A Merit eyeglass aperture, fits onto your shooting glasses with a suction cup and allows you to adjust the size of the aperture. With the eyeglass aperture it allows you to see the rear sight, front sight and target all in focus. I am a wrong eye shooter, I am left handed and right eye dominant, this makes things interesting. Normally I will close my right eye and use the left eye to shoot, this normally works good.... But with aging eyes when I close my right eye wearing 1.5 power safety glasses, I can't see the rear sight, front sight and the target. When using the aperture that does not happen, everything is in focus. What I did, was using electrical black tape, punched out a 9/16 disc then punched out a 1.5mm hole in the disc. I attached the tape disc to my left lens of my glasses and now I can see everything in focus. A Merit sight is very expensive, around $80 plus shipping. My homemade electrical tape aperture costs 1/2 a penny, and it works.