I have a Sigma 18-250mm F3.5-6.3 DC Macro OS lens that I use exclusively with my Nikon D5300. I went to Denmark last year, and visited Rubjerg Knude Lighthouse. If you've ever been there, you know that the lighthouse is surrounded by a type of sand that is so fine that the grains cannot be seen separately. Long story short, some of this fine dust went into the lens apparently. Since that moment, the autofocus motor on the lens makes strange noises when it wants to work, and most of the time, it is stuck until I give it a push by moving the manual mechanism a bit. Is there a way that I can clean-up inside of the motor so that the lens goes back to normal working condition?
Here’s what I did in the end:
First of all, opening up the lens is not a good idea. It’s a complex and delicate system, and if you don’t have experience in this area, avoid disassembling the lens.
To clear up the dust from within the focus ring you need to blow air from under it. In this model, this was not possible from outside. So I got a power duster spray, put the zoom to max (250mm in this case), and blew the air through the exposed parts of the inside of the lens, particularly behind the yellow cable.
I think I got almost all the dust out, and the lens is working fine now. But please note that the amount of dust was not a lot, and it didn’t really compare with lenses damaged from color runs.
No, there is nothing you can do yourself. This is one of those cases of, "if you have to ask, then you don't know enough about it to do it yourself".
The lens needs to be sent to Sigma (or a Sigma authorized repair center) for cleaning and repair. Lens motors and mechanics are not designed to be user-serviceable.
Damage or malfunction due to fine-dust conditions are some of the worst things to repair or clean. For instance, LensRentals.com has specifically banned uses such as photographing color runs, where colored fine powder is purposely shot at the runners (yeah, I don't get it. Seems like a bad thing for your lungs). Even LensRentals's full damage waiver, which covers damage from things like being attacked by a bear, doesn't cover color runs or events like Burning Man. For a good article about just how bad fine dust is for cameras and lenses, read the blog post by Roger Cicala, founder of LensRentals.com: How to Ruin Your (or Our) Gear in 5 Minutes (Without Water)