All normal design lenses cycle air in and out as they are focused. As
the focusing mechanism moves the main body of the lens forward to focus
closer, air is pulled in from the back. Focus back to infinity and air
is pushed out again. The whole thing gets more complex with zooms.
Internal focus lenses likely move less air in and out from outside
except when they are zooms that change length with fl, in which case
something has to fill the expanded internal space. Macros are perhaps
more vulnerable than regular primes due to the greater focusing
extention moving more air, but less so than many zoom designs.
In any case, the modest amount of dust found in most used lenses makes
absolutely no optical difference. Look at it this way. If dust were a
real problem, top end 'pro' lenses would incorporate filters (like hard
drives, where it makes a big difference), but they don't and never have.
Get a fine layer of reflective white dust in a lens and it will lose
contrast, but the few bits of darkish dust in most lenses just don't matter.
Moose
Daniel wrote:
I didn't know about the macro pulling in dust Thanks for the info
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