TargetTalk

question? how come the image seen through a small hole such as an iris is more defined and clear than without? does it have anything to do with the amount of light transmitted through? can any one provide a technical explanation of this effect, i tried to search or related topic on the web but none of the articles address exactly this question.


thank you

Try searching for the photographic expression "depth of field". The smaller the lens aperture, the more things will be in focus.

I'm afraid that's the limit of my explanation, other than to agree that it does happen.

It is not always a good thing, remember that if you reduce the aperture you will also be reducing the amout of light reaching the eye. That is why, on a camera, if you reduce the aperture you need to increase the exposure time.

Google up "pinhole" "pinhole effect" "pinhole camera" or even "sunspots*"

Or try Nygord's Notes (which I think have been copied to our TenP files?) for something more directly shooting-related.

Steve Swartz

*for reasons that will become obvious when you read about how to safely observe sunspots

Also check out http://www.starreloaders.com/edhall/nwongarts.html for a compilation of information provided by Dr. Wong an eye doctor and a very active bullseye shooter.

If your eye lens is perfect, then whatever you focus on will NOT be any clearer with the iris.

With the iris, depth of field is improved, so items you are NOT precisely focused on WILL be clearer with the iris. So although you cannot see the front sight any better, or the back sight any better, you will be able to see both sights at once better. This makes it easier to align the sights with each other. Technically the bullseye will also be clearer, however that is much less important.

Also, if your eye lens is NOT perfect, then the iris focuses the light through
a smaller portion of the lens. For most people, this will remove at least part of the lens portion that is different than the rest, improving the image.

At the extreme, with a zero size hole through the iris, you would be able to see all objects at once perfectly in focus regardless of eye lens imperfections or regardless of whether you eye had a lens at all. Of course, the image would then be completely dark, so you would not see anything. The properly adjusted iris is always a compromise between image clarity and image brightness.

The iris is only useful in relatively dim lighting. In bright sunshine such as outdoor Free Pistol shooting, the natural iris of your eye contracts small enough that you don't need one on the glasses.

One disadvantage of the iris is that it can straighten the light through your eye, to the extent of making visible small "floater" particles in the interior of the eye. I often have to wait for these to move out of the way before I can see well enough to make the shot.

I find it helpful to use an iris set at ~5 mm diam. Sufficient to help me keep my head in position, and my eye in line with the optical center of the lens, but not small enough to increase depth of field or darken the image.

Fred

Does anyone know if exercising the eye muscles outside of normal use (forcing a close distance focus in repetitions) helps or damages vision?

Sounds to me like the See Clearly Method. I have no idea personally whether or not this system works, but in general, moderate exercise of any muscle might help, while extreme exercise can damage something.

The See Clearly advertising indicates that glasses make the eye worse, but eliminating the need for the eye to keep things in focus so it gets out of practice. This would have a double effect, the focusing muscles get weak from lack of exercise, and at the same time the lens tissues that are no longer being focused tend to drift to a different permanent shape. Proper eye exercises (and I don't know what "proper" is) might correct these problems, reducing or eliminating the need for glasses.

Some claim See Clearly doesn't work for them.