"Diopter" sights?

[Slater]

Can anyone tell me exactly what is meant by "diopter"? I have a PTR-91 on layaway that supposedly has this type of rear sight.

[Outlander Systems]

Can anyone tell me exactly what is meant by "diopter"? I have a PTR-91 on layaway that supposedly has this type of rear sight.

di·op·ter
dīˈäptər/Submit
noun
a type of drum-rotatable, back up iron sight, with a selectable hole aperture of varying size; commonly thrown in the garbage for a flip-up iron sight.

[26 Inf]

Diopter - according to H&K it means 'as seen through.'

They generally consist of a rotary drum rear sight with different sizes of diopters, the appropriate front sight for the diopter system is a circle (aperture) with a post.

The principle takes advantage of the eye's (and brain's) ability to align concentric circles very precisely. The differing diopters are used to adjust the sight picture for shooter preference (amount of light around the circles - less is better) or cheek weld. I usually run/ran #2.

Here is a page that at least illustrates the sight alignment principle:

http://www.hkpro.com/index.php?optio...opics&Itemid=5

A traditional aperture sight with a front post only also takes advantage of the eye's ability to place object's in the center of a square or circle, although not as precise as a concentric circle sighting system.