MrChipShoulder,

The idea is to obtain best contrast for your aiming, and to get the light intensity correct for you.

The 'theoretical correct' aperture is apparently ca. 1.2 mm.

'Modern' thought is to leave the aperture at its optimum value (1.2 mm), and to adjust light intensity solely by the use of filters in the rear sight and/or in the foresight tunnel.

WRT the rear sight, the filters can be in the aperture unit, and/or in the glare tube out front of the sight if fited.

The twin polarisers will allow you to adjust the light from ca. 60% (polarisers in phase) to zero (polarisers are crossed).

This is mostly useful for use with the modern Sius etc. targets, which are very brightly illuminated, though can be useful on very bright days outdoors.

For me, filter colours such as yellow & vermillion are best used outdoors.

Have fun

Best regards

Russ