fwb dioptre's

[GB2017]

The adjustable foresight is also an iris so you can twist it and effectively change its diameter. Alongside the adjustable bar it was a very useful piece of kit.

had taken it all apart and wondered if it would allow focus, still cant establish exactly what the diameters are though, and finding concise documentation is a royal pita

[tim s]

had taken it all apart and wondered if it would allow focus, still cant establish exactly what the diameters are though, and finding concise documentation is a royal pita

The foresight doesn't really have any input on focus - that's all in the rear peep, or glasses.

We use a foresight ring because it gives a 360° reference against a post.

On a basic foresight iris the numbers are the diameter of the inner aperture in mm. For the 10m target, start somewhere over 4mm. You want enough space around the target that the foresight ring doesn't overlap it as you aim. That's the risk with a foresight that's too tight around the target.

Irises can also be used for 22rf shooting. Prone shooters mostly use a smaller aperture as the position is much less wobbly; 3.6-4.0mm is recommended for a regular Anschutz barrel.

Some irises also adjust the outer diameter of the ring too. Basic irises tend to get thicker as you reduce the aperture. Don't like this? Then tweak the outside so it's a constant thickness, or a proportionate thickness to the aperture. Dual adjustment irises tend to come as a tunnel.