TargetTalk

The trick is to have the prescription made so you can also see the target. A sharp front sight might leave the target so out of focus that you can't see it at all...at least for my bad eyes. This is not obvious at first. My eye doctor and I had to experiement with different prescriptions to get the right compromise.

GaryN wrote:The trick is to have the prescription made so you can also see the target. A sharp front sight might leave the target so out of focus that you can't see it at all...at least for my bad eyes. This is not obvious at first. My eye doctor and I had to experiement with different prescriptions to get the right compromise.

By seeing the target what do you mean? The target should be blurry and not in focus because it is imposible to focuss on both the target and front sight at the same time.

Richard H wrote:

GaryN wrote:The trick is to have the prescription made so you can also see the target. A sharp front sight might leave the target so out of focus that you can't see it at all...at least for my bad eyes. This is not obvious at first. My eye doctor and I had to experiement with different prescriptions to get the right compromise.

By seeing the target what do you mean? The target should be blurry and not in focus because it is imposible to focuss on both the target and front sight at the same time.

Even though the target will not be in focus when you are looking at the front sight, it still needs to be seen with enough clarity to place the sight in a consistent position.

As eyes age, it is not unusual to need so much correction to see the front sight that the target is so hopelessly blurred as to make aiming impossible (with any level of consistancy).

Lee Sellers wrote:Even though the target will not be in focus when you are looking at the front sight, it still needs to be seen with enough clarity to place the sight in a consistent position.

As eyes age, it is not unusual to need so much correction to see the front sight that the target is so hopelessly blurred as to make aiming impossible (with any level of consistancy).

I'm afraid I don't agree. I well remember one Centre-Fire training session when I was shooting at my peak (many years ago). I couldn't work out why fellow team members were watching and laughing, until I tried to scope my shots. My target had fallen off and I had been shooting at the hole in the backing board without noticing the difference.

David Levene wrote:

Lee Sellers wrote:Even though the target will not be in focus when you are looking at the front sight, it still needs to be seen with enough clarity to place the sight in a consistent position.

As eyes age, it is not unusual to need so much correction to see the front sight that the target is so hopelessly blurred as to make aiming impossible (with any level of consistancy).

I'm afraid I don't agree. I well remember one Centre-Fire training session when I was shooting at my peak (many years ago). I couldn't work out why fellow team members were watching and laughing, until I tried to scope my shots. My target had fallen off and I had been shooting at the hole in the backing board without noticing the difference.

But you still had a discernable aiming point, even if it was only a hole in the backer. The fact that the blurry black image was not the target bull, but rather a hole in the paper is immaterial to what I am describing.

There are hundreds of service rifle shooters who would love you to be right. Once you get to around minus 3 to minus 4 correction in your shooting glasses to see your front sight clearly you can not resolve the target at 600 yards. In fact you cannot even see the frame well enough to use a frame hold. This is why many who shoot this discipline make comments like “my eyes are too old for service rifle so I’m moving to match”. It is easier to discern a front aperture (less correction) than the front post that service rifle requires.

The same holds true at pistol distances as well, I’m not sure of the numbers (correction) but I know of shooters who now have to use an Aimpoint scope for bullseye rather than irons just for this reason. I can get the numbers from them if you care.

The problem is that the eye can only focus on one point (distance) at a time, and that the depth of field is limited when the focus is on nearby objects. When you place a corrective lens in your shooting glasses (or sight) to sharpen the near focus you blur the images beyond the depth of field more than without the lens. The more correction the more blur at the target. Eventually the target becomes totally indiscernible at which point you can no longer shoot with consistency.

If you think you need a black circle to aim at to shoot tight groups, how do you explain most shooters being able to shoot tighter again on a reversed target?

Look at the front sight, keep it aligned with the rear sight, and aim somewhere below the blurry black circle. Squeeze trigger.
All you need to do.

