Do the light inspection test by taking a small flash light and, while looking through the rear of the lens, shine the light through the front and start to take a good look around the different elements. Many things you've never seen before about your lens will instantly become visible. If there is haze, it will appear as a soft glow on the culprit element. Some of this can be cleaned, it's not damaging but aggravating in terms of what it does to overall image quality. Adds a soft diffuse glow to a lot of subjects in certain lighting situations.
Pay attention to any dust particle with any kind of circle forming around it, that's early stage fungus developing. It's best to get any lens with fungus off the camera and leave it off until the fungus has been cleaned as it can spread to other pieces of gear, although contagious cases are rare unless some thing is severely affected by it.
Any dust particle which has moved in to the stages of looking like a spider web or small frost formation may be well to the point of actually causing damage to the glass. Fungus growths can emit an acid which can and will etch in to the coatings first, then the glass.
If you find any of the above afflictions inside of your lens, especially fungus, remove it from the camera and refrain from using it have it CLA'd by a professional who has the tools to take the lens apart and clean internal elements.
These defects all tend to start from moisture being introduced some where in the lens. Either a wet, dew soaked morning, the same goes for night time photography where dew has formed on the camera, or just simply being stored in a not-so-optimal environment with no silicon dessicant.
I have experienced a lot of issues with some plastics and how they interfere with external elements. Ever notice how when you buy a filter it has a ton of haze all over it, yet it's brand new? It's the off-gassing of the plastic and foam used inside that is reacting with the glass and the coatings to create some sort of haze on the surface of the lens or filter. It's incredibly common on older filters and lenses when stored in older containers utilizing products like old open-cell foam, foam padded filter cases, etc. the foam and plastic material break down over time and will cause glass to draw these materials to its surface as a 'reaction' to the breaking down of the petroleum based materials.
There is no science to it all and this information is in regard to the literally hundreds of used and new lenses that pass through my hands on a weekly basis. If you think your lens is affected by any thing at all, try and find a local person who is willing to work on things for a fair price.
-mb
