondering if I was going to be asked what I mean by "sweet spot"...

What I suggest you try is : while aiming, move your head around very slightly so you are looking through different bits of the peephole - don't worry about whether the foresight is central in the peep, just look for the clearest, sharpest image of the ring and aiming mark. THAT is the sweet spot and it might or might not be central. Now as you open up the rear iris, it is my experience that the "sweet spot" becomes larger or easier to locate.

The reason I believe that this happens (for me) is that the lens in my eye is old and full of defects. In everyday use these defects are not visible because the pupil is sufficiently large that light passes through a relatively large area of lens and the effects of the flaws are averaged out.

When you stick a 1.1mm peephole in the way, the area of lens in my eye that light passes through is drastically reduced and the effects of these flaws in my eye can produce optical distortions that degrade the sight picture. Moving your head around, you are looking for a bit of lens in your eye with least flaws to give you the best sight picture.
Opening the iris up increases the area of lens in your eye that light passes through - reducing the distortion but also reducing the depth of field making it more difficult to get the foresight sharp and target sharpish.

Adding a lens of +0.5 diopter on top of your distance prescription should give you back that depth of field - if you search this forum for "hyperfocal distance" you should find more info on this. This prescription lens would usually be worn in a frame (shooting glasses) or attached to the rifle somehow.

I hope that's not too much to take in...

Cheers,
Ken.

[Cumbrian]

[quote="KennyB"]Hi Humpers, I was wondering if I was going to be asked what I mean by "sweet spot"...

What I suggest you try is : while aiming, move your head around very slightly so you are looking through different bits of the peephole - don't worry about whether the foresight is central in the peep, just look for the clearest, sharpest image of the ring and aiming mark. THAT is the sweet spot and it might or might not be central. Now as you open up the rear iris, it is my experience that the "sweet spot" becomes larger or easier to locate.

Ken,

Forgive me for barging in on your very interesting discussion, but, can you please tell me whether you actually aim and shoot on the basis of the 'sweet spot', even though it may not be central in the iris? I can imagine that this might well work as I have sometimes, in an 'oh bother the whole thing anyway' type mood, taken a shot knowing that the target was slightly off centre and found that I still hit a '10', but it would be good to have confirmation of your practice. Thank you.

Roger

[KennyB]

Roger, I have been doing this as part of my process.

I started after Ross Mason pointed me towards this article on parallax suppression : http://dougkerr.net/Pumpkin/articles/Aperture_Sight.pdf
The images on page 12 are quite thought provoking.

If it all goes wrong, don't blame me though....

K.

[Humpers]

Hi Kenny,

Another great explanation, thanks, and still with you so far.

Just getting ready to go down the range, so I'll read your link later. Have you seen there's also a follow-up article - http://dougkerr.net/Pumpkin/articles/Ap ... t_Demo.pdf (assuming as a new boy my link doesn't get stripped out).

[Wynne G Oldman]

Roger, I have been doing this as part of my process.

I started after Ross Mason pointed me towards this article on parallax suppression : http://dougkerr.net/Pumpkin/articles/Aperture_Sight.pdf
The images on page 12 are quite thought provoking.

If it all goes wrong, don't blame me though....

K.

Very interesting. Thank you for posting that link.

[Humpers]

What I suggest you try is : while aiming, move your head around very slightly so you are looking through different bits of the peephole - don't worry about whether the foresight is central in the peep, just look for the clearest, sharpest image of the ring and aiming mark. THAT is the sweet spot and it might or might not be central. Now as you open up the rear iris, it is my experience that the "sweet spot" becomes larger or easier to locate.

Hi Ken,

I tried what you suggested last night, and for me the sharpest image was when everything was central. This was the same if I looked through the peephole off the rifle, just looking at something at 50 yards. I guess my eye is different to yours. But I will try it again on Sunday morning as the light wasn't brilliant last night.

I also tried moving the rear sight back slightly to reduce the eye relief for some cards, and the target seemed more in focus which was good. But I wasn't shooting brilliantly (I just couldn't seem to get the position of my right elbow right) so it's difficult to judge if the results on the cards were better or worse. But I'll keep trying.

