Dust BEHIND front element, how?
 

I saw some dust particles on a PD-150 lens and then noticed that they weren't blowing away with air. Upon further inspection, I realized that they were behind the glass of the front element. Now, how did that happen? These are rentals so there's no telling where all they have been and what all they've been exposed to.

A lens is not a closed system. Lots of openings for dust to enter. Especially if someone uses air to blow off the lens instead of a vacuum.

Thanks for the reply Mike. Now, that makes sense but I have never seen an AC cleaning lenses with a vacuum, only a soft cloth or canned air. Air is sometimes absolutely necessary for larger particle removal before wiping with a cloth.

That is the front surface and those tend to be fairly well sealed. The sides of the cheaper lens systems, and I place the Sony firmly in that camp are not so well sealed. The front element on the Sony may not even be a 'real' lens element but only a dust shield anyway.

Several people have removed the front element to clean it and may be able to better comment on its optical contribution to the lens, if any.

The front element of the PD150 is a plain, parallel sided piece of glass James, as so too is the second element in line. They form the outer limits of the vari-angle prism and contain and mould the wedge of fluid between them. This OIS VAP is simply placed in front of the zoom's true front element and you're probably seeing a foreign body on that surface. I'd not expect that it was too difficult to remove the VAP asembly, though like Mike says, there's lots of openings for dust to enter simply to allow the zoom's 5 moving elements room to zip about in there.

tom.

Thanks for the info. As far as disassembly, that's too invasive for me to do and is better left for the techs at the rental house.

Trying to remove the dust behind that element is like having your keys locked in your car in plain view but not being able to do a damn thing about it.