Question About Rear Of 50's 21 Biogon

[wjlapier]

I'm curious to know what the oblong piece on the back of the 1950's 21mm Biogon is for. It's near the rear element. It has two screws holding it in place and it rotates. I was told it could be removed to make it fit in the chamber of a Leica LTM body with an adapter ( I have one ).

Bill

added photo.

 

[ernesto]

It is a rear shade, that blocks possible reflecting light rays coming from the film surface, or film gate borders, that otherwise could be bounced from back metalic parts in the lens. Wides are very tricky because the light rays reaches the film gate at very narow angles, and this may produces all kind of artifacts in case there is a shining surface interacting inside the black chamber.

Most rear elements in longer lens are bigger, so this possibility is really small, but in the biogon case where the rear element is so small in diameter, if this "shade" wouldn´t be in place, there would be plenty of dangerous space just arround its outer border.

Ernesto