Under good lighting conditions (relatively easy to focus), I compared the Pentax-A 150mm f/3.5 prime lens to the Pentax FA 80-160mm f/4.5 zoom at its 150mm setting.
The 150/3.5 is a nicely compact lens, and would easily be my preference in terms of carrying it. Its manual focus characteristics are also much superior to the 80-160 zoom. And the extra stop of brightness is very important for focusing by eye, especially in low light conditions (autofocus is simply not up to the task in my view).
I focused both lenses manually, multiple trials. The results shown here appear to be optimal or nearly so for both lenses at center, and since there is no way to know for sure, the results speak for themselves as a practical matter. Lighting was not a limitation, not to my eye in the viewfinder.
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