The Leica 40mm Summicron-C was originally created to be paired with the analogue Leica CL, a compact rangefinder that was made between 1973 and 1976, and has an M bayonet mount. Leica offered the compact camera system that consisted of 1 body and 2 lenses, the other being the 90mm f/4 Elmar-C. In order to keep the included standard lens compact, it was agreed upon that the unusual focal length of 40mm would replace the need for a 35mm and 50mm lens. The 40mm Summicron-C is the second version of this lens and the first being the 40mm f/2.8 Elmarit-C, which is even more compact, and was produced in much smaller quantities compared to the Summicron version. The 40mm f/2.8 Elmarit-C doesn't perform quite as high in performance as the Summicron due to its optical design, but it laid the initial groundwork for the design of the 40mm f/2.0 Summicron-C.
With the lenses aperture stopped down, the 40mm Summicron-C performs sharp throughout the frame, and when opened up, the lenses' characteristics begin to shine. At f/2.0, this lens offers an even glow in the highlights and minimal swirl effect in the out of focus areas. The 40mm Elmarit-C has performs similarly to the 40mm Summicron-C stopped down but is slightly softer wide open.
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Both lenses are great options if you are looking for a standard lens that is ultra-compact, lightweight, and offers good performance. The 40mm Summicron-C was produced in much larger numbers and is more accessible in price point, which ultimately makes it a great starting lens for one's first Leica if you don't yet know if the 35mm or 50mm frame is right for you.