Optical puzzle

[Pheyden]

Optical distortion line running through middle of rifle sight picture.

Checked rear sight, but as it has no optics, I cannot see a problem or obstruction
Checked iris, and as it is a plain iris, it is just an opening. no optics. Made sure it was clear, no obstruction.
Checked front sight. has Anschütz pink insert, clean and undamaged
Check my shooting lens rotation orientation. Turned it 180 degrees with no change.

The problem is a focus aberration that runs right through the middle if the sight picture. As I move the sight I can see a change in focus between the top and bottom halves of the sight picture, particularly at the cnter orifice. Plays hell with trying to maintain front sight focus.

I have tried different front sight inserts and three different iris discs, all manifest the same aberration. So I have come to the conclusion that the problem may very well be with my eye and not the front rear sight combination. As a final test I changed shooting eyes (left instead of right) and the problem is gone. Any sports opticians/opthamologists want to share their thoughts?

[TenMetrePeter]

This is easy for me to say as being over 60 I get free eye examinations in my country, but I would book an eye exam pronto and have the eye checked out. Hopefully its a temporary foreign body on the cornea and simple dry-eye drops (artificial tears) may help.

[mtncwru]

Yep, I second that suggestion. Sounds like you've pretty well isolated it to your right eye, so, assuming it's a recurring problem, probably worth paying a visit to your eye doc.

[Pheyden]

Update!

Well, had my exam today. Generally I have reasonable vision, as below:

Right Spher. 0. Cyl. .5. - 120
Left Spher.+1.0 Cyl. 0

However, the right eye has the befinnings of a very small, posterior cataract, which unfortunately is located precisely in the center of the lens. My opthamologists opinion is that as my vision is for the most part very good for general vision, he would be inwilling to perform cataract surgery just to accommodate my shooting hobby. I tend to agree with him.

So what are my options?

1. Ignore the artifact and just learn to deal with it.
2. Shoot right handed, aim with left eye
3. Change everything over for left handed/left eye shooting

Has anyone used options 2 and 3?

Regards Paul

[TenMetrePeter]

if you're still hitting the target I would go with 1. And an annual eye check.

[KennyB]

Bad luck. I sometimes have a feeling something similar is happening with me (eye exam imminent).
I'd go with option 1 but investigate option 2.
A crossover sight or offsets are a lot cheaper and easier than changing your whole setup and relearning your technique.
Not a perfect solution but...

Regards.
K.

[Pheyden]

Had another chat with my eye doctor and formthe moment we are going to try #1. I am having Neal at ISS make a couple if new lenses for both rifle and pistol, and I will see how I go. I have taken the position that it is not the end of the world and good things may result from all this.

[Markrem]

This might seem an obvious explanation but worth putting forward - I experienced something similar at times while I was using my Champion glasses and found that it was the alignment of the lens around the aperture and placing the rim of the lens across the sight picture. Adjusting the alignment was not easy because of the position of the lens being so close to the nose piece. I now use the Ahg lens holder fixed to the rear sight and no further problems.

[Pheyden]

Markrem, thanks for the suggestion. I am already using the MEC Monocle system mounted on my Anschütz rear sight. This assures that I am looking through the centre of the lens. Still, this anomaly was there. I have been playing around with a variable opening size in the iris. When I use a larger iris size (1.2instead of .8mm), I can "tune out" some of the effect due to changing the overall depth of field. I was given this idea by Neal Stepp in the USA, and it seems to work. He also indicated that due to varying light intensities at different ranges, it is vital to use an iris that can be varied in size. Neal is mqking me some new shooting lenses, so we will see how these work with a variable iris.

[TenMetrePeter]

I have noticed in rifle and pistol ( when I experimented with a knoblich iris) that too small an iris aperture will magnify any eye defects. I have no idea why but it happens even if its a stray eyelash. on rifle a 2mm iris makes little difference to accuracy at my level compared to 1.1mm.
0.8 will be severe in this regard.

[Pheyden]

TenMetrePeter - I concur that very small iris apertures can have this effect. I have switched over to an iris with a variable aperture and am playing around with larger sizes. Will see how I go over the next few weeks.

[TenMetrePeter]

Slightly more learned info from an American shooting eye doctor!

https://www.ssusa.org/articles/2017/1/2 ... -aperture/

[ShootingSight]

When you look through your unaided pupil, its diameter is about 3mm, and you have about 8mm^2 of lens area. THe image that forms on your retina is the superimposition of the images that are formed from each individual square millimeter of lens surface. If you have a defect on your lens that encompasses 0.5 square millimeters, the composite image on your retina would have 'good' image from 7.5mm^2 of good lens, and this would wash out the 0.5mm^2 worth of bad image.

