Fungus question

I'm looking at buying a lens on Ebay. The owner states that there is a small spot of fungus inside the lens but it won't affect the photos. Can someone explain this subject to me? How does fungus get inside of a lens? Does it get worse over time? Does it affect the photos? Would you buy a lens that has fungus inside of it?

Fungus usually is a result of moisture, and yes it can get worse. I would pass; however, they can be cleaned but then you would need to include the cost of cleaning with the total to help decide if it is worth the purchase.

It is good he is up front about it. Sometimes it is a non issue, but other times it can spread. Unless the price is really cheap I would avoid it.

As said, its caused by moisture and yes it can get worse. Im assuming the seller said it dosent affect the image because its on the inside of the front element. Do you live in a really humid area? How great of a price are you getting on the lens?

Fungus comes from moisture and air and yes it will grow. Fungus on the outside can be cleaned but inside would require professional dis-assembly and cleaning.
The seller probably did not follow the advice in these articles.

www.ehow.com/how_6719397_fungus-removal-lens.html
www.ehow.com/how_6345894_keep-fungus-out-camera-lens.html

And this is a good article on what it is and how it will ruin the lens.
www.chem.helsinki.fi/
On the The Department of Chemistry - Laboratory for Instruction web site,
Toomas Tamm offers this info.

Does it affect Image Quality?
In its early stages, fungus as seen above does not show up on the photos. It is actually pretty dark, and since images of no lens surfaces show up on your picture, you will never get an image like those above by accident. The fungus usually starts its life in the baffles and dust in the lens barrel, and no visible effects may be seen for years. Once it reaches out onto the glass surface, it will slowly cover it with a mesh of lines. The first effect is probably very slight loss of image brightness, followed by increased flare from light reflecting off the growth. In its final stages, the lens surface may become etched by the chemicals excreted by the fungus, at which point image sharpness becomes affected. You will probably be able to use the lens for several years before it reaches this stage. Once you get there, you will need to replace the lens.

and
What Can I Do to Remove the Fungus?
Once it is there, there is very little you can do yourself. You may slow down or stop the growth by following advice from the previous section. I have heard suggestions of using ultraviolet radiation (sun rays) to kill it, but since parts of the growth are outside of the glass area, this is probably only a temporary solution. Some types of glass are not transparent to parts or the whole of ultraviolet spectrum, and the radiation will be absorbed before ever reaching the fungus. As a chemist, I have also thought of poisonous gases, but (1) they may kill you before they kill the the fungus, and (2) they may corrode the mechanics of the lens, so better do not try this.

Summary - Just say NO.

The lens was in an environment that encouraged fungus spores to grow: high humidity, spores present and food. Fungus spores are everywhere so all you need is the moisture (high relative humidity) and food. The food is organic dust -- a classic food source is organic fabric camera bags.

If the fungus is very minor it will have a negligible effect on the photos, just as a little dust inside a lens is no big deal. As the fungus increases in size it of course shows up more in the photos, primarily as a loss of contrast. If it's really bad the lens becomes unusable.

Remove the moisture and the fungus stops growing, but fungus spores are everywhere so....

The fungus itself can be cleaned from the lens however if it's on internal surfaces the lens must be taken apart -- big ($$$) job with a modern zoom lens. Cleaning the fungus however will not remove damage caused by the fungus. As the fungus grows it excretes acids that etch the lens coating and even the glass itself. This damage is permanent and if it's big enough the lens is compromised.

I would not buy a lens with fungus inside it. Fungus inside a lens dramatically devalues a lens.

Darrell wrote: It is good he is up front about it. Sometimes it is a non issue, but other times it can spread. Unless the price is really cheap I would avoid it.

Yea, I was also thinking that's it's good the seller is honest about the fungus, however it's not IMO cheap enough for having a fungus issue.

john_m wrote: As said, its caused by moisture and yes it can get worse. Im assuming the seller said it dosent affect the image because its on the inside of the front element. Do you live in a really humid area? How great of a price are you getting on the lens?

I don't live in a really humid area, the lens is going for $500 for a 70-200 f4

Thank you to everyone who responded, after reading all your answers I think I am going to pass on this lens. Sounds like it will give me more trouble then it's really worth. Thanks.