TargetTalk

I need to order a new lens for my daughters shooting glasses. What adjustment should be made to her every day prescription?

Right Eye: -1.50 Sphere, +0.25 CYL, 075 AXIS

Thanks in advance for the advice.

Rifle or pistol?

The prescription would be modified by +0.75 diopters (for pistol), which would result as:

Right Eye: -0.750 Sphere, +0.25 CYL, 075 AXIS

I have always modified my non-dominant eye by the same amount (+0.75 diopters) since I shoot with both eyes open and don't use a blinder.

Ordering glasses through Zenni optical is very economical to try out a prescription modification. The most trouble I've had when ordering glasses is finding the correct size frame (lens height and overall width), but that can be estimated from a set of current frames. The pupil distance (which is the distance between pupils of the eye) can be measured via a mirror with a ruler or using a dedicated tool.

FWIW, I've found this prescription modification (when applied to both eyeglass lenses) makes reading more pleasant.

jmdavis wrote:Rifle or pistol?

Exactly. Rifle doesnt often need as much correction, if any. The diopter gives a bigger depth of field

https://shootingsight.com/product/test-lens-kit/
shooting sight.com
TEST LENS KIT
$60.00

Red,
My optician simply asked the measurement from eye to front sight to get the correct focal plane for my monocle (rifle). If you deal with a business that does shooting glasses, they should know the best corrections to her regular prescription that will be most affective for her.

And this is why opticians often get it wrong. You do not want to set the focal point to the front sight, or the target will be too blurry.

You want to set the focal point to the hyperfocal distance, which is 2x the distance from your eye to the front sight on a rifle, or 2x the distance from the eye to the rear sight on a pistol.

This will centralize your depth of field between the sight and the target, so your eye can see them both at the same time.

In practice, this ends up calculating to adding +0.50 diopters for most rifles, smallbore, M1, etc. You add +0.75 to shorter stuff, like AR-15, or pistol. When I say adding, I mean these values are added to your distance prescription.

While someone recommended my test lens kit, above, and I appreciate it, the reality is that you don't need it. I started selling those kits before I figured out the math so I could just calculate the correct answer from a theoretical perspective, rather than going at it empirically by just trying different lenses. Some people do still like the lenses to play around with, but the addition of +0.50 or +0.75 will get you to the correct answer most quickly.

If you go to Zenni, pay the extra few dollars to get the polycarbonate lens upgrade. Polycarbonate is impact resistant, the standard CR-39 lenses are not.

Thanks for the great replies.

I emailed Shooting sight and got a fast response. They answered all my questions. Lens should arrive in 2-3 weeks.

ShootingSight wrote:You do not want to set the focal point to the front sight, or the target will be too blurry.

"Too blurry" is a variable concept.

I did my best shooting at a time when I couldn't read the numbers above the targets (I used the b/w or w/b colouring to differentiate) and was even known to shoot at the cutout in the target board when the target centre had blown away in the wind.

Whether it is too blurry depends on what your mind can accept.

I'll go with Dave on this one. I find that if the bull is too sharp, the shooter tends to look at it while aiming instead of at the front sight. The eye going back and forth from bull to sight just doesn't cut it.

When I shot smallbore in college at UK, back in the early '80s, we used the Anschutz 1813 rifles with a circular front sight and peep rear. The target was in sharp focus along with the front sight. The FWB 300 air rifles were configured similarly. I haven't shot my FWB 300 in some time to say if this is still the best approach for me, but expect it may be.

I've ordered eyeglasses from Zenni for many years. My personal preference is to order an inexpensive frame with the 1.57 refractive index (free lens) and see how it works out. If that is good, then a more expensive version (with coatings, etc) is ordered. Not to imply I make a lot of mistakes and don't get things right the first time, but I've found taking the inexpensive route and tweaking any values (primarily the spherical diopter magnification) to work out best for me. The polycarbonate lens is a good idea for safety.

Optically you can't have both the target and foresight in sharp focus, even with aperture sights. The rear aperture does extend your depth of focus so you can see both (try without the rear peep, and one will be almost invisible), but one will be slightly fuzzy. Young good eyes can flick back and forth quickly enough to create the impression that both are in focus together. Older eyes find this tiring.

I'm long sighted, and also find it tiring to make my eye focus on the foresight not the target. +.50 on my distrance prescription works for me, and many others, although it's not absolute. Rifle shooters do generally want their focus ahead if the foresight, so the target is clearer. However a prone rifle is almost still on the aim compared to an AP. I know of one succesful shooter who had a new lens replaced because he wanted to focus on the target, as that's what he'd always done, and couldn't untrain this habit after 40-odd years.

There is an experiment somewhere on the interweb, can't find it now, an assistant holds a piece of card to obscure the target completely, the AP shooter can only see the sights, he scores mostly tens and nines, with one or two eights.

FWIW, I expect the ability to focus on both the front rifle sight and target clearly was due to an increased depth of field from the rear adjustable sight iris. Back in the day, I wore hard contact lenses which provided 20/15 vision with all astigmatism removed. We shot three hour matches and I had no eye fatigue. I can get a similar effect from my Gehmann adjustable iris which fits on my eyeglasses. But with pistol I have become accustomed to focusing on the front sight and having the target a bit blurry.

Young eyes do not flick back/forth. Young eyes naturally focus at the hyperfocal distance of the sight, so both the target and the sight are in reasonable focus at the same time.

