It seems Sigma 17-70mm have a fair reputation of getting dusty between the front group and middle focusing groups; and it can get really dirty fast in the wrong places. For some users it has been a problem. You can have it cleaned or save yourself a lot of money for what is a very easy job. However I have not found as many complaints as some non Canon L zooms, seems they get dusty in days (some models).
Credit goes to Zaldidun at dpr PentaxSLR forums for showing me the ring tools.
After about 9 months of occasional use mainly outdoors my Sigma 17-70mm had too much dust for my taste. I had it cleaned and adjusted before the warranty ended but in two months it got more dust
oh yes anal I know. The dust does not hurt IQ at all so if you don't feel good about taking your lens apart don't
1) You need a tool to grip the purely decorative ring around the lens that hides the screws. Take a look at the tools at the link I provided. I made my own homemade tool. Some have no luck, but remember my lens has been taken apart by Sigma before so its probably not as tight as lens never touched. The tools don't cost a lot. I don't know the right size to order, they have a universal one for $18, thats suppose to fit all, does it fit the Sigma (?).
Turn the ring toward the left (counter clockwise) to remove it. Below it is about 6 or 8 (I forgot) little screws, you will need a special Philip's screw driver made for small parts. Those screws hold the hole top piece down when you take them out you can just lift off the elements and plastic top all in one piece, there are spacers so be careful, I mean don't forget to put the spacer back on, if you do you probably should not be doing this anyways
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Gently blow out the chamber and glass, set it back on the same way it came off, put the screws back in, spin the ring back down, done. I can now clean my Sigma 17-70mm in less than 10 minuets. Its a very easy job, and you can't as far as I can tell hurt calibration. But do this at your own risk.
I would strongly advice taking the time to get the right tool to grip the ring. You don't want to touch the glass when you spin the ring. Look at the tools in the catalog. If you can make something at home great (as I did, but mine was loosened before during warranty!). This is the hardest part, the rest is easy.
There are lots of Sigma 17-70mm owners I know I read one here where their dads Sigma got full of pollen, well now you know how to get it out and never have to worry again. Hope this helps someone.
I have posted the place to get the tool they sell a black ring set for most sizes that are hollow so they don't touch the glass, this one I may get myself. The other single ones are cheaper but you have to hallow them out yourself if you don't want them touching your glass, sound harder than the job itself
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The two tools you can order, I made my own. But, my was already removed before (get the right tool
).
https://www.micro-tools.com/store/mainframe2.aspx
And a Pic with my clean Sigma of a Pentax lens. Does it take better pics? Nope, does it take worse pics? Nope, but I love having a spotless lens
And I have read some cases that were the dust was so bad it could affect IQ. But again you don't have to, and if your nervous don't do it and just enjoy your lens.