My favorite 10/22 has a Williams FP rear aperture and a Lyman 17A front sight. I have been thinking for a long time about giving up the iron sights as my aging eyes made it quite difficult to see the front sight clearly, no matter how hard I tried to focus on it.
A student a few months ago had a large eyecup on his rear sight; he told me it was a Merit adjustable iris, the advantage of which is that like a pinhole camera, it has a long depth of field. Found one at Brownell's ($44 with my instructor discount) and tried it for the first time this weekend. Sure enough, both the front sight and the target were in clear focus! Depending on your eyes and the light conditions, you can adjust the size of the rear aperture from .022" to .125".
I may have a few more years of iron-sight shooting left in me after all. Of course, it doesn't fit on Tech-Sights or Nodaks, but should work on Redfield, Lyman and other sights with the same threads as the Williams.
And Merit now makes one of these specifically for an AR platform rifle:
http://www.meritcorporation.com/via.html
(http://www.meritcorporation.com/images/meritvia.jpg)
Pinhole camera theory can work great under bright light conditions!
For anyone with vision issues, take a sheet of aluminum foil, make a small pinhole, and look through the pinhole without using your glasses. One's blurry vision suddenly becomes very sharp looking through a single pinhole in a brightly lit area!
It works by eliminating all the other blurred images save but one!
The drawback is that you only get one image, instead of hundreds of correlated images, and the image brightness is greatly reduced. But, the depth of field is great!
In the days before the invention of glasses, pinholes were used by some (monks, mostly), to be able to continue to read. These poor fellows could only read around mid-day. Reading by candlelight was not possible with pinholes!
Reminds me of this thread in the Adaptive Appleseed area: http://appleseedinfo.org/smf/index.php?topic=14010.0
UL