Lee Sellers wrote:Once you get to around minus 3 to minus 4 correction in your shooting glasses to see your front sight clearly you can not resolve the target at 600 yards.

In my world that is classed as perfect vision ;-)

The incident I referred to was back in the days when my normal prescription was -6 and my shooting prescription was -5.5

Lee Sellers wrote:[The problem is that the eye can only focus on one point (distance) at a time, and that the depth of field is limited when the focus is on nearby objects. When you place a corrective lens in your shooting glasses (or sight) to sharpen the near focus you blur the images beyond the depth of field more than without the lens. The more correction the more blur at the target. Eventually the target becomes totally indiscernible at which point you can no longer shoot with consistency.

I don't believe that's correct. By changing lenses to give you near focus you merely make it easier for the eye to see the foresight. Glasses restore the eyesight to 'normal' vision such that eyesight is pretty much that for a person with 20:20 vision without glasses. It doesn't alter the depth of field. I know some people claim to have trouble seeing the target clearly (which they shouldn't anyway). The problem is probably because they have never got used to just looking at the foresight and have always switched from target to sights. As we get older our eye muscles can no longer shift focal length as quickly and I suspect it's that that causes the problems.

Rob.

RobStubbs wrote:By changing lenses to give you near focus you merely make it easier for the eye to see the foresight. Glasses restore the eyesight to 'normal' vision such that eyesight is pretty much that for a person with 20:20 vision without glasses. It doesn't alter the depth of field.

How do you know this? How would anyone know this? It's not exactly like we can stick an electronic instrument in the back of someone's eyeball to measure this. So you're guessing, based on an assumption that the lens and retina and our brain's ability to process an image is just like the model you have in mind of how ordinary optical systems work. Suppose you're wrong.

Personally, I think you are. My personal experience agrees with the Elmas's original comment, which is that as you get older, depth of field diminishes and that there is a need to choose a correction that "satisfices" between crystal clarity of the front sight and an acceptable focus on the target. The prescription I use with AP at 10m isn't the one that works best for me with FP at 25m, nor vice versa, even though the front sight is roughly the same distance for both guns. I think my depth of field has shrunk but I don't know why. But the difference is, I don't refuse to acknowledge this happens only just because I can't explain it.

I'm not sure if I didn't read the replies all correctly, but from my experince, sharpness of focus at a particular distance is corrected by using the correctt lens. A lens works with your eye optimally at one distance only. It is absolutely impossible to see two distances sharply at the same time. Depth of field (which is what you are talking about in order to see the target also) is acheived by using a variable iris. Without the correct iris setting, many people would find a free pistol target exceptionally difficult to see at all, when the fron sight is in focus. So, you need the correct lens, and a variable iris also. I use a variable iris for free pistol, air pistol, and the precision portion of centre fire. For the rapid fire portion, as well as military rapid fire I use only the lens; no iris.
The same is true for rifle shooting. A variable iris would be very helpful for service rifle, but, that is not part of the game. When the iris of your eyeball cannot become small enough on its own (as you get older), you cannot acheive the depth of field without artificial help.

Nicole Hamilton wrote:How do you know this? How would anyone know this? It's not exactly like we can stick an electronic instrument in the back of someone's eyeball to measure this. So you're guessing, based on an assumption that the lens and retina and our brain's ability to process an image is just like the model you have in mind of how ordinary optical systems work. Suppose you're wrong.

Personally, I think you are. My personal experience agrees with the Elmas's original comment, which is that as you get older, depth of field diminishes and that there is a need to choose a correction that "satisfices" between crystal clarity of the front sight and an acceptable focus on the target. The prescription I use with AP at 10m isn't the one that works best for me with FP at 25m, nor vice versa, even though the front sight is roughly the same distance for both guns. I think my depth of field has shrunk but I don't know why. But the difference is, I don't refuse to acknowledge this happens only just because I can't explain it.

