Lenses and apertures

Centerfires, rimfires, pistol cartridges and everything in between.
ShootingSight
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Lenses and apertures

Post by ShootingSight »

Brand new to the site, so I thought I should introduce myself. My name is Art Neergaard, I never shot cowboy anything, but I am a Service Rifle shooter.

I am not a doctor. I am a mechanical engineer, photographer, I know optical physics some. 50 years old, presbiopic, can't see the front post anymore, so I studied the optics of apertures and lenses to figure out what a rear sight ought to do, and developed a couple of products to help shooters see better, and aim better. I started a small company, ShootingSight LLC to sell these for Service Rifle use.

Recently, a cowboy silhouette shooter contacted me about fitting some of my products to a Williams sight for his rifle, which I did, so I figured I would poke around on the forum and see if any of the lessons from Service Rifle shooting apply, and I'd like to ask your help. I'll give a description of what I know about shooting vision, and end up with some specific questions, and then look to you for general input to see if it is worth anything to your discipline.

Rifle shooting is about focus and depth of field, because you are trying to juxtapose a nearby object (the front sight) and a far object (the target). People who prefer to memorize quips, rather than actually understanding a subject like to throw out "the eye can only focus at one distance at a time". This comment is at best shallow, and under certain circumstances is downright wrong. Yes, in a theoretical sense, a lens has only one focal length, but when the image sensor (your retina) has a maximum resolution, degrees of blur which are less than your maximum resolution will be perceived to be perfectly in focus. So, there is a range of distance that you can move an object slightly closer than your focal point, or slightly further than your focal point, where the amount of blur will be invisible. There is an even bigger range of distance where the amount of blur is so small that the image is good enough for your brain to use. Furthermore, there are certain things you can do to minimize the blur on something that is theoretically not in perfect focus, like reducing the aperture size.

In order to get the best vision of both a front sight and target, the first thing you want to do is focus your eye on a point between the front sight and the target, so that while the eye is relaxed, the amount of blur on the front sight (because it is closer than your focal point) is equal to the amount of blur you see on the target (because it is further away than your focal point). This focal distance which exactly divides your depth of field between the front sight and infinity is known to photographers as the ‘hyperfocal’ distance of the front sight, and you can calculate it quite simply because the math works out to exactly 2x the distance from your eye to the front sight. When I shoot an AR-15, the sight radius is 20” and there are usually 2” between the rear sight and my eye, so eye to FS is 22”. That means my hyperfocal distance is 44”.

Next question is how do you focus at 44”? In the relaxed state, a healthy human eye will focus at infinity. The ciliary muscle around the lens can then exert itself to squeeze the lens to make it focus up close. When you are young, and the eye lens is soft, you can focus as close as about 10”, so making only a slight shift from infinity back to 44” is a breeze (it is not zero effort, but most young people ignore it).

As you get older (usually around 40), the lens starts to get harder, and the eye muscle can only focus up close with great effort, or else can’t focus up close at all. You can still focus at distance fine, but need reading glasses (or longer arms). These people lose the front sight, especially at the end of the day, where they have forced the eye muscle to maximum exertion all day long, and then tried to get the muscle to hold still in that exerted state while they aim. After 20 or 30 shots, the muscle can’t hold, and starts to slip or tremble, and your focus goes away. I’m sure at my age I can still do a snappy pullup. But he second one will be slow, the 5th one might not happen, and the 20th attempt will be not much more than slight muscle twitch as I hang on the bar.

If the eye muscle can’t exert to get your eye to your hyperfocal point for the rifle, the other solution is to add a lens. Positive diopter lenses will shift your focal point closer, so you can see up close without the eye muscle having to make the effort. This is what reading glasses do. Difference is that reading glasses shift your focus way close, to arm’s length, which is much too close for shooting. To shoot, you need only a mild corrective lens to shift your focus from infinity to the hyperfocal point. To be clear, older shooters might NEED this correction to see, but young shooters can benefit from a lens as well - it will give them the same image they see with their unaided eye, but they will see it while the eye muscle remains relaxed, rather than having to hold the eye muscle still in the flexed state.

