Nikon leaped into the professional SLR arena in 1959 with the F1 and a fantastic assortment of accessories, such as a 250 exposure back and a Polaroid adapter. Both of these made a removable back necessary, which some Nikon fans found inconvenient. The first 21mm lens was adapted from the 21mm lens for the Rangefinder Nikons, and protruded far into the camera body, making mirror lockup necessary. A few early Nikkor fisheye lenses also required mirror lockup. Efficient use of the mirror lockup required planning ahead to avoid wasting a frame of film. It may seem that I don't like the idiosyncrasies of the Nikon F: not so! A series of F1 bodies and accessories were my main SLRs for 50 years until digital photography became more practical. Usually I preferred the simple prism finder.