However when you start looking through a reduced aperture, as in shooting, where you have a 1mm diameter opening, or about 0.75mm^2 of area, only a small fraction of your lens is contributing to the image formation. If you have a 0.5 square millimeter defect, and it happens to fall in-line with the 0.75 square millimeter of lens that is forming the image, you will have 2/3 of the image being distroted, and the 1/3 'good' image cannot compensate.

Thus, micro defects in the eye that are not visible in daily view can become very visible when shooting. The only option short of surgery is to use a larger aperture - try a 1.5 or 2mm aperture instead of a 1mm aperture. You will lose depth of field, and you will need to make sure your shooting lens is the right power, in order to optimize the small amount of depth of field you have left, but it might begin to diminish the effect of the cateract. If it works, it's a much easier solution than surgery.

The other thing you could try is to play with non-round apertures. In the olden days, eye doctors used these, and they were referred to as stenopaeic slits. If you have a horizontal defect on the cornea, rather than using a round aperture that allows the defect to take up a large percentage of the aperture area, use a vertical slit, so the defect only cuts across a small amount of the opening. I can laser cut small non-round apertures, but it is probably easier to experiment by getting two vertical strips of electrical tape on your shooting glasses, and make the space between them 1/2mm. Now, when you open up the diameter of the rear sight aperture, it will get taller, but the width will be limited by the tape strips. This should diminish the impact of the horizontal defect.

For lenses, you want to take your distance vision and add +0.50 diopters for rifle, and add +0.75 diopters for pistol with iron sights. So for your right eye, your distance vision is 0 SPH, +0.50 CYL. Your rifle lens would become +0.50 SPH, +0.50 CYL, and for pistol you would use +0.75 SPH, +0.50 CYL.

[TenMetrePeter]

Anyone else see Purkinje's Tree (below) when using rifle iris and if so was it a result of a cataract? Its driving me potty. Worse if I move around
Going to hospital next week to get checked out as other eye is fine.

images.png

[Pheyden]

Art, thank you for the advice. I reqched close to the same conclusion independently. Now using a+0.5 sph. + 0.5 CYL lens mounted on rear sight, with rather large1.5mm iris opening.

For pistol +0.5 +0.5 Cyl with iris and vertical slot. This works really well.

Paul

[GeraldC]

Slightly more learned info from an American shooting eye doctor!

https://www.ssusa.org/articles/2017/1/2 ... -aperture/

This article clears up many questions I have asked over the years. Recommended reading for older shooters

[TenMetrePeter]

Slightly more learned info from an American shooting eye doctor!

https://www.ssusa.org/articles/2017/1/2 ... -aperture/

This article clears up many questions I have asked over the years. Recommended reading for older shootings.

Yes and I will take a copy to the hospital as the spectacle shop opthalmologist had not heard of it even though experiments with pinholes and entoptic phenomena in 19th century were documented. Pinholes are known to magnify eye defects, I just want to know if mine is catatact or start of AMD which my mother had.

[Martin H]

Anyone else see Purkinje's Tree (below) when using rifle iris and if so was it a result of a cataract? Its driving me potty. Worse if I move around
Going to hospital next week to get checked out as other eye is fine.

images.png

Hi Peter
I usually refer Purkinje's Tree as "cobwebs". I see the cobwebs whenever I have my iris set below 1.1mm.
I compensate by using a larger than recommended iris diameter, 1.2mm to 1.5mm. This gives me a relatively clear sight picture free of cobwebs.

However the downside of the larger iris size is that I suffer from loss of of depth of field and have to be be very careful to get the correct lens power to keep a sharp foresight and reasonably sharp target image. It can be a balancing act to get the correct lens power for each range or different light conditions.
On ranges with shaded or poorly lit targets the target becomes a completely indistinct grey blob which is impossible to centre. I need to adjust my lens power so I can see the target better (more negative power). I just have to be careful the foresight doesn't start blurring out, definitely not ideal.
Oh to have the vision I had 30 years ago.
Cheers
Martin

[TenMetrePeter]

Thanks Martin . That's two other shooters that see the cobwebs.
My friend's cobwebs went after a cataract operation replaced his natural eye lens. I am feeling less alarmed now. I see better with 1.6mm and a yellow filter.