Sharpening up sight picture

[BobGee]

I have a question related to sighting that I hope someone can answer. Might be one for Art Neergard.

At the range this morning shooting 50m (free) pistol, I found (as I have before) that I was unable to see my rear sight notch properly. My pistol is a Hammerli FP60 with a wide front sight and a fairly wide rear notch; I use Knobloch glasses with an adjustable aperture on my shooting eye lens, which is stopped down, and an occluder over the other eye. The lens was prescribed by an optometrist and has astigmatism correction. I have a mild cataract.

Looking at my sights both before and after settling in the target sub-six area, the right-hand edge of the notch was indistinct which I assumed was something to do with refraction though why it was only on one side I’m not sure unless it was to do with the prevailing light direction. The range points north by north-east and here, in the southern hemisphere, at this time of the year (spring equinox) the sun rises in the east, moves towards the north and sets in the west. Thus in the morning, when we were shooting, the sun was above and behind our backstop shining towards the firing line putting the targets in shade. Increasing or decreasing the aperture had no effect. I don’t have this problem with my air pistol (FWB P44) on our indoor range which is lit with a mixture of natural and artificial light.

However, this morning, I discovered that if I looked at the sights through the hazy fringe at the edge of my shooting glasses aperture, very close to the black edge, I got a sharp image. I mentioned this to my shooting companion and he said that he’d discovered this too and has been doing it for a long while now. I thought it might have been due to the nature of the rear sight notch but as I have a flat sided notch and my colleague a bevelled edge notch (Morini CM84E), that does not seem to be the cause. Neither of us can figure out why this happens and why the solution is what it is; we’re both engineers.

What’s going on? I really cannot figure the optics. Any thoughts anyone?

Bob

[Rover]

The purpose of the diaphragm is to maintain a uniform "pupil", not to adjust "depth of field" (as many think).

While you may have a good "prescription", the lab making your lens may not be giving you what was prescribed (hey, that's good enough!).

Also, your scrip may not be bringing your focus close enough.

Good luck messing with this stuff!

[Brian Girling]

"The range points north by north-east and here, in the southern hemisphere, at this time of the year (spring equinox) the sun rises in the east, moves towards the north and sets in the west. Thus in the morning, when we were shooting, the sun was above and behind our backstop shining towards the firing line putting the targets in shade."

Here in the northern hemisphere our ranges face mainly north i.e. the sun (when it shines) is behind the firing point. I had assumed that in the southern hemisphere that would be reversed Having only shot in the southern hemisphere once in New Zealand I do not recall a problem (so the ranges must have faced south) although I do remember the shadows moving in the opposite direction!

[thirdwheel]

Rover has made a couple of good points, lenses are often out a tad from the lab and that is why Don Nygord gave up and set himself up to check all lenses with a Lensometer, I've had four years of not getting the front sight to look as I wished and had several shooting lenses made but none of the opticians said they need me to take my pistol in as they could judge what I needed mathematically. Having discovered Dons take on the vision thing I went down his road too obtained a complete set of opticians trial lenses, test frames and a lensometer to measure made up lenses, none of my shooting lenses were spot on to the different prescriptions. What I now do is use a set of lightweight trial lens holders and the trial lenses instead of having one lens made up, in the frame I have a lens to correct my astigmatism (cyl) and another to bring in the front sight (sph). I also found the normal + 0.75 on the distance correction that the opticians have used does not really do it for me and have found +1.0 for me is good. I now do not have to get new lenses made up as my eyes change i just swap the lenses and I keep the next powerful lens in my shooting kit ready if my sight shifts during a match or if the light is too bright and my pupils close up too much and bring the target into focus. Gave up using the variable iris a very long time ago.

Widen up the rear sight a bit to see if that helps as you probably know light can be bent through a narrow slit - remember your physics from school.

On a fun note the lightweight trial lens holder I use as my shooting frame is puce lime green and glows in the dark, really daft!

[d-cuttler]

Try this.
Unscrew the clamp in the lens holder a little, and rotate the lens a little in both directions. It may help you.

Here is the reason. When you have your eyes examined generally they have you rest your chin on a pad in a machine while changing lenses etc. Finally everything comes together and is sharp, and they write the perscription. Here is where it can go bad. If your head is not perfectly vertical when they are dialing everything in, when you get your glasses made, they will be off by the angle of your head in the examining machine.

I found this out the hard way, because I have so much arthritis and disk degeneration, my head was never vertical, and all of my glasses would come out wrong. It took a long time to figure it out. The good thing is that unlike regular glasses where the lenses cannot be rotated your shooting lenses are round.

