Leica M3, with 50/1.5 Summarit
Improvements over the IIIf , the current flagship of the Leica line are numerous. The finder was vastly improved. It became a larger, combined rangefinder and viewfinder with framelines that are keyed by mounting different focal length lenses, and which are parallax corrected. The framelines may be preselected for framing with a front mounted lever. The finder shows the field of view of the 50mm lens, with framelines for 90mm and 135mm. Wide angle lenses had optics attached to the top of the barrel that would convert the finder to 35mm. These optics are referred to as "eyes" or "spectacles".
This was the first model of Leica to use a rapid lever advance instead of a knob. Early cameras were "double stroke", requiring two short pulls on the lever. After camera number 915,251, it was changed to a single stroke. The engineers thought that a single stroke would tend to tear out sprocket holes in the film.
This was also the first Leica to use the new shutter mechanism in which all of the shutter speeds are on one dial, which does not spin during winding and firing. On previous models the shutter speeds were split, with the slow speeds on a dial on the front, and the fast speed dial turned with the shutter, which allowed you to set the shutter speed accurately only when the shutter was wound. The non-spinning shutter also allowed for a clip on exposure meter to couple with it. The shutter speeds changed at camera 854,001 to a more mathematical progression.
The lens mount was changed from the old 39mm thread to a quick change bayonet. Adapters are available to use the screw mount lenses. Back focus remained the same, and they still couple to the rangefinder.
The camera still loads from the bottom, but a back door of sorts opens to take some of the guesswork out of loading. The openning the door also resets the film counter.
There are some smaller variations, such as early teardrop shaped, and later round strap lugs, and a change from early glass pressure plate to metal. Production started at camera 700,000, and very early cameras have many differences than later ones. Production ended at camera 1,158,995. Many people consider this the finest rangefinder camera, or even camera of any sort ever built, not just for features and design, but for quality of construction as well. Cameras with serial numbers over 1,000,000 command a premium, and cameras over 1,100,000 a greater premium yet.