For me it was the motor drive. Although not as many the Kodak IIIS had 4, 35, 50, 80 and 135, the 28mm needed a separate viewfinder as did the 200 which did not couple to the rangefinder. The lens were interchangeable with the Retinaflex I think the only lens set that could be used either on an SLR or rangefinder. With the lens with cams that coupled to the rangefinder the correct viewfinder was selected. The downside is the top shutter speed of 1/500 and fiddley way it works, setting the frame count, the EV metering, although metered it is not coupled. The SP was much easeir to use as was the Canon 7.