Regards,

Humpers

[Cumbrian]

Ken,

Thank you very much for that reference. Looks very interesting - if I can understand it all. Most grateful.

Roger

[redschietti]

I think a simple rear aperture is the first thing you should buy after you buy a gun.

[Humpers]

I think a simple rear aperture is the first thing you should buy after you buy a gun.

Hi redschietti,

What do you mean by "a simple rear aperture"? I thought that's what I've got, i.e. a fixed 1.2mm aperture.

I was thinking that after I've developed a good, consistent position, etc and have improved my scores a bit more (I'm currently getting 92 on average) I'd buy a less simple one, i.e. with adjustable iris and probably coloured filters and maybe twin polarisers too. At the moment I'm not certain that I'd benefit from all those extra goodies. As a relatively new person to the sport there are so many other variables in my shooting which are limiting my scores. Is that a sensible approach or were you thinking of something different?

[Tim S]

I think it's a typo for "a simple adjustable rear aperture".

I agree that concentrating on building a steadier position is sensible, but having a fancy iris unit won't hurt you. If you shoot primarily indoors you won't use polarisers, nor many colour filters (although yellow-green can improve contrast under fluorescent light), but the adjustable aperture is useful.

The only caveat is that some fancy irises can get quite long, so the rearsight must be placed further forwards to compensate. You often see the older Anschutz rearsights with the clamp cut short so it doesn't obstruct the breech.

[Humpers]

I think it's a typo for "a simple adjustable rear aperture".

I agree that concentrating on building a steadier position is sensible, but having a fancy iris unit won't hurt you. If you shoot primarily indoors you won't use polarisers, nor many colour filters (although yellow-green can improve contrast under fluorescent light), but the adjustable aperture is useful.

The only caveat is that some fancy irises can get quite long, so the rearsight must be placed further forwards to compensate. You often see the older Anschutz rearsights with the clamp cut short so it doesn't obstruct the breech.

Thanks Tim.

I currently just shoot outdoors (from an indoor firing point), where the glare can get quite bright when the sun is on the target, but also where we need to use floodlights when the evenings get dark. So quite variable light conditions. But I may also shoot indoors in the future, so want to be prepared for most eventualities, hence my thoughts about coloured and polarising filters.

I suppose one reason I'm not rushing out to buy an all singing/dancing iris immediately is that I feel I ought to serve my "apprenticeship" on the more basic setup I've already got. I'm then hoping that when I get a fancier iris it will help my scores to go up further! At least I can hope.

Off to the range again this morning, so another opportunity to try things out.

[Tim S]

I see. With relatively new UK shooters, I usually assume indoor ranges. Is this an open 25 yard range or 50m/100yds?

Whatever the distance, with an outdoor range an adjustable iris and bits will be more useful than on a totally indoor range. I'm going to disappoint you and say a fancy iris won't give you a 99 average. It won't compensate for not having a good steady position, good trigger technique, mental concentration (and good wind reading skills), but it does help.

If you are still using a club rifle, I would wait until you get your own. While almost all German sights have a common eyepiece thread, you may buy a rifle with an iris and save yourself some cash.

[ShootingSight]

Eye relief will have almost no impact on focus for a fixed aperture. The definition of 'entrance pupil' in an optical system (in this case, your eye) is the projection of the smallest aperture onto the first lens surface (your cornea). So when considering the front sight, take an example of a 30" distance from the front sight to your eye. With 1" eye relief (sight at 29" from the front sight), the projected aperture would be 30/29 or 103% of the actual aperture. With 2" eye relief, the entrance pupil would be 30/28 or 107% of the actual aperture. So in theory, closer to your eye is better, but in practice, if you have a 1.2mm fixed aperture, one solution would give you a 1.24mm entrance pupil, the other a 1.28mm entrance pupil, so almost no difference. Since the distance to the target is much greater, the difference would be even smaller.

So, in regard to focus, eye relief makes no difference.

However, in regard to field of view, it makes a much bigger difference. You usually want to make sure you have enough field of view that you can see the outer ring of your front sight, and likely also the target numbers. Any small focus benefit is easily offset by the negative impact on your score from firing on the wrong target.

