OM-30/OM-F: You set the switch to whatever number is closest to the maximum aperture on your lens. It adjusts sensitivity for the focus assist light. It's kind of a dog OM, like the OM-88/101. Taking 5 batteries really bothered a lot of people. For OM quality, stick with a single digit OM, actually designed by Maitani (he had very little/nothing to do with double digit OM bodies). Try OM-1, 1N, 2, 2N, 3, 4 etc. John