Shooting Lenses

[william]

Disclaimer: I'm not an engineer, physicist, optician.... WI am a wearer of eyeglasses for over 60 years, a competitive shooter for nearly 20, an avid experimenter, and a firm believer in the value of empirical results.

The result of more than a little time and money spent with cooperative, understanding optometrists and opticians is that Your Results May Vary. The formula for pistol may very well state +0.75, but my experimentally derived optimum combination of clear front sight and unclear bull is +1.25.

Over the years I've developed what I think is a reliable, repeatable test procedure which I'm happy to share with anybody who asks via PM.

I'm posting this here and in the Shooters Lounge. No offense if a moderator decides it should be in one only and removes one.

[Silvershooter]

but my experimentally derived optimum combination of clear front sight and unclear bull is +1.25.

Exactly the same for me, I use +1.25 . Discovered this by accident when trying my computer glasses (+ 1.5) for shooting, found they were better than the +.75 shooting lens. So had the optician make me a lens a step down for shooting.
+.75 on distance prescription gives me a very clear target.

Yes, please pm me your test procedure.

[Robbie01]

I'd like to know more about how you came up with your results. I started shooting 10m air pistol last year and have been using a +1.25 pair of inexpensive cheaters from Costco. I put a very small piece of band aid tape on the lens of my non dominant eye. My target is fussy while the front sight is crystal clear. It seems to work great but was looking for expensive shooting optics since that's what everyone is using.

[Gwhite]

I also use ~ +1.25D for indoors at 33 and 50 feet. I think my 50 yard correction outdoors is a little less. I also use an adjustable aperture to set the depth of field. Between the too, I can see my front sight very clearly, and the rear sight is also reasonably sharp., but the target is blurry.

I've found the adjustability is essential in a long match, or with varying light conditions outdoors. I've just started competing in air pistol again this yer after about a 30 year break, and found that my eyes aren't stable over the course of a match. As they tire, I find I need to reduce the aperture a tiny bit to keep the front sight sharp long enough to get a good shot off.

I wear contact lenses, and my right eye is set up for distance, so a plain +1.25D lens in a pair of Champion frames does the job. Over the years, I've amassed a set of lenses in 1/8th diopter increments that go from +.63D to +1.5D, along with a +1/8D and a -1/8D. That allows me to play optometrist at the range under actual lighting conditions without being rushed. As I've mentioned in another thread, getting lenses ground that accurately is a bit hit or miss, and I measure the strength & quality of every lens I get and mark it with the true strength.

[dulcmr-man]

and I measure the strength & quality of every lens I get and mark it with the true strength.

How do you make the measurement? What kind of test instrument or procedure do you use?

Dennis

[TenMetrePeter]

Focus the sun or distant lamp on a white card using the lens. Distance of lens to card in Metres is focal length f.
Diopter value =1/f

[Gwhite]

From a previous post on this topic in another thread:
-------------
You can set up a home brew measurement system to check your lenses using a long dark hallway or basement, and a small bright light source (I use a Mini MagLight with the reflector removed). You park the light source as far away as possible, and move the lens back & forth until you get a sharp focus on the opposite wall. The Diopter rating of a lens is given by:

D = (1/O) +(1/I)

Where "O" is the distance from the light to the lens, and "I" is the distance from the lens to the focused image (everything in meters). If you have a light that is far enough way, the "O" term becomes negligible, but it has to be bright enough to allow you to see the focused image clearly. My basement is ~ 15 meters long, so neglecting the "O" term adds a ~ 6% error. If you can wait until mid-day, you can also try to focus the sun on the ground. That is bright enough that you can also add an aperture to evaluate only the portion of the lens in the center that you will be looking through. A piece of opaque tape with a 1/4" (6mm) hole is good, but you may find it hard to see the focused spot. Some shooting glasses lens vendors will sell lenses in 1/8 diopter increments. I've tested all my lenses, and even the "fancy" ones are often off by more than 1/8D.
----------------
If you do this indoors, it helps to turn out the lights near where you are focusing so you can see the relatively dim image. A really bright LED light source might make this easier. Shooting glasses lenses typically have a long enough focal length that you can't simultaneously hold the lens and observe the image. I actually mount the lens on a stand located roughly 1 focal length out from the wall, and then adjust a white sheet of paper back & forth until I get a good image. It's impossible to measure a low value (like 0.25D or 1/8D) lens directly, so I will measure how much it changes the focus of a known lens, like +1.0D.

