This is my lens, I'm sorry to say. See below for pictures.
It's my favorite modern lens, and I keep it on my camera most of the time. If my wife had her way, it would be on there ALL the time -- when she picks up the camera with a Takumar on it, she puts it right back down again. Her loss. Anyway...
None of my other lenses have this problem: very many, probably hundreds of little pin-head-sized dots on the front surface of the front element. For a while I thought they might be water spots from fog or mist, but they don't clean off at all, with brush, cloth, or Lenspen. Looking closely, it almost looks like the lens coating is missing from those spots, as they are brighter-looking than the surrounding glass.
I don't know whether they are affecting image quality -- maybe a bit. This lens is pretty susceptible to flare and glare anyway, and this may be making it worse. Pics are still plenty sharp and contrasty under normal conditions.
I have kept this lens in the best possible condition. I ALWAYS use the hood and keep the lens cap on when not in use -- the cap is on even most of the time while I'm carrying the camera around. So it's really inconceivable that any physical damage did this.
Has anyone else seen this before? On a Sigma or any other lens? I'm going to send it in for service, and I'm hoping it will be warranty service. But I'm afraid they're going to claim that I did something to it, which would be disappointing.
Here are the pics. The only way I could get the spots to show up was to have the sun reflecting right off the front of the lens, so sorry for the flare. Because the glass is otherwise perfectly clear and these appear to be just tiny spots of altered reflectance, they can't really be seen in a normally-lit photo.
And so, ugh, on to the pictures:
P.S. This is NOT an old lens. I bought it during the Great Sigma 24-60mm Discontinuation Blowout in 2008.