Originally posted by newarts Orly,
Thank you for the informative posts.
Is it true that a sharp lens must always have hard edged Bokeh or is it simply a general rule?
Is the sharp-edged aperture shape seen on most out-of-focus light points simply mechanical vignetting of a wider blur function or are things more complex than that?
Dave
I'm no optical expert
The hard-edge OOF highlights are indicative of overcorrection for spherical aberration. If you correct all the residual SA in focus, then the lens will be overcorrected for out-of-focus areas. Since most people like maximum correction for the in focus image (heh!) then generally yes the OOF image will have hard edges.
But there are exceptions... in general simpler lenses have less correction but can remain sharp (at the center) e.g. Tessar formulations.
Have no idea about the vignetting etc. But do consider that when you stop down, you're chopping off the edges of the lens.. and lens designers spend most of their time correcting for off-axis aberrations. Almost any lens (even a singlet) is sharp at its center. So my guess is, when you stop down, you're only taking the center where the correction is already very good (and where correction/overcorrection is not needed as much) which is maybe why the hard edges become less apparent.
That said I've never seen Gaussian bokeh, at best what I've seen is neutral bokeh (e.g. the edges are not brighter than the center).