Aperture and bokeh

[Keith B]

No, but I am tired of the strange stares I get with my ear pinned up against the monitor.

[Jon Ruyle]

Okay. Here is my question.


Suppose two lenses have the same focal length and aperture and are both perfectly corrected. You shoot them both wide open. Do they have the same bokeh?


I guess what I'm asking is, what are the parameters that determine bokeh? Correction, aperture, blades, sure. What, if anything, else?

[samoksner]

The optical design is first and foremost the deciding factor being bokeh. The camera to subject and subject to background distance ratio also plays an important part. Lens coatings play a part, although minor and lastly when stoped down, even 1/3rd of a stop, the roundness of the aperture blades. The more blades, the easier it is to have it round.

[Jon Ruyle]

I don't think I was very clear when I asked my question. I guess what I'm getting at is that


1) it *seems* that given the image of point a some specified distance on both sides of focus, one should be able to determine bokeh exactly, modulo off axis aberrations (which are, as far as I know, always considered undesirable).


2) the well corrected out of focus image of a point (not counting diffraction) is the aperture itself.


This seems to indicate that the only way to modulate bokeh is by shape of blades, or by over/under correction. I've often heard, however, that the shape of the blades isn't really that important... that the design of the lens is more so.


It would be interesting to see a comparison of the 50mm f1.2 and 50mm f/1.8 stopped down to, say, f/2.8 using an external round aperture. I wonder how different, if at all, they would be then.