I have a 'kit' Ricoh lens that is K mount but was made for use with the KR-5. Its specs are 2.2/50 and there is something different from this lens that confuses me. The rear element set (behind the aperture) is rigid, does not move when lens is being focused. Only the front element set (in front of the aperture) moves. The closest focus is about 25 inches, certainly not impressive. At infinity the lens is tack sharp, even at f2.2. At the CLOSEST distance, the viewfinder indicates that 29" is in 'focus' when, on the negative, 25" is in focus. At infinity, the viewfinder indication matches the reality on the negative. That discrepancy, alone, surprises me, as always test lenses for both infinity (I have wonderful, distant skyscrapers outside my window) and closest focus.
The overwhelming number of prime SLR lenses are not like this, as all elements are moving as the lens is being focused (because the entire element set is housed in one set). My question is this: there has to be an advantage to the rear elements moving, right? But what is this advantage, in detail, and what is the disadvantage to the rigid rear element set that I have? Many early rangefinders lenses also had this 'fixed rear set' also. - David Lyga