The notion of focus is fuzzy, excuse the pun.

There is a limit to the smallest thing your eye can resolve. If a blur line around an object is smaller than the photoreceptor cells on your retina can resolve, you cannot see the blur line, and the object will appear to be in perfect focus. 20/20 vision is defined as being able to see 0.015mm of blur on the retina (Actually, it is defined as being able to resolve a line 1MOA wide. However that is a tolerable circle of confusion 2MOA in diameter, which is 0.015mm on the retina). When looking through a 0.040 aperture with a 25mm lens (about a human eye), if you focus at the hyperfocal distance of the sight, neither the target nor front sight will be in 'perfect' focus, but the width of the blur line on the retina will be about 0.023mm. This is not 'perfect' focus, but it is pretty good, like having 20/30 vision. It basically says you can resolve an item as small as 1.5MOA. Since the target is 6MOA, this is enough to get you close.

The other two things your eye/brain uses to aim are brightness, and the observence of symmetry. For brightness, your eye can detect about 16 different shades of brightness. With a front aperture, you get a ring of white around the target. The white line has a certain brightness, so if you have 3MOA of white on both sides of the target and you can only resolve down to 1.5MOA, you could not hold an X ring. However if you are off center, so one side has 2.5MOA and one side has 3.5MOA, the blur around the 2.5MOA edge will be less bright than the blur around the 3.5MOA edge, because there is more light coming in on one side. This is how good shooters can hold groups smaller than the resolution limit of the eye.

With a post, your brain can also use symmetry. If the post is centered under the target can be judged, regardless of the degree of focus. Unfortunately, there is no symmetry when you judge elevation - there is only one top horizontal edge to a post. THis is why a typical symptom of shooters who are struggling with their vision is that they will string their shot groups up/down, even though they are holding tight side/side.

10M_Stan wrote:FWIW, I expect the ability to focus on both the front rifle sight and target clearly was due to an increased depth of field from the rear adjustable sight iris. Back in the day, I wore hard contact lenses which provided 20/15 vision with all astigmatism removed. We shot three hour matches and I had no eye fatigue. I can get a similar effect from my Gehmann adjustable iris which fits on my eyeglasses. But with pistol I have become accustomed to focusing on the front sight and having the target a bit blurry.

You are correct. The width of a blur line you see around an object that is not at the perfect focal point is directly proportional to the size of the aperture.

With your naked eye, the pupil is about 0.125" in diameter in good light. With a match sight, the aperture is about 0.040 (1mm), so that is a reduction of aperture size by a factor of 3. Consequently, the width of the blur line that you see is reduced by a factor of 3.

You can achieve the same thing with a pistol by getting a piece of black electrical tape (even better is self adhesive aluminum foil) and drilling a small hole in it, then sticking it to your glasses so you can look through the hole as you are aiming. This actually allows you to do an interesting experiment, because you can tip your head slightly to compare sight picture looking through the hole, versus looking over the tape so it is just the pupil of your eye. The difference is amazing.

ShootingSight wrote:Young eyes do not flick back/forth. Young eyes naturally focus at the hyperfocal distance of the sight, so both the target and the sight are in reasonable focus at the same time.

The notion of focus is fuzzy, excuse the pun.

Art,

You miss my point. Young shooters can see a good sight picture without glasses, but the target isn't completely in focus at the same time as the foresight. That was my point, that the rear aperture can only achieve so much.

I don't think I missed your point. You said that the aperture can not put the sight and the target in perfect focus. I agreed. The rest of my post was trying to do the math to quantify how much out of focus things are. The only thing I was disagreeing with is that to aim, you do not flick your eye back/forth, but rather let it settle at the average point so you see the sight and target at the same (albeit imperfect) focus.

I consider that for shooting, there are two parts to your depth of field. There is a narrower range, where the width of the blur line is smaller than you eye can resolve, ie the blur projected on your retina is less than 0.015mm in width. Over this range the image truly appears in perfect focus to the resolving limit of your eye. Then there is an expanded range, where focus is not perfect, but still pretty good. Good enough to aligh a sight with a target consistently.

With an aperture, you need to be VERY small to get your sight picture in the smaller DOF range where there is no blur. Like, 0.7mm or smaller. Most people shoot with a rear aperture that is in the 1mm range, and will get a blur line that is in the 0.025mm range. This blur is visible, but pretty small.

So you are correct. An aperture alone can usually not get you to perfect focus, but it gets you close.

The issue with young shooters is the ability to maintain a close-up focus. The hyperfocal distance where your eye needs to focus to evenly distribute your depth of field is at a distance that is 2x the distance to your front sight. So if you have a sight radius of 1m, you want your eyes focused at 2m. As we get older, the lens in our eyes gets hard, and the eye muscles have to strain harder and harder to pull our focus in close. Getting a sight picture where you are forcing your eye muscles to pull your focus in to 2m and then hold still is difficult for old people, while young eyes can do it easily. THe solution here is to add a lens (like reading glasses), but you need to calculate the lens strength so it puts the relaxed focal point of your eye at 2m. That way, you are focused at 2m and your eye muscle is not doing any work (or minimal work anyhow).

Luckily, lens calculations are easy. Assuming you are starting out with good (or corrected) distance vision, The strength of the lens is simply the inverse of the focal distance. So to focus at 2m, you need a 1/2 diopter lens.