In article <19970105174100.MAA27277@ladder01.news.aol.com>, ksin8808@aol.com says...

>First, let me preface things by admitting that I'm a relative novice. I
>have a Sigma 28-70 f2.8-4.0 lens for a Canon EOS mount. Yesterday, I took
>it off the camera body and was looking through it with a lamp in the
>background (from both directions). While I was zooming in/out, I noticed
>that around 50mm there were several spots (looked like dust particles)
>visible (when looking through from the back of the lense to the front).
>At the other zoom points, I think they were blurred out.
>My question is this: are those spots defects in the lens or dust
>particles? If the latter, how can I clean it? I received a very high
>quality lens cloth (some new patented microfiber thing) which I tried on
>both the front and back lens surface. It helped somewhat, but not with
>these particular spots; I'm guessing it cleaned off the dust on the front
>and back lens surfaces but maybe there's some trapped inbetween somewhere
>(or on the inaccessible sides of the lens surfaces).

Don't worry about it - dust (and other minor flaws) inside lenses are
not uncommon, but rarely cause problems (oil and other large-area events
on the glass will degrade image quality - but do not get into frequent
lens cleaning, at least before knowing how to do it with a reasonably
low expectation of resultant lens damage... [consider scratches as
permanent BAD "dust"...]).
Hope This Helps