Re: Opton Lenses
Well, f/8 is kind of slow for a 500mm lens. Preset diaphragm and lack of autofocus is a big downer, too. Stick a x2 teleconverter on it and it becomes f/16 - that will give you a seriously dark viewfinder, I don't think you will find it at all easy to focus.
This looks like the kind of lens that was sold around 1970. The design probably hasn't advanced much, either, so I'd be astonished if the image quality is up to much.
If you want a 500mm lens and are only prepared to pay around £100, a good used Tamron Adaptall-2 SP 500mm f/8 mirror lens plus the required adapter should cost about the same, the image quality will likely be better (except for the doughnut out-of-focus highlights which all mirror lenses suffer from), it will be lighter and consume a lot less space in your kit bag and/or storage shelf, and if you decide you don't like it the Tamron should at least keep its value in the secondhand market.
Serious modern autofocus 500 mm tele lenses, or zoom lenses including 500mm settings, do of course tend to be rather expensive.
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