At shorter focal lengths poor spot sizes will yield soft images while it becomes less important as focal length increases.
For instance, at 16" F/3.8 has a focal length around 1544mm. If used under the kinda average 2" seeing conditions that we've all grown to know and love, a star will cover 15 microns.
An 8" F/3.8 has a FL of 772mm and stars will cover 7 microns at 2" seeing.
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