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Do you have access to a lens collimator and other equipment needed to ensure proper alignment when reassembling the lens? If not then you may just as well use a sledgehammer to disassemble the lens; it'll work just as well afterward.
Before you start disassembling your lens, let me ask this: Do you have a lens collimator and other equipment needed to ensure proper alignment when reassembling the lens? If not, you may as well disassemble the lens with a sledgehammer; it'll work just as well afterward.
Dust and other small debris is drawn into the lens due to the vacuum
created when zooming in. You can reduce the chances of drawing in dust
and debris by slowly zooming in as opposed to rapidly zooming in.
Now that you how it gets in and how to help prevent it from getting in
again, comes the bad news. There's really no way to disassemble the
lens to clean it up at home. These are precision optics with many small
moving parts that really need to be serviced in a dust free
environment. You best bet it to contact Tamron (or authorized servicer)
to learn how much it will cost to be cleaned by them. This will keep
the warranty (if any) intact, too.
The dirt has gotten onto the front element of the rear lens group. You do not want to try partially disassembling the lens to clean it. Either Tamron or a good camera lens repair technician should be able to perform what is called a partial internal CLA (cleaning, lubrication and adjustment) of just rear portions of the lens for fairly cheap. Alternatively you may be able to pick up a used Tamron 28-200 in excellent condition for less than the repair cost of your current lens. If successful, then you could turn around and sell your 28-200 for parts or sell it to someone who knows how to service it.
You're probably right about the lens. Tamron lenses have a flexible circuit that drives the aperture blades. The aperture might need to be cleaned or the flex might be worn out. Either case needs a repair shop to correct.