What lens to use? Lens math for this is pretty simple. Lens power (measured in diopters) is simply the inverse of the focal length in meters. Using the AR example, if you want to focus at 44”, that is 1.12 meters, and would require a +0.9 diopter lens (1/1.12 = 0.9) to shift your eye’s relaxed focus from infinity to the hyperfocal point. Since lenses typically come in ¼ diopter increments, you would round this to a +0.75 diopter. (I won’t go into why you always want to round down, but you do). Unfortunately, reading glasses from the drugstore usually start at +1.25 diopters, which is much too strong – these will focus you at 32” – it does not seem like much error, but in fact it will give you a really sharp front sight, but the target will be gone in a blur.

If you wear glasses for corrective purposes, the good news is that diopters simply add, so if you are farsighted and need a +1.00 sphere to see distance, you would simply take the +1.00 in your prescription and add the +0.75 we worked in the above example, and your ideal shooting lens would be a +1.75. You do have to beware of signs. If you are nearsighted, and need a -1.00 to correct for distance, adding +0.75 to the -1.00 value would net you a -0.25 lens.

After you have achieved focus at the hyperfocal distance, the next step is to reduce blur across the whole range of distances, and this is done by reducing your aperture size. The amount of blur you see on an object that is displaced from your focal point is linear with aperture diameter. Your eye naturally has an aperture (the pupil) of about 0.125” on a bright day. If you try and aim without a rear aperture (ie using notch sights), you will see considerable blur on the front sight and the target, even if your eye is at the hyperfocal distance. Using a 0.060” diameter aperture will cut the amount of blur by half.

The dark side of apertures (no pun intended) is that the total open area of the aperture limits how much light reaches your eye. As you make the aperture smaller, less light reaches your eye. In theory, if you used a 0.012” aperture, the focus on the front sight and the target would both be at the resolving limit of the eye, and would be in perfect focus. Unfortunately, most people need about 10x as much light as this to see a good image, so a 0.040” aperture is about as small as most people can tolerate.

There is a subnote on apertures, which bears on my products: apertures focus in an axis, based on how big they are in that direction. In other words, a horizontal line has its vertical blur limited by how tall the aperture is, while a vertical line can blur side/side based on how wide an aperture is. If you look at a front sight post through a standard round aperture, it will exhibit the same amount of blur on horizontal and vertical edges. However if you keep the total area constant, but make the aperture wider and shorter, so it becomes oval, you will improve focus on horizontal lines, and give up focus on vertical lines, even though you have not given up any brightness. For a post shooter, this is a benefit. Windage aiming, and elevation aiming are two different things for your brain. In windage, you are judging the symmetry of the target to the front sight. Centering a target side/side between two sharp vertical edges of the sight, and centering it between two fuzzy edges is the same thing, so focus is not important for windage estimation. Elevation, however requires determining exactly where the top edge of your sight is, and for this you need sharp focus. Net, a slit shaped opening will bias your focus on the top horizontal edge, giving you improved focus ion elevation, without any sacrifice in windage aiming.

My questions to you:

What is the barrel length you typically shoot?
What type of front sight do you usually use (bead, post, etc)?
Do you shoot a center of mass, or a 6 o’clock hold?
Are lever actions the only class that commonly shoot iron sights? Seems from reading around, most of the small bore and HP guys use scopes.

Thanks,

Art Neergaard
ShootingSight LLC
http://www.shootingsight.com
email: shootingsight@nuvox.net
Last edited by ShootingSight on Mon Feb 14, 2011 6:28 am, edited 2 times in total.
nomad
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Re: Lenses and apertures

Post by nomad »

Art,

I've enjoyed your posts on the NM site. Welcome to silhouette. IMO your expertise will be valuable and welcome -- especially to those of us on the back side of 30. (OK so I'm 30+ years on the back side of 30. Let's not quibble over small numbers! lol)
Check your email and I'll try to provide a thumbnail sketch of the various silhouette disciplines and their attendant sight problems.
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Bob259
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Re: Lenses and apertures

Post by Bob259 »

Great post Art.... you will find many optically challenged folks over here and I can relate to everything you have said. :shock: :-B

As stated, usually it's the lever gun activities where the problem is found as they are normally the only ones we use open sights on.

