Kodak Retina C Convertible Lenses

In 1954 Eastman Kodak Company's German subsidiary, Kodak AG, produced versions of the Retina camera that had interchangeable wide angle and telephoto lenses in addition to the normal lens. 

From Rudolf Kingslake, A History of the Photographic Lens

Retina IIIc, lenses, viewfinder and cases

Normal 50mm lens in place

Preparing to remove 50mm lens

Match red dot on lens to red dot on body to remove lens

Lens removed

Match red dot on lens to red dot on body to replace lens

Twist to match red dot on lens to white dot on body

Telephoto distance scales are on the left.  Wide angle distance scale is on the right.

The rear half of the lens was fixed to the camera body and the front half had interchangeable f/5.6 35 mm wide angle, f/2.8 or f/2 50 mm normal and f/4 80 mm telephoto lens front components. Here we have a Retina IIIc with its normal f/2 50 mm lens, a 35 mm lens, an 80 mm lens and a viewfinder for the 35 and 80 mm lenses. The auxiliary viewfinder slips into the accessory shoe on top of the camera. You can flip a switch on the top of the viewfinder to change from 35 mm to 80 mm. The 35 mm and 80 mm lenses also fit the Retina IIc, IIC and IIIC. One difference between the "small c" and "Big C" models was that the "Big C" cameras had frame lines for the 35 mm and 80 mm lenses built in the camera viewfinder and did not need the auxiliary viewfinder. You have to be careful to match the red dots on the lens and body when attaching a lens front component because you can damage the camera if you don't. Focusing the auxiliary lens is a procedure. First you get the distance to the subject using the viewfinder in the camera, then transfer the distance from the scale on the top of the camera to one of the scales on the bottom of the camera. The bottom scale having white letters on a black background is for the 80 mm lens. The index mark is a "T" to remind you it is for the telephoto lens. The scale with gold letters on a black background is for the 80 mm lens plus a close focusing attachment for portraits, which was a separate item. The scale with black letters on a white background is for the 35 mm lens. The index mark is a triangle to remind you it is for the wide angle lens. The scale on the barrel of the lens is a depth of field calculator. In 1956 Kodak brought out a single lens reflex camera that used the same interchangeable front component system. The Retina Reflex got a new f/4 35 mm lens for easier focusing. The new f/4 35 mm would work with the rangefinder "c" or "C" Retinas, too.

The focal length marked on a camera lens often is just a nominal figure. A +/- 5% variation usually is considered acceptable for ordinary picture taking. A 50 mm lens could be anywhere from 47.5 mm to 52.5 mm, and an 80 mm lens could be anywhere from 76 mm to 84 mm. Some lenses focus by shifting internal lens elements to vary the effective focal length and the lens would be at its nominal focal length only when focused at infinity. At closer subject distances the effective focal length would be less.

In 1958 Kodak switched to a solid body Retina IIIS that let you interchange the complete lens instead of just the front components, and was less complicated to focus because the range finder automatically adjusted for different lenses. These new "S" lenses were shared with the Retina Reflex S and later reflexes.