eye-iris-lens-target

[vin]

I've been told that the lens should be between the target and the iris.
Can anyone explain why?
Thanks,
Vin

[Shooting Kiwi]

The iris should be as close to the eye as practicable, otherwise the field of view would be too small.

[vin]

So, if my field of vision is acceptable, then having it on the other side is just as good?

Thanks for the help,
Vin

[zoned]

The iris should be as close to the eye as practicable, otherwise the field of view would be too small.

I use a custom mount Merit adjustable iris forward of my glasses and have a clear view of a bullseye target at 25 yards. Also no problem for 10-meter AP.

[vin]

That's great. If the only reason is field of view then I'm putting my iris on the outside where I can adjust it easier.
Thanks,
Vin

[Shooting Kiwi]

As long as it works for you, put it where you like.

However, if you end up constricting your field of view too much (because you need a tiny aperture to produce the desired depth of focus), just be aware that the edges of the field of view will be fuzzy and distorted because of the diffraction effects from the aperture edge. The closer the iris is to the eye, the less troublesome these effects will be.

[RobStubbs]

Umm, and you use the iris because ? Large depth of field is generally considered a hindrance not an advantage. You want the foresight sharp and the target slightly out of focus, ditto the rear sight. Getting them all sharp prevents you concentrating fully on what's most important, i.e. the foresight.

As with most things in shooting there are different opinions and of course there's no one 'made-for-all' solution that works for everyone.

Rob.

[vin]

My home range has natural light so I use the iris to control the amount of light I get. When the range light is too bright the front sight whites out on the edges.
Thanks!
Vin

[Rover]

The iris on my Champion glasses is designed to be on the inside.

According to Nygord, the iris is used to to ensure a UNIFORM pupil size under varying lighting conditions NOT to increase "depth of field" (sights and target sharp).

[visitor]

Oh, the law of unintended consequences! We want the iris to give proper light level, but (unintended consequence) as the pupil is made smaller the depth of field increases. It would be nice if, like a photographer, we could manipulate shutter speed to prevent unwanted iris effects; but, alas....

[vin]

Hi Rover,

If I use my iris to control light levels does it matter if it is on the front or back of the lens?

Thanks,
Vin

[David Levene]

According to Nygord, the iris is used to to ensure a UNIFORM pupil size under varying lighting conditions NOT to increase "depth of field" (sights and target sharp).

Out of interest, both the Champion and Knobloch web sites say that the iris is used to increase depth of field.

IMHO a correctly prescribed lens is used to ensure sharp focus on the sights, filters are used to reduce the amount of light & change contrast and an iris opened to 3-4mm can be useful to ensure you are looking through the centre of the lens.

[Rover]

Damn, you guys are going to make me actually have to think! My background is as an industrial photographer, so I'm fairly up on optics and their effects.

I bought my shooting glasses from Nygord and he gave me a few "lessons" with them. Because they are set up with the iris on the inside of the glass I never thought to question it. However, I coached a guy who had been nearly blinded in a smelter explosion and we ended up using one of the Merit irises on the outside of his glasses to enhance his really poor vision. He quickly reached Master class.

Because the eye is "lumpy" you do not get uniform vision as the pupil size varies. That's why Don recommended the iris; to keep the vision uniform. It had nothing to do with light levels or depth of field. Of course, it does affect both those things. It may also cause diffraction, in effect lowering your quality of vision if used too small.

As far as "centering the eye" in the glasses, that's is why a tool is available to ensure centering the lens in front of the eye while in shooting position (if you have the fancy glasses).

I know of no one who uses the iris to control "light levels". When setting up my glasses I use a diameter that gives me the best vision, then I mark it and leave it.

I recommend that you snoop around a see what "Nygord's Notes" has to say on the subject. Don was an engineer and he thought like one. Everything was subject to close examination and testing. I could only pick up so much from a few lessons and chatting over a couple of beers.

This is an interesting subject, though, and there is not a lot of info floating around about it.

[David Levene]

As far as "centering the eye" in the glasses, that's is why a tool is available to ensure centering the lens in front of the eye while in shooting position (if you have the fancy glasses).

