Any lens may develop internal fog if stored in poor (damp) conditions. I own all the iterations of the 50mm Summicron M and cannot say that any one has a greater tendency to fog (apart from greater age). The position with Elmars is different - this is a Tessar-type (4-element) lens, Leica chose to place the iris diaphragm right behind the first element instead of between elements 2 and 3, this lens, especially in the f2.8 version, is notorious for fogging due to evaporation of oil from the iris. I have scrapped two examples over the years because of this, the very latest version may be better (no experience of this).
Originally Posted by Hawkeye