As I'm headed out for a couple weeks of hiking I thought I would give my equipment a bath. I started with the rocket and gave every nook and cranny a good blowing then took one test shot to see how much dust the sensor really had. (pic #1) Not too shabby as the pic was taken at f/22 and auto leveled.
I set off for the first wet cleaning. Now I hadn't cleaned the sensor in maybe four months so I was expecting some hangers on and knew the sensor would be needing multiple wet passes before getting the sensor reasonably clean. I started with a couple of passes and took another test pic. (pic #2). Much better but that blob in the center bugged me.
Off for another wet pass, and check again. Spot still there. (pic #3) I've now cleaned the sensor maybe 5 or 6 times and the blob won't budge. One more cleaning and this time I pop on the 100mm, took another test pic and the sensor is perfect. I put on the 17-55 and the blob comes back. It is so clear that it can be seen on the LCD at f/11 (pic #5).
After a few more tests with a couple other lenses I narrow it down clearly to the 17-55. I've had this lens for a bit over a year and its seen lots of use so it has picked up some dust and it looked like a speck of dust on the front element was causing the problem.
I ran across a post a few days ago that had a link to this page:
http://www.pbase.com/lightrules/drp
So I though I would give this procedure a try. It turned out that this lens was a breeze to take apart. The inside ring is just stuck on with a bit of sticky. Once the label ring is removed the three screws are clearly visible and come right out. A tilt of the lens and the front element slides right off.
I hit the front element with the rocket, give it a wipe with a moistened pec pad and the lens is once again spotless. It went together just as easily as it came apart. The total time to take apart, clean, and put back together was less than 15 minutes. Another test shot and the blob is gone.
I never thought that dust on the front element could manifest itself as a spot on a pic and POTN common wisdom has advocated that dust on the front element would be invisible. I have heard that ever since I joined. But clearly, a rather small speck on the front element does impact a picture and can clearly be seen at f/11 and even at f/8 there is a hint of a shadow.
One other mystery solved as well. I had always thought that the front of the lenses are rather tightly sealed, but this is not the case for the 17.55. What looks like a solid plastic ring screwed in is simple a thick label and there is quite a gab between the label and the barrel of the lens.
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