There is indeed the lens centring device for setting up the glasses initially and checking them thereafter, but thats not what I am suggesting using the iris for.

By having an iris set at 3-4mm you actually know that your head position is correct from shot to shot. It's one of those things that you don't notice if your position is correct, but it jumps out at you if it's wrong.

[Shooting Kiwi]

There's a lot of conflicting opinion out there about the purpose and use of the variable iris. So I'll just stir up the already-muddied waters...

Everyone knows that your pupil diameter increases in low light, to let enough light into the eye for the photo-receptors in the retina to work well enough to produce a usable image. It may be less well known that, as we age, the sensitivity of the retina to light decreases. This drives our pupils to open wider than a young person's, given the same light level. However, old pupils can't dilate as much as young ones, so us oldies are attacked from both ends. Incidentally, the inability of older pupils to dilate as much as young ones means that impressively enormous exit pupil diameter specifications for scopes is overkill - there's no point having an exit pupil larger than your eye's pupil.

So, in many range conditions, old eyes will be working with relatively dilated pupils. The optical quality of the eye, like a photographic lens, is worse at large aperture. The shooting glass iris therefore acts as an artificial pupil, so that it doesn't matter what the eye's pupil is doing. It is adjusted to get the best balance between optical performance and light admission.

As a consequence of using a small aperture, focal depth is increased. As is well known, this is not always a desirable effect, but, particularly for us oldies, an increase in depth of focus over what we would get with our dilated pupil is generally worth having. At least one can see where the bull is!

The prime purpose of the iris is therefore not to achieve increased depth of focus - that's just a spin-off. Neither is it to set desired light level to the eye, for which neutral-density or crossed polarizers should be used. Its prime purpose is to negate the effect of the eye's variable aperure, and to allow the eye to operate at its best achievable optical performance. How you decide when you've got that is a matter for experiment and/or personal preference. Since the iris is trying to replace that in your eye, its best position is as close to the eye as possible.

[Rover]

What he said!

[RobStubbs]

There's a lot of conflicting opinion out there about the purpose and use of the variable iris. So I'll just stir up the already-muddied waters...
<snip>
The prime purpose of the iris is therefore not to achieve increased depth of focus - that's just a spin-off. Neither is it to set desired light level to the eye, for which neutral-density or crossed polarizers should be used. Its prime purpose is to negate the effect of the eye's variable aperure, and to allow the eye to operate at its best achievable optical performance. How you decide when you've got that is a matter for experiment and/or personal preference. Since the iris is trying to replace that in your eye, its best position is as close to the eye as possible.

I don't know where you get that idea from as it's a new one on me and sounds odd.

Most people use an iris for increasing depth of field. Some for cutting down the amount of light <and some for head centering>. The depth of field has been done to death many times, suffice to say the only situation I could see it of any benfit is to someone who really cannot see a black blob that is the target. But you really need to be careful not to overdo the iris and make everything sharp. It looks 'very nice' but will be counterproductive to good shooting.

Cutting down the light with an iris just leads to eye strain. You still get the same bright light, just in a narrower 'beam' and of course your eye's iris opens up to get in more light = worse still. Cut down light with a filter if you need to, if you get lots of glare / reflections then try a polariser.

Rob.

[vin]

Do people use polarization to reduce glare in 10m air pistol?
Thanks,
Vin

[Shooting Kiwi]

Rob wrote:

The depth of field has been done to death many times, suffice to say the only situation I could see it of any benfit is to someone who really cannot see a black blob that is the target. But you really need to be careful not to overdo the iris and make everything sharp. It looks 'very nice' but will be counterproductive to good shooting.

Cutting down the light with an iris just leads to eye strain. You still get the same bright light, just in a narrower 'beam' and of course your eye's iris opens up to get in more light = worse still. Cut down light with a filter if you need to, if you get lots of glare / reflections then try a polariser.

Quite. Which is why the prime purpose of the iris is NOT to achieve these things! Where do I get that idea from? An understanding of the optics.

[vin]

Is polarization the best way to reduce light glare in bright light conditions?
Thanks,
Vin