Hello all.

Recently I bought a second-hand Canon EF 16-35mm f/2.8 L USM (Mk1). I could see it was pretty much used, but the image quality and sharpness seemed OK, and the price was good. Well, I could see some semi-transparent specks on the front element, but I thought they were stains of some kind that will go off with the cleaning. And even if they don't - I thought - some front element blamishes should not effect the image quality... Unfortunately, I was wrong in both.

R.O.R. doesn't clean the specks, nor isopropyl alcohol, not even acetone (used carefully on the spot).

Do the specs affect the images? Well, yes. Especially in high contrast situations, and against strong sources of light there's much light dispersion and even light stains visible around light sources or objects reflecting strong light. This makes the lens practically useless for night photography, as the light helos/stains around street lamps etc. are just too strong.

In normal or low contrast, the stains become visible only when stopped down to F10-11 or more.

Is there anything I can do for this lens? Is it possible that the AR coating might be peeling off in those places? Could it look like that? Otherwise the lens is OK - the rear element and the inner elements (judging by a thorough inspection with a torch) seem totally OK, the images come out very sharp and all the mecanisms (AF, zoom, etc.) work fine.

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Another problem is that there's some light dispersion overall ("softness", but not related to sharpness), visible at 100% magnification - not only where the stains are, but around any lit objects. Could this be related to quite a bit of dust particles visible behind the front element when I pass through the torch light? I unscrewed the plastic lid around the front element (by 2 little holes in it) to clean the front element, but - unlike with other lenses, the front element doesn't come out easily. Looks like it's hold by 3 locks that I marked on the 3rd pic. Any suggestion to how to open the locks?

I've read somewhere that after removing the front element on this lens, it needs recalibrating. Is it true, and is it possible to do at home?

Please have a look at the pics: http://s1273.photobucket.com/user/bonini12/media/_MG_8005-2_zps90d4ca68.jpg.html?sort=6&o=0