It's remarkable how flimsy a lens can be and still produce good pictures when there's an autofocus mechanism holding it in perfect focus. When piggybacking, that autofocus mechanism is turned off, and what's more, the camera and lens move and tilt as the telescope tracks the stars. Lenses that are very good for everyday photography can perform poorly in such a situation.
That's why older manual-focus lenses appeal to me, as well as professionalgrade autofocus lenses that are built to survive rough handling. Almost any lens from the 1970s will seem to be built like a tank compared to today's products.
Another advantage of manual-focus lenses is that they are easier to focus manually. That sounds like a tautology, but it's important. Autofocus lenses can be focused manually - some more easily than others - but they are often very sensitive to slight movement of the focusing ring. Older manual-focus lenses are easier to focus precisely.
Of course, optical quality is also a concern, and it's where older lenses often fall down. Only the best lenses from yesteryear are likely to perform well by today's standards; too many of them had excessively simple designs or loose manufacturing tolerances.
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