[spektr]

All of this calculation is good, but I have to carry many different lenses and change back and forth as my blood sugar moves around

[Gwhite]

There is a company that was making variable lenses for shooters. They used a clear fluid, and by pumping it in & out, they could make the lens bulge by varying amounts:
http://www.variolens.com/english.html

There's also this, which is a lot cheaper:
http://borelsport.com/product/variable- ... g-glasses/

I tried something similar a while back. The problem I had was that the region of good focus/vision isn't very large, and is centered in the lens. I had to turn my head quite a bit from my normal position to be able to see well. Because these are wrap around lenses, they may work a tiny bit better, but I doubt it.

[David M]

The formula for pistol may very well state +0.75, but my experimentally derived optimum combination of clear front sight and unclear bull is +1.25.

Your formula of +0.75 has a bit missing.
You need a lens to defocus +0.5 to +0.75 over and above your normal distance script.
So after a eye test if your distance script is +0.5 then add +0.5 to +0.75 becomes +1.0 to +1.25.

[Rover]

I took William's advice once and found it excellent.

He told me, "NEVER fry bacon in the nude."

[Coolmeester]

In my experiment different lens is needed when shooting indoors than outside. Outside is always bright (even on cloudy days), so you have better depth of field, meaning that you need more + in your lens. Indoors in low light conditions pupil is large, depth of field poor so only very small amount of + is needed to get the bull unclear.

Hope this helps.

[Rover]

That's correct; which is the point of adjustable iris fitting for your shooting glasses. That way you have a uniform "pupil". It is NOT for adjusting the sharpness of the bullseye or to control
"depth of field."

[Gwhite]

Not quite. If your eyes change during the course of a match, it gives you a simple adjustment that can help to compensate. I'm not sure if they are getting tired, or the muscles in the eyes stretch over time, but I definitely have trouble seeing my sights as well towards the end of an air or free pistol match as I do at the start. Having an adjustable iris has made it possible for me to tweak things enough to see OK and finish the match. It doesn't take much, but it's definitely a help.

I also find it useful to compensate for varying light outdoors. I use filters, but they are in fairly coarse steps. The adjustable iris is also much quicker than swapping filters on and off my glasses when a cloud passes overhead.

[Coolmeester]

Iris is good for some shooters but not the answer for everyone. Your balance gets dramatically worse if you can't see everything surrounding you. Second it's much more uncomfortable to shoot when your head must be exactly the same position all the time to see the sights and the target. Third it's almost impossible to shoot rapid fire stage on CFP or RFP when using iris.

I've used iris in 10m and 50m pistol but for me with right lenses things got much easier.

[ShootingSight]

Optically, iris and lens do two different, and independent things.

Iris, or aperture size, determines how wide your depth of field is, and also regulates light and field of view. These are a trade-off against each other. Smaller aperture gives a bigger depth of field, but lets in less light and gives a smaller FOV.

Lens diopter determines where your eye's relaxed focus falls, and your depth of field is centered about your focal point. The normal human eye has relaxed focus fall at infinity. You have to exert the eye muscle to pull your focus closer to get it to fall at the optical average of the target and the sights, so you can see both together. Adding a lens will shift your focal point, and the goal is to add the right power, so your relaxed focus falls at this optical average point, allowing you to see an optimal sight picture without having to exert your eye muscle at all, so there is no eye strain or fatigue.

Multiple things can happen during a match that might cause a drift of one or the other: if you have the wrong lens, such that your focal point is not quite close enough, and your eye is making a part-effort to set up a sight picture, you might do it fine at the beginning of the match, but the eye can fatigue, and your sight picture will fade. For this, you simply should have started with a slightly stronger lens, but not everyone has the right lens to start with. Also, blood sugar and hydration will affect focus. If you hydrate or dehydrate, so the eye swells or shrinks slightly, it will change the focal distance between the cornea and retina. I don't know how much the eye's diameter changes on a daily basis, but 1/100 of an inch would result in a 0.5 diopter shift in lens powe, which is noticable. Also, sugar levels in the body affect glucose levels in the ocular fluid, so the refractive index of the cornea changes, and shifts your focus. I think this is usually not an issue for healthy people, but if you are diabetic or have a wandering sugar level (or try to shoot a day-long match while skipping lunch), it can affect. I know blurred vision is one of the symptoms of a diabetic attack. Lens will also affect the balance of focus on the sight versus the target. Using a stronger lens will sharpen the sight and blur the target. Finally, not everyone starts out with relaxed vision that falls exactly at infinity, just like everyone is not exactly the same height as the average. The +0.75 recommended lens is added to an assumption that your eyes focus at infinity, though some people might need slightly more or slightly less.

In terms of aperture, changes of brightness might require a compensating change of aperture size, change of task from slow fire to rapid fire might require a slightly larger field of view.

Net, 1 - 1.2mm aperture with a +0.75 lens is a reasonable starting point, and might work for a lot of people, but tweeking those values for personal needs or preferences is certainly valid.

While you typically