Again... WELCOME :-bd
F Troop - Southwest Outpost

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Re: Lenses and apertures

Post by Tlee »

Art -

Thanks MUCH for your informative post... I've both a PM and an email coming your way.

-Tim
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Jason
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Re: Lenses and apertures

Post by Jason »

Very good information. What would you suggest for glasses for targets at four different distances in the same match? For instance, the smallbore and pistol cartridge cowboy rifle targets are at 40, 50, 75, and 100 meters. The centerfire rifle cowboy targets are at 50, 100, 150, and 200 meters.
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Re: Lenses and apertures

Post by Jason »

Hey Art.. check these numbers if you have a sec to see if I understand this correctly. For my Marlin 57 that I shoot for smallbore cowboy, it's 4.25" from my eye to the rear aperture and 27.25" from the rear aperture to the insert in the front sight, for a total distance from my eye to the front sight insert of 31.5". Since hyperfocal distance should be twice that distance, hyperfocal distance is 63" or 1.60 meters. Ideal diopter = 1/ hyperfocal distance (m) so ideal diopter = +0.625. After Lasik surgery for nearsightedness a decade ago, my vision is pretty balanced but -0.25 diopter makes my distance vision very slightly clearer. My optometrist said he only had lenses in quarter diopter graduations so my ideal was probably less than a quarter. That means that my ideal correction in a lense would probably be right at +0.5 for shooting that rifle, correct? Does the fact that the targets are fairly close (40, 50, 75, 100m) change that at all?
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Re: Lenses and apertures

Post by ywltzucanrknrl »

Just a plug for Art. He helped me last year with my sights and vision for service rifle. His products/ideas work and he took time and provided me great information. My front sight was disappearing. I'm older, farsighted in my non aiming eye and near sighted with astigmatism in my aiming eye--so there were plenty of issues to work on.
I don't shoot any lever action silhouette, I shoot SB and HP where I can use a scope, but I've been tempted to shoot the lever action stuff but I'm a little confused by what kind of matches there are and what can be used.
Anyway, if you are shooting iron sights, I 'm sure Art can help.
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Re: Lenses and apertures

Post by boats »

Art

Here's one for you.

I shoot CLA with post front (Required) use Center of mass hold, do know guys that use bottom of mass like in bullseye but not many. Also Shoot Schuetzen with Irons, Aperture front and rear both adjustable. Schuetzen uses Bulls eye targets real easy to see compared to CLA. I used to shoot smallbore prone with a Anschutz diopter with some success also bullseye targets. Gehamnn and Anschutz rear sights have all sorts of tricks available and have used them all looking for sight picture in that match.

Single shot rifle bought a diopter from one of the service rifle suppliers for the MVA tang sight and tried it on CLA Rams at 200. It screws in the front side, real small about the size of a kitchen match head. +.37 I think it is. Seems to limit available light, even when I open up the rear aperture and ended up not using it much. Sharpened the front sight but made the irregular shaped black ram too hard to pick my spot on. It works good using aperture front and rear on bullseye Schuetzen targets.

You can get a Merrit Disk (which I use for CLA rear aperture) that's set up for a diopter insert. Wondered if my problem with the small one on my MVA tang is unusual are related to the small size of the lens itself. Or is it the fact that CLA you need to pick a spot on a irregular shaped target that has little contrast front sight to target itself.

Rifles are accurate and I hold good, scores are limited by the sight picture without a doubt.

Comment appreciated

Boats
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Re: Lenses and apertures

Post by ShootingSight »

Jason,

You do not need different lenses for different distances to the target. In optical math, lens power is the inverse of focal distance, so 1/40 meters, 1/100 meters, and 1/200 meters are all very small numbers, and the differences in the 'theoretical' lens power you need to focus at these different distances is much smaller than your eye can see. This is why the eye doc can test your distance vision using a chart that is 25 feet away.

As you get closer, differences can become significant, so a rifle with a 20-24" barrel will require a different lens than a 30" barrel, but even here, we are only talking 1 step different in lens power.

