Irises

[PatrickS916]

Newbie question here, what is the purpose of irises for the rear sight? i have seen a few people shooting with them and keep forgetting to ask what they are. Would they help me shoot outdoors or indoors more? Thanks in advance.

[justadude]

The stock fixed aperture that comes with most sights is a little on the small side.

The iris on the rear sight helps to control the amount of light coming through to the eye. By controlling the light you can clarify and sharpen the image of the front sight and overall sight picture.

While others may offer slight variations on their use, you get in position and from a point where the sight picture seems impossibly dark start opening the iris, the image will start to brighten, the front sight will likely appear crisp. Keep going and you will notice the front sight starting to get fuzzy. Stop

Going down the other way, once again the front sight will get crisp, then as you continue, the overall image will start to darken. Stop

Open out the iris a bit and you are likely at the sweet spot for those lighting conditions.

An iris will help you both indoors or out. If you are shooting only indoors then one with a few colored filters might be helpful. If you are mostly outdoors some colored filters and some neutral gray filters can also help with fine tuning.

There are some irises out there with colored filters, neutral grey filters and polarizing filters. Some folks find the polarizers to simply be too dark so that feature is never used. The does everything irises are quite expensive and also, that level of complexity is seldom needed.

Start simple and work up.

'Dude

[PatrickS916]

[quote="justadude"]The stock fixed aperture that comes with most sights is a little on the small side.

The iris on the rear sight helps to control the amount of light coming through to the eye. By controlling the light you can clarify and sharpen the image of the front sight and overall sight picture.

While others may offer slight variations on their use, you get in position and from a point where the sight picture seems impossibly dark start opening the iris, the image will start to brighten, the front sight will likely appear crisp. Keep going and you will notice the front sight starting to get fuzzy. Stop

Going down the other way, once again the front sight will get crisp, then as you continue, the overall image will start to darken. Stop

Open out the iris a bit and you are likely at the sweet spot for those lighting conditions.

An iris will help you both indoors or out. If you are shooting only indoors then one with a few colored filters might be helpful. If you are mostly outdoors some colored filters and some neutral gray filters can also help with fine tuning.

There are some irises out there with colored filters, neutral grey filters and polarizing filters. Some folks find the polarizers to simply be too dark so that feature is never used. The does everything irises are quite expensive and also, that level of complexity is seldom needed.

Start simple and work up.

'Dude[/quote]

Thanks. appreciate the help. May hold off on getting one until it starts to seem necessary.

[justadude]

starts to seem necessary

Interesting choice of words in this sport...

You can get a basic Gehmann 510 iris (iris only, no filters) usually for between $50 an $60. Depending on where you are you might find it to be immediately helpful.

Is there someone around who might let you borrow one for the afternoon, then you can evaluate for yourself how helpful it might be.

My eyes are old, I would not be able to shoot without one.

'Dude

[ShootingSight]

This is all straight photography theory.

Lenses move your focal point in/out, and aperture drives your depth of field.

Shooting is all about optimizing depth of field so you can see both the front sight and the target at the same time.

In order to optimize your sight picture, you want to use the right power lens to centralize your depth of field between the target and the front sight, so the front sight is just at the near edge of the depth of field, while the target is at the far edge. Optimally, for most rifles, you want to use a +0.5 diopter lens to do this, so you do not have to exert the eye muscle, though young shooters often don't bother, and just rely on young eyes. Carbines and pistols take a +0.75 lens, because the front sight is a little closer to you.

Aperture size then drives how big the depth of field is around that ideal focal point. Smaller aperture gives you a bigger depth of field, though as pointed out above, it is not a free lunch. Smaller aperture also reduces incoming light, so depending on how brightly the target is lit, you will eventually start to dim the image.

[Martin Catley]

I have seen your comments on this subject before-I hope the poster doesn't mind me jumping in- the.5 you mention is added onto your distance prescription if I get it right. I use 2.5 roughly for reading so that the prescription needed for shooting will be a fair bit less than this?

[RobStubbs]

We're mixing up lenses and iris's (apertures) here, when the OP just asked about irises. I'm of the opinion that a normal non-adjusteable iris is fine for 99% of the time, left at what most are made to which is normally 1.1mm. I never adjust mine except in extreme light conditions, where the light is in my face and it appears to make the iris smaller, in which case I open mine up, to get the same sight picture. One thing to bear in mind, is that if you fiddle with it according to the light conditions on the day, the sight picture will look different. And that is counter to the maxim of keeping everything as consistent and repeatable as possible.

Don't worry about filters at this stage as again, they are far from essential. They can though help in certain circumstances, or odd lighting situations, but are not needed indoors.

Unlike shootingsite, I don't agree that shooting is about maximising depth of field. Shooting is about concentrating on the foresight and maximising focus on that. The rear sight and target are of a lower importance in terms of optical sharpness, they can (and should) be slightly fuzzy.

Rob.

[PatrickS916]

We're mixing up lenses and iris's (apertures) here, when the OP just asked about irises. I'm of the opinion that a normal non-adjusteable iris is fine for 99% of the time, left at what most are made to which is normally 1.1mm. I never adjust mine except in extreme light conditions, where the light is in my face and it appears to make the iris smaller, in which case I open mine up, to get the same sight picture. One thing to bear in mind, is that if you fiddle with it according to the light conditions on the day, the sight picture will look different. And that is counter to the maxim of keeping everything as consistent and repeatable as possible.

Don't worry about filters at this stage as again, they are far from essential. They can though help in certain circumstances, or odd lighting situations, but are not needed indoors.

Unlike shootingsite, I don't agree that shooting is about maximising depth of field. Shooting is about concentrating on the foresight and maximising focus on that. The rear sight and target are of a lower importance in terms of optical sharpness, they can (and should) be slightly fuzzy.

