Recently I saw a too good to be true deal on a Leica 50mm F2.8 Elmar lens. It was a M Mount and collapsible, which would be a good compliment to my 50mm Summicron DR that is heavy but very precise. The only issue is that it had haze behind the front element but for $160 it was a small risk for a potentially great lens.
Fix Old Cameras has always warned me that haze can't be taken lightly, it can either be really easy or ruin the lens. Most of my experiences with hazy lenses has been really easy. You open up the lens, use some alcohol to wipe away the haze and you're on your way. But older collapsible Leica's have a known issue where the oils in the aperture blades evaporate over time and etch lens. Etched is not haze, it looks like haze but is more like running sandpaper over the element. This ruins lenses.
At first I thought no big deal, let's put a couple of rolls through and see what comes out. Surprisingly it handled some situations really well and really bombed on others. Direct light made it extremely soft and almost unusable. When light didn't hit the lens directly it took some amazingly sharp and clear images. I waffled over keeping it but David Hancock woke me up, "Return it. There are ones out there without unfixable haze. If you want that look with a future lens, grab a UV filter and scratch it up." This had me walking back from the cliff.
Collapsible Leica lenses are an enticing bargain. They're cheap, easily found and sometimes a great value. But turn down any one you see with haze. Every Leica repair person I contacted wouldn't even take a look at it. I thought I could live with it too, but it's not fun to shoot with something your always going to worry about.