Yeh, they have tendency to accumulate. My second pair is Zeiss 7x40BGA #G053143 (1989vintage) with FMC lenses, 7.5deg fov and 22mm eye relief and +/-7 diopter settings per ocular (no eye glasses necessary). I have used them very hard and had to replace both ocular and objective rubber cups with those scope "pop up" lens covers (they work great). The only useless thing is extra prism piece in right barrel with 2 different range-finding reticles.
I was going to have it removed, but chose not to in order to preserve waterproofness of the piece. Once the seals are broken the inert gas escapes one gets fogging und mold growth.
The 7x42 are for social gatherings such as pony racing, und ball games while the 7x40s are purely for hunting (commandobubba all the way).
