I’m trying to build a full-frame DSLR kit on the cheap by buying used. When looking at old lenses on eBay (for example, the Canon 100-300 L), they are often described as having a low amount of haze.
How bad is this? I’m thinking in terms of how it affects the picture and how cheaply it may be fixed.
How bad is this?
It depends.
How cheaply it may be fixed.
You almost certainly would not want to spend more to fix it than a more pristine copy of the lens is worth.
Again, it all depends on the lens. Obviously one can spend several hundred dollars getting a $1,000+ lens cleaned internally cheaper than buying a new one. On the other hand, a $200-300 lens is cheaper to replace than to have disassembled, cleaned, and reassembled.
If fungus has not etched the glass, it can be killed without opening the lens up by exposing the fungus to UV light. To remove the existing haze, though, requires opening up the lens and cleaning it. If the fungus has etched the glass, the damage to the glass is not repairable short of replacing the lens element(s) in question.
If lens elements glued together are separating the elements in question must be removed, cleaned, reglued, and the lens reassembled. Probably not worth the cost unless it is a lens worth close to or more than $1,000.
If a lens is full of dust to the point it is affecting image quality it probably needs to be totally disassembled and cleaned.
Unless you know what you are doing, have the proper tools and diagnostic equipment, and know how to optically adjust lens elements and measure the results as you reassemble the lens, then taking the lens apart and cleaning it yourself will probably make it totally unusable. Once you take it apart yourself almost no repair shop will put it back together for you. Even if they do, they'll usually charge more because it is harder and more time consuming to put together a lens you haven't disassembled.
Some lenses can be partially disassembled enough to clean certain elements without necessitating involved measuring and adjustment when reassembling, just be sure to mark positions of adjustments and shims as the lens is taken apart. That is totally dependent upon each lens' particular design and where in the lens the cause of the haze is located, so you'll need to search for instructions on how to do that on a lens-by-lens basis.
Take for example, the Canon EF 100-300mm f/5.6 L you mentioned in the question. Very good copies from reputable used dealers can be had for around $300. It's probably not worth it to try and get a better price by accepting a lens with haze. Just getting anyone to open the lens up will probably cost more than $300. Canon factory service won't even service that lens. It has been out of production since it was supplanted by the EF 100-400mm f/4.5-5.6 L IS in 1998. Canon generally supports lenses for seven years after they are discontinued.
I'm going to take a different approach to this question for general use.
I’m trying to build a full frame dslr kit on the cheap by buying used. When looking at old lenses on ebay (for example the canon 100-300 L) they are often described as having a low amount of haze.
I'd suggest that this idea (buying a full frame and lenses on the cheap) is flawed.
A modern crop frame sensor with decent glass is going to perform just as well in practice as an old full frame unless you have very particular requirements. Some combinations of older used crop frame might even be better for some purposes. The combination of modern high ISO and dynamic range performance on a crop frame is really very good and while you might squeeze more out of some older full frame models, my impression is that your budget will compromise you too much for this.
I'm not opposed to cheap glass because there are some pretty good older lenses that are cheaper but do a great job. I am very wary of older lenses when you're willing to consider haze. This suggests you're willing to risk poor mechanical condition as well and both are a bad idea. Haze is asking for trouble - I'd rate this as equivalent to buying a car with a transmission problem or a house with obvious damp issues.
If you're an AF user I'd also be wary. Older lenses can develop problems with AF motors and that can game over.
Older crop frames may also have much more wear and tear on the shutter. Now typically full frame systems would have a longer expected live that the entry level crop frame DSLRs - they'd be rated for e.g. 100,000-150,000 shutter actuations instead of 50,000. But the problem is that they can be very heavily used. I'd rather have a consumer grade shutter with 15k of 50k actuations than a full frame with 100k of 150k.
Having started on 35mm film and moved to crop frame digital and used e.g. APSC MILCs with focal reducers I honestly don't see an enormous advantage to full frame, particularly older full frame models. Obviously YMMV, but I think you're better off buying decent kit that works well and reliably than trying to aim for compromised kit that may have limited shelf life left.
