Originally posted by Not a Number Depends on the lens.
Originally posted by UncleVanya Overall the conclusion reached by most forums is to store lenses front element down - primarily for stability - they will not fall over easily this way. However it is clear from all the reading that there is no universally accepted camera manufacturer recommended method.
I think this is basically it. Its a confused issue, but it shouldn't matter if you use the lenses fairly regularly. If they just sit there for years on end, then you should consider such things carefully. Is the barrel heavier than the moving, outer element part? Is there lots of grease in the lens? Is there dust in the environment (can enter through the mount more easily than the front element)? How stable is the position in which the lens is put?
I just don't like putting the lenses sideways, because I fear they might roll and fall, somehow. But I don't have very long or heavy lenses. My biggest lens is probably Samyang 14mm, which I store mount side up. It wouldn't be stable the other way