You nailed the math, and I agree, a +0.50 lens is where I would start.

Art
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Re: Lenses and apertures

Post by ShootingSight »

Boats,

Lens power is always a trade off - more positive lens power will improve the front sight, but give up target, less lens power improves the target at the expense of the front sight, which is why it is important to get the right balance. If you got more front sight and insufficient target with a +0.375 lens, it sounds to me like you are slightly near sighted. The recommendation would be to drop the lens power, though since 1/8 diopter steps are considered the smallest that the eye can see, you can't drop it far before you get to zero, which is the equivalent of not using a lens. Try a +0.25 and see if that does the trick.

The other step to look at would be reducing the aperture size, which will improve your depth of field. This is easy in adjustable apertures, and most have a scale on it, so you can see what works. Most people end up at around a 1mm opening (0.040"). I don't typically make round apertures, but have some ideas for how I could prototype something if the smaller sizes are not available.

Art
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Re: Lenses and apertures

Post by ShootingSight »

Boats,

Lens power is always a trade off - more positive lens power will improve the front sight, but give up target, less lens power improves the target at the expense of the front sight, which is why it is important to get the right balance. If you got more front sight and insufficient target with a +0.375 lens, it sounds to me like you are slightly near sighted. The recommendation would be to drop the lens power, though since 1/8 diopter steps are considered the smallest that the eye can see, you can't drop it far before you get to zero, which is the equivalent of not using a lens. Try a +0.25 and see if that does the trick.

The other step to look at would be reducing the aperture size, which will improve your depth of field. This is easy in adjustable apertures, and most have a scale on it, so you can see what works. Most people end up at around a 1mm opening (0.040"). I don't typically make round apertures, but have some ideas for how I could prototype something if the smaller sizes are not available.

Art
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Re: Lenses and apertures

Post by boats »

Art

The problem that everybody has to solve with CLA is Post front and a large indistinct target that often presents very little contrast to the front sight. Shooters have always been told sharpen the front sight and let the target blur, that's fine if the target is round and the sight can center or bottom the bullseye. However with Silhouettes all 4 are different and with CLA they are very large. You need to pick a spot on the critter, can't trust holding just any-wear on the animal. This complicated by different backgrounds and the targets can be painted white or black at match directors option

I crank my rear aperture during a match often, anytime the light or background changes and even were I get some eye fatigue. That seems to be the best solution. My eyes are corrected after cataract surgery. Right is farsighted left is nearsighted right handed shooter. Rifles use opaque spot on the left shooting glass lens so that eye can stay open with no cross vision. Pistols in action matches that require a fast sharp front sight focus use glasses with opaque spot on the right lens forcing me to be left eye dominant with a better sight picture.

Still I think a little correction would help, it did well for me in smallbore prone. That's a different match entirely were great precision is required and plenty of time to take a sight picture though. Have you ever made a diopter for Merritt's adjustable disk ? + .25 ?

Boats
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Re: Lenses and apertures

Post by krgriggs »

Art, What do you think about the new superfocus glasses that have came out. Would these glasses help our kind of iron sight shooting
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Re: Lenses and apertures

Post by ShootingSight »

Boats,

I've never made a lens for a merit so I don't know how they mount, but I cut them on a lathe, so I can make any diameter you want. You'd have to send me the merit, or else tell me what diameter you need. I charge $40 for lenses that are AntiReflective coated polycarbonate.


I read about a pair of $900 adjustable glasses, but I don't believe they are a help. Their deal is that the focus is adjustable, and they tout that for daily activity you can adjust them to whatever you need. For shooting, you only need to see at one distance, so they make it easier to dial them in initially, but after that, the adjustability does not do anything for you. $900 is steep for that one time benefit. It is far cheaper to simply get the right $40 lens. Even if you have to go to the eye doc for an exam, and then buy 10 lenses till you get one you like, that's still $450 cheaper than these glasses.

Art
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Re: Lenses and apertures

Post by boats »

Thanks

Don't have one in hand Merrit recently listed the disk with a cavity for a lens. I will order out one and get you on your site once in hand. From the pictures it looks pretty simple just a extra cover on the front or screw side of the sight.

Boats
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