TargetTalk

THe greatest depth of field is achieved by centralizing your focus between the sight and the target. In optics, diopters were conceived as a unit of lens power exactly to make this simple, because it becomes a mathematical average.

For a rifle, the rear aperture drops out of play, because you are never going to focus that close, so you average between focusing on the target, and focusing on the front sight. Let's take a hypothetical 30" sight radius rifle, shooting at 50m, and further assume that the shooter has perfect 20.20 vision.

Diopters are the inverse of a lens focal length, in meters. So a 30" sight radius (note, this should be measured from the eye to the front sight), is 0.762 meters, so to focus perfectly on the front sight requires a 1/0.762 diopter lens = 1.31 diopters. To see a 50m target perfectly, you would need a 1/50 = 0.02 diopter lens. To centralize your depth of field between the two, you simply average the two diopter values, so (1.31 + 0.02)/2, or a 0.67 diopter lens.

Since your eye can always exert slightly to add power, but can never subtract power, you would take your 0.67 solution, and round down to the nearest available lens power, +0.50, or if you can find a source of 1/8 step diopter lenses, a +0.625.

Since the target is always far enough away that the diopter value of the target lens is always close to zero (a 10m target only requires a 0.10 diopter lens, and the human eye typically cannot see steps smaller than about 0.125 diopters), averaging the lens to see the sight with a near zero value will result in a shooting lens which is half the power to see the front sight. Since diopters are inverse focal lengths, a lens half the power to focus on the front sight will result in a focal point that is 2x the distance to the front sight. So the recommendation to get the maximum depth of field possible will require a lens that focuses at 2x the distance to the front sight is not some old wives rule of thumb, but is actually an exact calculation of what is known as the hyperfocal distance of a lens.

For pistol, in my experience fitting shooters with lenses, the same math is applied, but you base it on the distance from your eye to the rear sight. THis will centralize your depth of field between the rear sight and the target, so the front sight will appear slightly sharper than either the rear sight or the target, which is how most pistol shooters like it.

Art, I agree with your physics, however in pistol shooting with open sights (no aperture), the goal is different. Since the rear and front sights are far enough from the shooters eye to focus on both and the alignment of the front and rear are so critical to pistol, the target becomes much less significant. It is the pistol shooters goal to achieve the sharpest sight alignment possible and place that alignment in position to the blurred target. In rifle shooting the rear aperture greatly increases depth of field which will allow the target to be in relative focus and it provides a precise alignment reference. This is a luxury pistol shooters do not have. This is why we sacrifice the sharpness of the target to attain the sharpest sight alignment focus possible. In other words, our optics must yield the sharpest front and rear sight alignment possible, which by definition will yield a blurry target. Hence focal length should be just behind the front sight in pistol.
Gort

Gort,

You are correct; I cannot speak to personal preferences. My math is based only on the physics of achieving the greatest depth of field possible.

However, that said, I have tested many pistol shooters with different lenses, including some who were trying out for the Olympic team, and the empirical result matched the physics, with the caveat that the depth of field math is based on the rear sight. This puts the front sight more solidly in the depth of field, so the front sight will be more crisp than either the rear sight or the target. Again, this is what I have found with most people, however I'm certain there are some who prefer it differently.

If you do want to get your focus in close, the cheap way to try it is reading glasses. As I mentioned, diopters are the inverse of focal length, so assuming you have good distance vision, you can go to Wallgreens and buy reading glasses. Typically, these are available in 1/4 steps from 1.00 or 1.25 up to about 3.00. The focal lengths are as follows:

1.00 diopter will focus at 1.00 meter = 39"
1.25 diopter will focus at 0.80 meter = 31.5"
1.50 diopter will focus at 0.66 meter = 26"
1.75 diopter will focus at 0.57 meter = 22.5"
2.00 diopter will focus at 1/2 meter = 20"
2.50 diopter will focus at 0.40 meter = 16"
3.00 diopter will focus at 0.33 meter = 13"

If you want to get your focal point behind the front sight, that will be somewhere around 26" away (for me, the rear sight is 24" from my eye), which would suggest a +1.50 diopter. So with all due caution, because reading glasses are not rated for imapct, so I would not actually shoot with these, but you can do a $5 experiment by buying a pair of +1.50 glasses and just aiming with them to see if you like it.

Final point about depth of field being a luxury pistol shooters don't have. It is easy enough to put a small aperture on your eye glasses by getting opaque tape with a small hole on the lens. I make some of these, and if you want a few, email me your address and I'll send you some. Someone makes the 'Eye Pal' that they sell for $80 which is exactly the same thing. Mine don't have the fancy packaging, which is why I geve them away for free.

Art, I use An adjustable iris (aperture) on my Champion shooting glasses, however it is a two edge sword. Yes it increases depth of field but it sharpens the target enough that without the discipline to keep intense visual acuity on the sight alignment and just be aware of the target in the background, one will start looking between the front sight and target, with disastrous results. That is why most do not recommend apertures and relatively few use them.
Gort

Gort wrote:Ragnar Skanaker is a special case and a special person, however I was inquiring about us mere mortals. I doubt it is in the cards for most of us in the 60's, 70's and beyond to be as competitive as we one were, no mater how rigorously we train. I was talking to an Orthopedic Surgeon yesterday about this topic and he told me that after one hits 35 we lose about 10 percent strength and stamina per decade. Fitness may determine a starting point, but the rate of decline is somewhat inevitable. I am hoping to find some morsels of wisdom and technique to stave off the march of time.
Gort

I hear you. I am 60 this year, and still at times shoot as well as ever. I live at high altitude and tend to shoot poorly when I go even higher, or if is hot, and windy and I get dehydrated.

I go down to the Banana belt, and I shoot fine.

Don't just worry exclusively about your arms and shoulders. They help but your core strength is just as big a factor in your hold and your match endurance.

I have found squats the most efficient way to build core strength, but anything that gets your legs strong, and your heart rate down will help a great deal.

The elderly people I have seen whose skills deteriorate the fastest, are the ones who get on a boatload of pills to control the conditions that can usually be managed far better with diet, and exercise.

I am particularly suspicious of type 2 diabetes medications, and most blood pressure medications, based on what I have seen happen to some of my fellow elderly shooters.

Your mileage may vary.

Isabel dear, 60 is far from "elderly".

Being "pre-cataract surgery", indoor lighting is usually too dim for me to see the sights clearly. I also have "floaters" in my eyes, which are more apparent with dim lighting at the range. One other thing, is that the older I get, the more emotional, so match pressure rears it's ugly head more often. Keeping generally fit and muscles toned is more of a chore than when younger. I get that little twitch once in a while, when ready to break a shot which results in a poor shot. I do enjoy participating in matches, especially when very young folks are competing. I get a lift from seeing them there. It isn't about winning any more for me, more about just the experience and breaking the daily routine.