I picked this Karat-Xenar up at a Photorama show for $30, no camera. It was made for the Karat camera, the serial number puts it as World-War II vintage. Fully coated optics, had internal haze. I always thought the Xenar was a Tessar formula lens: this one is not. It is 5 elements in 4 groups. Later Xenars used the classic 4-elements in 3 groups Tessar formula, as did the Elmar. The Tessar formula was "stretched" to reach F2.8, and did not do well until higher index of refraction glass was used to reformulate it in the late 1950s. This Xenar does quite well at F2.8, better than the Tessar of the day. I also think it is better than my 1947 coated Elmar 5cm F3.5. The lens is converted to RF coupled Leica mount using a Canon 50/2.8 focus mount. The glass in the Canon was badly etched, was not repairable. I tried. This is a great use for the mount. Filter size is 29.5mm, same as the Kodak Retina. I need to try this lens out on the M Monochrom. Pairs of images at F2.8 and F4, single images at F2.8.