Focus wise, your optimum focal point is at the hyperfocal distance for your sight. This is minimally affected by target distance, but sight distance has a strong impact. Typically, for a rifle the math solution is that you want +0.50 diopters added to any distance correction your eye needs (or just a +0.50 diopter lens if you do not need distance correction). This will shift your eye's relaxed focal point to a distance 2 meters from you, and this represents the optical average between focusing on the sights and focusing on the target. Thereby, your focus is centered between the two, allowing you to get the best use of your depth of field. In the interest of full transparency, I sell both lenses for shooters, as well as a rear sight adapter to hold the lens in the rear sight, though this math applies to my lenses or to eyeglass lenses you can source anywhere.

For pistol, the optimal math is usually a +0.75 diopter added to your distance correction.

All lenses will shift focus and will magnify or shrink, all at the same time. An eyeglass lens is no different from a magnifying lens, other than in power. However, the math equation for how much is driven both by lens power, and by the distance of the lens from your eye. In most cases, where the distance to your eye is small, and the lens power is under about 3 diopters, the amount of magnification is so small that it is lost in the rounding, and is ignored. However when you take a low power lens, like the 0.3 Adlerauge, and put it at a great distance by placing it in the front sight, the magnification becomes apparent. With these front sight lenses, you get a magnification of the target, though you also get a blurring of the target. Many shooters find that the benefit of magnification, especially as it makes the number boards easier to read, is a greater benefit than the negative impact of the slight loss of focus.

As a side note, if you shoot with regular glasses, avoid progressive or no-line bifocals. Since these have a constantly changing focal length, even slight changes of head position, or the position of glasses on your face can impact your vision, and could change your impact point.

Art Neergaard
ShootingSight

[Humpers]

I see. With relatively new UK shooters, I usually assume indoor ranges. Is this an open 25 yard range or 50m/100yds?

Whatever the distance, with an outdoor range an adjustable iris and bits will be more useful than on a totally indoor range. I'm going to disappoint you and say a fancy iris won't give you a 99 average. It won't compensate for not having a good steady position, good trigger technique, mental concentration (and good wind reading skills), but it does help.

If you are still using a club rifle, I would wait until you get your own. While almost all German sights have a common eyepiece thread, you may buy a rifle with an iris and save yourself some cash.

Hi Tim,

We do have a 25yd indoor range at the club, but I usually shoot on the outdoor range (albeit with indoor firing points), which has a choice of 25yd, 50yd, 50m and 100yd. At the moment I'm usually shooting 50yd as I'm doing the Polar Bear this summer. And it's my own rifle rather than a club one.

Don't worry, I'm not expecting 99 average with a fancy iris; 98 will do for now (only joking).

[Humpers]

Eye relief will have almost no impact on focus for a fixed aperture....

Thanks Art for taking the trouble to answer my question about focus, and a whole lot more. That's encouraged me to continue experimenting with the position of my current fixed aperture until I get an adjustable one, to see how much space around the outside of the foresight tunnel I prefer.

After Ken mentioned hyperfocal distance earlier in this thread I read some of the other enlightening posts you've written on the forum. Although I usually wear my funny inverted (non-bifocal) glasses for shooting as mentioned above, I wear contact lenses instead when shooting in the field as they are +0.5 diopter compared to my glasses. So I'll try those to see if it improves focus on the target and front sight.

[dlinden]

As KennyB posts that there is probably nothing to worry about in terms of parallax error, what he doesn't relate is that he is an incredibly skilled shooter who has a high degree of consistency in his set-up and process. Although the author of the cited article on parallax suppression dismisses the impact of head position in his discussion, I would disagree because inconsistency in head pressure or angle of pressure will have an impact on recoil/shot placement. I would assume that Kenny is able to maintain very controlled head placement/pressure during each shot and from shot to shot, something we all should strive for. However, I am of the firm belief that many 9's that shooters claim are due to wind changes or bad ammo are in fact really due to changes in head pressure that they are simply unaware of. Keeping everything centered up every time can be a check for this variable.

Dennis L