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I typically remove the glasses of cheapo filters and use the remaining rings as stackable/adjustable unobtrusive lens hood. Sometimes I attach one ring to the filter thread of the lens to protect the front element so that I can attach the lens cap nonchalantly.
I have done the same with a filter that was cracked - I pushed the broken glass out and use the empty filter ring a mini hood. A hood that is only ~4mm deep doesn't seem to offer much protection, but it's a useful "bumper" on the front of the lens. It protects the lens filter thread from knocks, especially modern lenses which have plastic filter threads which are easily damaged. The aluminium alloy filter ring of older lenses is stronger but still benefits from some protection, and helps to keep your valuable lens in good condition. It doesn't provide much shade, but is often the difference between your fingers touching the front element or not. Plus it is very compact so hardly takes any extra room in your camera bag, and normal lens caps fit over the top - very convenient!
Thanks Akira, Roland, Nasos...@Akira, Roland, the hood is a good idea indeed! Again, I like the brushed aluminum a lot so I have sanded a couple more rings and made a small hood. Thanks for throwing the idea!
I have one older lens whose metal filter threads have been damaged and straightened with only partial success. I found an old filter with a pretty tough brass filter ring, removed the filter glass and permanently mounted just the brass ring on the lens itself as a replacement thread for attaching accessories & filters. This is not my idea - I don't quite know where I came upon it, but it works like a charm and it is actually tougher than the original thread on the lens itself.
Hugh, your post reminded me of doing the same thing to Nikkor 28/3.5 that had a bit deformed filter thread but functioned flawlessly.
I have the same lens with the same issue