A nice illustration! FWIW 50mm is considered "normal" (i.e. neither wide angle nor telephoto) for 35mm cameras. (I'm not sure if there needs to be a reduction factor in there if you're talking digital SLRs, most of which have a different size sensor.)
If you wanted to buy another one, say. What size would be good to get? Depends on a lot of things. Also that lens you mention is a zoom lens and can take any length between 18 and 200. But some lenses are not zooms and have a single focal length--like the 50mm my old manual camera came with.
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