TargetTalk

I understand what an iris does for increasing depth of field. Could someone explain why you WANT increased DOF if you're suposed to be focused only on the front sight?
I just turned 50 and have 20/20 vision as always with a rather thick (+7.5 & + 8.5) lens. I wear bifocals and have been experimenting with a pair of perscription reading glasses that are several years old- making them probably +.50 to +.75 weaker than what I now require for reading fine print. I can easily focus of the front sight with the old reading glasses, though my groups are landing higher. I suspect this has something to do with this script making the (fuzzy) bull look smaller somehow???
Any advice or comments welcome, thanks, todd.

It's no wonder you're confused...
Many think the purpose of the iris is to increase depth of field just because it does.

As Don Nygord told me when I bought my Champion shooting glasses from him: "The purpose of the iris is to maintain a uniform "pupil" size under varying lighting conditions."

You were correct in observing that changing the focal length of the eye (with glasses) will change the apparent size of the bull, thereby changing your zero. The fuzzier the bull (or bringing the focus of the eye to the front sight), the larger it appears.

Thanks for the reply, Rover. So, is the byproduct of increased depth of field via the iris a help or hinderance? I would think if the bull were in sharp focus along with your front sight, your eye would be more easily drawn to it.

Some people (including mysef) also profit the reduced iris diameter to insure a constant head position.

I use the maximum iris diameter that works for me. I find "stopping down" and making the bull sharper is a distraction because I can see it equally as sharp as the front sight. If you use too small a hole it makes your eyesight worse due to diffraction.

Okay, so I can buy the Knobloch iris for about $73 or the Knobloch Centering device for about $20:

http://www.champchoice.com/detail.aspx?ID=124
http://www.champchoice.com/detail.aspx?ID=123

If the primary useful function of the iris is to ensure a consistent head position, then is it worthwhile to just get the centering device?

Roger

I bought the Knobloch centering device many years ago. It is very useful to get the precise optical center of the related to your sigthline.

But I suppose you can achieve practically the same results with the aid of a fellow shooter watching you from the side, in order to notice any canting or turning of the lens from the perpendicular in relation to the sightline.

Just borrow the centering device. You only need it once and for a few minutes. You can also have someone work with you and skip the device altogether.

i use the iris to sharpen the sights (and red dot) i have an astigmatism so without the iris my red dot is like a w, or comet. Add this to head position and uniform pupil size. works for me, individual results may vary!

Put a prescription lens in your shooting glasses to correct your astigmatism. You shouldn't need an iris to shoot a red dot.

Personally, I use use a prescription for my astigmatism plus 0.50 diopter to pull my eye to the front sight...a single lens + iris. I just use eye protection when shooting a red dot since I'm only looking at the bull anyway. (re: Brian Zin)