TargetTalk

I bought an adjustable iris from my shooting glass eye doc. This gizmo is used to battle aberration. I know what aberration is, but i dont understand why its not fixed with the normal lens? Why dont they grind it so i get all the light hits where it is supposed to hit?

Some ppl, me included, have used the adjustable iris to get perfect clarity on all planes; sight, front sight, and target. As i understand, what happens is that all light exept that closest to the optical axis is "allowed" into the eye. This must mean we should get tired faster? I havent noticed?

- Not sure what you mean by "aberration." I have never heard that an iris was used to battle "aberration." Please clarify?

- Using the iris to increase your depth of field is defintely *not* recommended. The distraction bull is *supposed* to be out of focus; otherwise, you will be tempted to look at it. That is Very Bad. The problem is not that you would get tired faster. The problem is that you will be distracted by aiming and 1) lose the improtant stuff (like aiming) and 2) develop a serious case of chicken finger and/or try to release the shot when the sight picture is optimal (this is very, very bad).


- We ahve several excellent threads on the use of the iris in this forum that discuss the reasons/background for the above opinions/observations.


Steve

Ooops.

" . . . distracted by aiming and 1) lose the improtant stuff (like aiming) . . . "

should be

"distracted by aiming and 1) lose the important stuff (like aligning)"

sorry

the adjustable iris is used to cut off the light that is furthest from the optical axis, this light will/can converge infront of the retina and cause "blur on the front sight".

_Axel_ wrote:the adjustable iris is used to cut off the light that is furthest from the optical axis, this light will/can converge infront of the retina and cause "blur on the front sight".

But if your eye doctor has given you a proper prescription, then there will be no need for the iris since the front sight will be in focus. As Steve points out, the iris should NOT be used to bring the bullseye, front sight and rear sight all into focus together.

Under low ambient light/bright target lighting, the mechanical iris can be used to bring the front sight into *sharper* focus. Ditto for high/high (outdoor, sunny day) conditions.

Steve Swartz

Axel,

As Steve Swartz said, there's a lot of discussion about irises on this forum.

There are many different types of optical 'aberrations', so it's not clear which one it is to which you refer. You are right that a small aperture can reduce the problems of sperical and cylindrical aberration. If you are a spectacle-wearer, you will know, all to well, how much worse your uncorrected vision is in conditions of low light. This is when your pupil is wide open. Also, camera lenses show better performance when 'stopped down' (but not too far!). I remember my father, an hypermetropic and astigmatic physicist, broke his spectacles at work. He swiftly fashioned a pair, made from cardboard, with an aperture somewhat larger than a pin-hole in front of each eye. These allowed him to carry on working, without too much problem, however, of course, he needed all the light he could get, and he had effectively tunnel vision. Must have done wonders for his concentration!

The reason why this is the case is because, over the diameter of a pin-hole, the curvature of corrective lenses approximates to zero, like plane glass, therefore, right on the optical axis, there is no need for 'correction'. Like most things in physics, this is a naughty simplification!

Your corrective lens should, indeed, be ground to correct cylindrical (astigmatism) and spherical (short or long sight) aberration. The iris is not intended to act in this way.

Search the rest of the forum for more...

Hi,

Don't know if this'll help you or not, but a thought.

I spent a couple years on NT, all whist wearing contact lenses to correct my beyond terrible vision (power -10.50). Because my eyes were so bad, I worked with an eye doctor who did research and testing on contacts and glasses. At one point, I was a guinea pig for some anti-abberation lenses with SUPER optics. I lost the excessive "fuzz" or "halo" that appears around light sources, but more importantly I had no streaks of light or fuzz coming through my sights.

Unfortunately, they decided not to market these lenses, but they did come out with a similar product under the name of 2Clear.

For the record, contacts don't disturb the sight picture any more than a Knobloch...

I had a serious problem with cataracs causing the apperance of a hanging half moon decending across the rear apperature and interfering with my ability to properly determine sight allignment and sight picture. Surgery to replace the lens corrected the problem and I am back to my normal 20 20 vision. You would not believe the amount of movement I can now detect in the sighting relationships. The movement was obscurred by the catarac and I was unaware of it. Being unaware ment I was not able to correct properly and calls were consistantly off. Aint medical proceedures great!!! Good Shooting Bill Horton