Nicole,
I'd welcome your scientific input into how a lens changing contributes to the depth of field - Rather than just blindly saying I'm wrong. The iris is responsible for depth of field and that can only increase by closing down - in exactly the same way a camera does it. Now it could be that with age the iris doesn't close down as much so depth of field gets worse from that perspective. I have never heard or seen that reported but if you have scientific evidence that it does then that may explain it and I for one would like to see it.

Rob.

steve maly wrote:I'm not sure if I didn't read the replies all correctly, but from my experince, sharpness of focus at a particular distance is corrected by using the correctt lens. A lens works with your eye optimally at one distance only. It is absolutely impossible to see two distances sharply at the same time. Depth of field (which is what you are talking about in order to see the target also) is acheived by using a variable iris. Without the correct iris setting, many people would find a free pistol target exceptionally difficult to see at all, when the fron sight is in focus.

Steve,
You seem to be confusing yourself here. Yes a fixed lens can only focus at one particular distance. The lens in the eye is not fixed, it's shape changes according to where you want to focus by the set of muscles surrounding the eye. As we get older the muscles get weaker and our eyesight tends to deteriorate. The reason being that the eyes can no longer accomodate both near and far objects because the muscles can't move enough.

As to free pistol / AP I know no-one who uses different lenses for both. I use exactly the same for AP, FP, RF, CF i.e. from 10M to 50M. As has been mentioned you do not want the target in focus, just not so out of focus that you can't see anything and that can be achieved without an iris, just a lens that enable the eye to focus comfortably at the foresight. In actual fact I find free and 25M events much easier on the eye than air because the light levels are so much greater. The iris in the eye closes down to allow the 'correct' amount of light to reach the retina and that of itself increases the depth of field.

Rob.

steve maly wrote:... many people would find a free pistol target exceptionally difficult to see at all, when the fron sight is in focus. ... When the iris of your eyeball cannot become small enough on its own (as you get older), you cannot acheive the depth of field without artificial help.

It's true that the FP bull is smaller (in MOA) than than an AP bull. The difference is about 35%, which I agree is significant. But I don't think that's the reason. I also find the same to be true about SP at 25 yards, that I do better with the same prescription I use with FP at that distance rather than with the one I use with AP at 10m even though the SP bull is about 32% larger. So I think it's something else going on.

One possibility that's occurred to me is that as we get older, the lenses in our eyes get cloudy due to age and lifetime exposure to UV and that this affects the quality of the projected image on the retina even at best focus, limiting our brains' ability to recognize the pattern outside a narrower depth of field.

You see how it becomes easier to believe this happens if I give you a plausible explanation? This is the problem that all of us face: We see what we expect to see. It's particularly a problem in research, where scientists routinely get experimental results that confirm the accepted results -- until finally someone discovers those results were wrong. Then miraculously, everyone gets the new results.

If "everyone knows" that depth of field isn't affected by age -- or better still, that it can't be, because everything we know about optics says it can't be true -- then that's what everyone sees in their results. All I can say is that this doesn't match my experience, I don't know why, but I don't think that arguments about ordinary glass optics are sufficient.

RobStubbs wrote:I'd welcome your scientific input into how a lens changing contributes to the depth of field - Rather than just blindly saying I'm wrong. The iris is responsible for depth of field and that can only increase by closing down - in exactly the same way a camera does it. Now it could be that with age the iris doesn't close down as much so depth of field gets worse from that perspective. I have never heard or seen that reported but if you have scientific evidence that it does then that may explain it and I for one would like to see it.

I don't believe I'm blindly saying anything. I'm simply reporting that my experience doesn't match yours and that I don't believe you're correct. I don't claim to know much of anything for sure, certainly not for sure which of us is right and, if I am right, I've tried to be quite straight up that I don't know why our eyes behave the way they do.