Rob.

Thanks for the help. i will look around this weekend to see if anyone at the match uses them

[topclass52]

I prefer to shoot iron sights, but with old eyes found the only way I could shoot 4P indoors was with a scope. Until I was loaned an adjustable iris and front aperture. I'm back to shooting with iron sights and much happier. My scores dropped a few points, but not enough to make we want to give up iron sights and go back to a scope. For me, it was a worthwhile investment.

[justadude]

The reply by ShootingSight added a complete dimension that was not part of the original query. Bottom line, get the front sight in focus and let the target be a bit blurry. If you need corrective lenses you should get that worked out soon. In my opinion, the special frames for shooters that allow you to adjust the lens so you are always looking straight through it are worth every precious penny you spend on them.

I also assume that since this is under the rifle section that the discussion is limited to aperture sights, peep rear and ring front.

The usefulness of an iris will of course somewhat be determined by your range and how variable the conditions. If you are on a well lit indoor range then RobStubbs is correct, the fairly common 1.1mm fixed aperture will be fine. If you are shooting on a dark indoor range you might really appreciate the extra light afforded by an adjustable iris. It is interesting that I use and change very regularly the setting of my iris for the vary same reason RobStubbs leaves his set in one place: To get the same clarity and appearance from one day to the next. Outdoors on a bright day the black will appear smaller than on a cloudy or dark day. While the iris does not compensate completely it does help to even things out from day to day.

Best to just borrow one and play a little bit.

'Dude

[PatrickS916]

The reply by ShootingSight added a complete dimension that was not part of the original query. Bottom line, get the front sight in focus and let the target be a bit blurry. If you need corrective lenses you should get that worked out soon. In my opinion, the special frames for shooters that allow you to adjust the lens so you are always looking straight through it are worth every precious penny you spend on them.

I also assume that since this is under the rifle section that the discussion is limited to aperture sights, peep rear and ring front.

The usefulness of an iris will of course somewhat be determined by your range and how variable the conditions. If you are on a well lit indoor range then RobStubbs is correct, the fairly common 1.1mm fixed aperture will be fine. If you are shooting on a dark indoor range you might really appreciate the extra light afforded by an adjustable iris. It is interesting that I use and change very regularly the setting of my iris for the vary same reason RobStubbs leaves his set in one place: To get the same clarity and appearance from one day to the next. Outdoors on a bright day the black will appear smaller than on a cloudy or dark day. While the iris does not compensate completely it does help to even things out from day to day.

Best to just borrow one and play a little bit.

'Dude

Thanks. I understand it a lot more. It sounds very similar to camera lenses which i understand well.

[ShootingSight]

It is EXACTLY like camera lenses.

The human eye is around a 20mm to 25mm focal length lens (the distance from your cornea to the retina). Call it 1 inch, 25.4mm to make the math easy.

On a bright day, your pupil can constrict to around 1/8" diameter. The f-ratio of a lens is the focal length divided by the aperture diameter, so on a bright day your eye is an f:8 lens. In dim light, the pupil can dilate to about 3/8", so at wide open pupil, your eye is an f:2.8 lens with considerably shorter depth of field. Here is an example of how identical focus can achieve a large or a small depth of field, depending on aperture size:




When you use a rear aperture that is 1.1mm in diameter, your eye becomes an f:22 lens (25/1.1), with greater depth of field than the naked eye ever had, even in bright light.

If you go into the depth of field math, you will see that the actual depth of field for a lens is a function of the f ratio, but also of focal distance. If you are focusing up close, even an f:22 lens will have a near distance and a far distance which are out of focus.



If you look at old camera lenses, you can see the middle row of numbers would actually tell you what the depth of field was for that focus and aperture setting. On this lens, which is focused at 4 ft, and using an aperture of f16, the depth of field of the lens would be from infinity (where the '16' indicator lines up on the right with the focus infinity symbol), all the way back to 2 feet, (where the '16' lines up between 1.75 and 2.5 on the focal ring).



If this same lens, focused at the same point, were using an f8 setting, the depth of field would only be from a little over 7 feet, back to about 3 feet.

For any given f-ratio in a lens, the greatest possible depth of field you can achieve is if you set the distant f marker at infinity, setting your depth of field from infinity back to some near distance. The focal distance, ie the distance at which the center marker lines up in this condition, is referred to as the hyperfocal distance for that lens, and at that condition, the near depth of field distance will always be exactly 1/2 the distance of the hyperfocal. This is why, with the f16 distance marker lined up to infinity, the near distance is 2 ft when the focus is at 4 ft.

So, circling back to your question: a small rear aperture gives you a higher f-ratio in your eye, improving depth of field. By setting the focus of your eye between the target and the front sight, this improves focus on both the target and the front sight at the same time.

Adding my build on your question, your greatest depth of field will be achieved if you focus at exactly 2x the distance from your eye to the front sight. I won't make my answer even longer by going into lens math and how to calculate diopter strengths, but the bottom line is that for a rifle, the lens that accomplishes this is +0.5 diopters over your distance vision correction. There are two ways to accomplish this: first is to flex the ciliary muscle in your eye, which will deform the lens in your eye to move your focal point closer. This is easier for younger eyes, but it does still have the disadvantage that getting your eyes to a consistent focal point is difficult. The second way is to leave the eye muscle relaxed and add a +0.50 diopter lens into the equation.

In the interests of transparency, I do sell lenses to shooters, so there is a degree of self-interest, though I'd also point out that as a professional in the field, and a High Power Service Rifle shooter, and an amateur photographer, I have a better understanding of optical physics and the needs of shooters than your average bear.