Is a lens with fungus spots worth buying?

[yayas]

There's a cheap Tamron 90mm Macro floating around in local market with noticeable fungus spots.
I've seen the sample images are great and not affected by the fungus.

1. If it's offered for very cheap, say $90-100, is it worth buying?

2. I'm interested to make it my long range portrait along with my 50/1.8, is the AF speed adequate for non-macro shots, e.g. kid playing?

Thanks all.
Andreas 

[Bob Atkins]

Fungus will typically affect contrast more than resolution. If it's severe or if the lighting is just "right" you'll see a drop in contrast. Fungus will also tend to increase with time rather than decrease if you don't do something about it. Whether it's "worth it" to buy a lens with fungus is an individual decision. Certainly if the price is low enough it could be a worthwhile experiment.

The only real way to deal with it is to take apart the lens and try to clean it off the glass. If it hasn't been there too long it may be possible to remove it without damage by simply cleaning the elements. The longer it's been there the higher the likely hood that it has etched into the coating or even etched into the glass.

Some people suggest that you can "kill" the fungus by exposure to UV light (even sunlight), but even if it does (which isn't certain), you'll still have it present (dead?) inside the lens and it will still lower contrast in some situations unless you clean off the residue.

[yayas]

Thanks for the explanation.

Lens cleaning is not expensive, primes would cost around $10 while typical zooms cost $25.
Of course it's not official Canon/Tamron service center. The official would cost higher, take longer, and not too reliable from what I've heard (at least here in my country)

Having no knowledge about lens construction, makes me worry about the lens performance after cleaning.
Are focus accuracy(front-back focus), lens alignment (uneven sharpness left-right) typically affected after cleaning (because of unperfect reconstruction and callibration)?

Thanks.
Andreas