Dust inside Lenses
Written by Dave Barth, 22 December 2008.
During use, dust can be introduced into a lens where it may adhere to the glass/air surfaces of internal elements.
There is dust on internal elements of nearly every lens (except for new lenses and the solid-glass Vivitar
Series 1 600 and 800 catadioptric lenses).
The reason a lens gets dust particles inside is because when it is focused from infinity toward a nearer focusing plane,
the barrel expands outward, and to prevent a vacuum from forming inside the body of the lens, restricting focusing,
there are air paths to allow outside air to enter the lens to equalize the pressure.
Even though most lenses have an air filter, fine dust particles in the outside
air will eventually enter a lens and adhere to the element surfaces that are exposed to air. These particulates are tiny,
but they can be seen if the lens is set to a low f-stop setting, such as f1.4, and held up to a light source.
Almost all lenses exhibit some dust particles, but they are usually inconsequential.
Dust is easy to see in 50mm lenses, it may be harder to see in longer lenses. These dust particles usually don't make any
difference in the quality of a photograph, film or digital. However, if a lens is used in a very dusty environment, such
as in dust storms, if it is a lens the owner cherishes, it can be disassembled and the elements dusted off by a reputable
lens technician. Dust particles can slightly reduce the amount of light entering a lens.
