ShootingSight Rectangular Apertures (patent pending) give you the best of both worlds! A bright, high contrast image, and crisp edges between the front sight and the target. No more compromising. Rectangular Apertures improve focus at the critical horizontal juncture between the top of the front sight post, and the bottom of the target, improving aim. Rectangular Apertures give you the bright, high contrast image of a large aperture so you can see clearly, even on a dim day. Rectangular Apertures give you bright and sharp at the same time to let you see better, aim better, and shoot better!
The first number in the drop down menu for the sizes indicates what round diameter gives the equivalent brightness of that rectangular aperture.