On mine I fitted the very neat Kodak light meter. This was not coupled to aperture control, but neither is the in-built one on the IIIC. In either case better to use a Weston V or Weston Euro Master.
Background
Kodak bought the German factory Nagel Camerawerk, Stuttgart in 1931 and owned it apart from the war years up to 1969. As far as I know all Retina cameras were built in this factory.
Interchangeable lens
The Retina IIc (big or little C) support interchangeable lenses (or to be correct, interchangeable front elements). So as well as the standard 50mm lens you can also shoot with 35mm and 80mm.
The lenses used were supplied by two different quality German optical companies
Schneider
Rodenstock
Note: You cannot mix these two types, a camera fitted with the standard Schneider Xenon lens cannot be used with the 35mm or 80mm lens from Rodenstock, and visa versa.
Standard Lens
The standard Xenon lens was f2.8 50mm, and some people believe this to be sharper than the f2.0 variant fitted on the Model III. It is a six element lens. The front three elements, just after the aperture stop are part of the lens removed when the 35mm or 80mm front elements are used.
The Rodenstock Heligon lens is a very similar design.
The front elements are closely matched to the rear elements and the serial number around the front of the lens should be the same as that above the lens that remains fixed to the camera body.
However if you change lens for 35mm or 80mm of course these have not been matched, but no one then seems concerned about this?
The standard Xenon lens was f2.8 50mm, and some people believe this to be sharper than the f2.0 variant fitted on the Model III. It is a six element lens. The front three elements, just after the aperture stop are part of the lens removed when the 35mm or 80mm front elements are used.
The Rodenstock Heligon lens is a very similar design.
The front elements are closely matched to the rear elements and the serial number around the front of the lens should be the same as that above the lens that remains fixed to the camera body.
However if you change lens for 35mm or 80mm of course these have not been matched, but no one then seems concerned about this?
How it performs
The IIc is a joy to use. Because the shutter is a leaf shutter it is very quiet with no discernible vibration.
The camera uses the EV (Exposure Value) approach. This takes a measurement of light say 4 which coincides to a specific aperture-shutter combination say F8 at 1/60 sec. This value is set and then you move both aperture and shutter locked together so that the exposure remains constant. For example as you turn the shutter ring to 1/125, the aperture will change to f5.6.
Different lens
Although this seems a good idea in practise it is very fiddly These are the steps to focus the lens once it has been fitted
Focus using range-finder in normal way.
Read the distance on the normal 50mm setting.
Turn the camera upside down and locate the scale for 35mm or 80mm lens.
Turn the lens so that the correct scale pointer has the same value as the 50mm.
Turn the camera correct way up and re-compose.
Take photograph (assuming subject has not moved).
Further if you have fitted the 80mm the lens, which is very large, then you cannot close the camera.