TargetTalk

Question about the position of iris diaphragm.

I can clip the iris in front of my Champion shooting lens, or clip it on the backside of lens toward my eye.

Why doe it have to be behind my lens between lens and my eye?

With the iris between your eye and the lens, you are seeing the aperture as a true size, if you fit it downstream of the lens, the aperture will potentially be increased due to the diopter factor of the lens.
The diopter is to correct your focal point in front of the front sight, not to correct the size of the aperture.
Hope that makes some sense.

Iris has an adjustable aperture diaphragm.

Let's say the effective apertures in front and behind the lens are adjusted so that effective light cone falling on eye retina are the SAME size.

What difference does it make if it is put in front or behind the lens?

The closer the iris is to your eye, the better.

[quote="conradin"]The closer the iris is to your eye, the better.[/quote]

That sounds a bit too simpleton.

As Don Nygord would say, the whole function of aperture diaphragm is to keep light entering your eyes constant under different light condition. Controlling the DOF is your own eye iris.

So I maintain it makes no difference whether adjustable aperture is placed in front or behind the lens, as long as block out coverage area in front is as big as the block out coverage behind is.

Seamaster, I agree totally, since Nygord told me the same thing, but the glasses he sold me had the iris behind the lens. (Whatever that's worth.)

conradin wrote:The closer the iris is to your eye, the better.

do no know about that, but for many the convex lens curvature on the outside stops the iris from sitting properly.

Thee reason I said that the iris should be as closed to your eye is possible is because
1. other light would reach your eye, between the lends and your eyes.
2. The close the iris, the more accurate you can set up for your eyes. It is about the irises of your eye, not the irises after lens.
3. I also did the same thing originally, but I realized that it was not effective, so I put the iris behind my lens, it made a huge difference.
4. Geometry.
5. Finally if you are a planning to put on filters, the filter should be on the other side of the lens. It is rather difficult to put both the iris and the filter together on one side of the lens. The filter does not affect the lighting effect whether it is front or left, as long as it affects the way you aim the target (front sight, target, etc).
6. If you look at Olympic rifles, the iris is inevitably the first item closest to your eye on the diopter.
I am not very good at Physics, so perhapss an optical engineer can explain it better. esp. 6 since I do not shoot rifles.

You can put it on the front of the lens just fine.

1. Yes, it could shrink or grow the apparent iris size slightly, but likely too little to notice. Depending on the lens power, you can go through the vertex magnification correction formula, but it will change by likely under 5%.

2. As long as the aperture is adjustable, size does not matter (that's what she said ...). Depth of field is determined by the f-ratio of a lens, which is the ratio of a lens focal length to the aperture diameter, or in the case of an aperture that is in front of your eye, the aperture distance to the aperture diameter. Net, a small aperture close up, or a big aperture far away will have the same depth of field.

The only advantage I can think of having it behind the lens is that it won't bump into the rear sight.

Depth of field is determined by the angle with which light rays converge on your retina. If you have a great big iris opening, the cone of light will have a wide angle, and the ideal focal point will have a very small margin of error in the distance it can be placed behind the lens before this wide cone of light is no longer focused to a point, but is a blurry circle.

However if you have a small iris/pupil, then the cone of light gathering the image has a much smaller included angle, and is pointier. Now, if you move the focal plane slightly, the concentrated point does not grow or shrink quite as fast. Net the sensitivity of focus to the focal distance is reduced - which is a bigger depth of field.