I now keep a record of all the numbers in my perscription with me when I have my eyes examined to make sure I don't come out with a bad perscription.

This may not be your problem, but it only takes a few minutes to find out.

[BobGee]

Thanks gents. I’ll try d-cuttler’s potential solution. I’d have never thought of that one. I’m aware that some of the optical labs can be a bit slap-dash and so it might be that Rover's explanation is correct. I plan to get my lenses checked by another optometrist so we’ll see.

However, I still am puzzled by the interim solution I and my colleague found.

Bob

[thirdwheel]

Getting that lens correct and in it's correct position is so important this is a precision sport after all, most shooting frame makers make a little gizmo to clip onto the lens to enable you to orientate the lens correctly when in the firing position. Another problem is getting the lens the correct distance from the eye as many people have it way too far from the eye and this kicks out the focal point onto the back of the eye, compare your normal glasses to your shooting lens, your optician is presuming you will have the lens the normal distance (10 -12 mm ?) from the eye and makes no allowance. I did this adjustment with one of my shooting pals a couple of weeks ago and now his sights are correct. Olympic frames used to sell clip on lenses with +/- diopter strength to correct exactly what you are suffering from. Next thing would to be to try is filters but being an indoor shooter have never needed to play with them so no comment.
My guess why using the edge of the iris is working is that you are shading a lot of light from entering the eye thus the pupil is not stopping down and changing your depth of focus. I shot recently on a new generation of electronic target at a comp that had integral led lighting that was pure white and bright and experienced exactly the problem you have. I reproduced the scenario at home using very bright light onto a white sheet of paper and corrected it by adding +0.25 Diopter power to my shooing lens, that lens now travels with me to all open competitions. I would come over with my lenses to let you try some stuff out but your are a bit far for my buss pass!

[BobGee]

Thanks thirdwheel; a very helpful post. I haven't seen the lens orientator gizmo unless you mean one of these cone shaped devices with the ring of holes - I have one of those and so my lens is orientated correctly for my normal head position.

WRT the vertex distance (lens to retina), I saw on a previous post that normal variation between reading and shooting glasses only makes a 5% difference at most. However, with the inherent approximation in one's distance prescription, the inherent variance in the actual shooting lens diopter value and now the vertex influence, it's hardly surprising that getting the front sight into focus is a bit of a struggle.

Your comment about "shading a lot of light from entering the eye" is probably true - we do have strong light down here. I already have a couple of filters and so I can try one of them (yellow or amethyst/grey). However, I'd have thought that too much light could be allowed for by stopping down my aperture further. Unfortunately, I have a limit with that because I have a developing cataract and as I stop down, the cataract starts to obscure the view (like a splodge of black ink). I found out about this effect from cataracts when I got a set of peep sights for my Anschutz .22LR target rifle (64 MPR) a while back. I was convinced that there was some fluff in the (fixed) aperture. I tried blowing it out to no avail. I asked a nearby range officer if he could see it: he laughed (good naturedly) and said that it was not fluff but caused by having a cataract(I am 71). I'm resisting a cataract op as the thought of fiddling with my eyes gives me the heebie-jeebies.

So thanks for all the responses. I now have a few things to think about and to try.

Bob

[thirdwheel]

You have it in one, the cone shaped thingy is the thing to get the lens at the right plane and central to your eye. Think how little you move the focus on a pair of binoculars to get it in focus correct for you and you will be amazed how much the distance the eye is from the lens makes, some shooting glasses do not have the ability to get the lens closer without resorting to bending stuff. The more expensive types usually have a cranked adaptor available and that can give you endless fun with adjustments and placing the lens where you want it. Try this, get hold of a +0.25 and +0.50 test lens from the internet, take the iris off your lens as you want the largest width of lens in your eye to collect light and focus it and sit down at the range with your pistol and rest it on something so you are not ending up fighting gravity and get your pal to first place the +0.25 and then the +0.50 lens right up close to your shooting lens and see what the sight picture is like, wild guess now if your original lens is correct : - the +0.25 will be better and the +0.50 will sharpen up the rear sights and fuzz the front sight. If you must use an iris keep the hole big and use it mainly as an centralising your vision in the centre of the lens and not for stopping down light transmission. Then mess about with all of the filters you can get your mitts on (including polarised) to see it what improves your picture the best as you are looking for the best work around for your slight cataract and the light "problem".
On the heebi - geebies bit, my dad has to be worse than you on this but says the cataract replacement was fine, he is a big tough Ex Docker that faints when having normal injections.