But I think your response makes my point: You'd be willing to change your opinion about whether depth of field changes with age if only there was a scientific explanation for it. You seem to be saying that once you knew for sure that was the correct observation, then of course you would get it. In the meantime, since the best explanation you're aware of says it can't happen, you don't see it. Duh!

Dr Norman Wong has written extensively on this subject - http://www.starreloaders.com/edhall/nwo ... ml#focused.

Re-reading his article 'Keeping Focused', it seems reasonable to me that some people could require different lenses for different combinations of sight distance and bullseye size. I know in my case for AP, going from a +0.75 diopter addition to my distance prescription to +1.0 diopter changed the sight sharpness/bull fuzziness ratio such I could more easily maintain proper focus on the sight. Dr. Wong discusses this effect. I do not know at this point whether there is any difference between the lenses for FP.

Just to complicate the issue, I also have days where the 0.75 D lens is better!

Expanding on the matter of how our brains process out-of-focus images, it used to be thought that if a picture was taken out-of-focus, that information was lost irretrievably, that there would be no way to construct an in-focus image. About 25 years ago, mathematicians discovered that's not correct, though reconstruction cannot be done using glass optics.

There's a branch of mathematics called transform theory and, in particular, Fourier optics, that can be used to describe the transforms of an image done by glass optics. If the incorrect transform used in out-of-focus shot is known, Fourier optics techniques can be used to calculate the inverse transform, reconstructing an in-focus image. As I said, it can't be done using glass optics because it's not possible to build glass optics that implement these transforms, but it can be done using a computer.

So another possibly plausible explanation for why depth of field could change as we get older might have nothing at all to do with our eyes. It could simply be that as we get older, that our brains' ability to process fuzzy images -- in effect, to do these transforms -- deteriorates.

As I understand optical phyisics, at a fixed power of refraction (magification ) the depth of field is fixed. An iris ( apature ) must be used to alter the depth of field. Keep in mind that an artifical iris the same or larger diameter as the iris of the eye in a given light condition will not do anything optical.
I also belive that the goal of corrective shooting glasses should be to allow the sharpist focus on the front sight, with the eye in the most relaxed condition of focus. If this state makes the target so blurred that the sight picture can't be alinged with the bull, then, that is the time for application of the artifical iris.

Gort

Nicole Hamilton wrote: But I think your response makes my point: You'd be willing to change your opinion about whether depth of field changes with age if only there was a scientific explanation for it. You seem to be saying that once you knew for sure that was the correct observation, then of course you would get it. In the meantime, since the best explanation you're aware of says it can't happen, you don't see it. Duh!

Nicole,
I was politely saying I think you are wrong - is that clear enough for you ? Scientifically none of what you say makes sense or fits in with my understanding of the science of biology of the eye or the physics of optics. Being a scientist I'm happy to alter my opinions based on scientific evidence. So rather than childish retorts how about finding something to back up your own arguments ?

Rob.

RobStubbs wrote:So rather than childish retorts how about finding something to back up your own arguments ?

I'm disappointed that you think I'm being childish, Rob. But okay, here's an interesting report on how vision changes as we age that suggests that comparisons to ordinary glass optical systems are likely simplistic: Eyes on the Workplace (1988), The Aging Eye, National Academies Press.

This article does concede that the pupil tends to be smaller in older eyes and can offer increased depth-of-field in some individuals. But it then goes on to outline a number of more significant issues specific to human vision as we age that reduce our ability to process an image. Some of these include reduced transmission of light and increased scattering in the cornea, yellowing of the lens, reduction in nerve cells and especially of the cones in the retina, slower visual processing in the brain, reduced ability to deal with glare, decreased sensitivity to contrast, decreased ability to deal with visual events in sequence, reduced peripheral vision and greater tendancy to be distracted from visual tasks.

My suspicion, and I think this article supports it, is that our effective depth of field, i.e., our overall ability to process an image containing both in-focus and out-of-focus elements probably diminishes with age. I don't think the whole story is just about optics. I think it's more complex.