Clipped bokeh circles at max aperture with wide aperture, short focal length lenses

Question

I recently purchased a Sigma 30mm f/1.4 lens for my Canon 450D and noticed soon that at least wide open, any bokeh circles in the lower half of an image (in landscape orientation) seem to have their topmost part cut off. The upper half seems to be fine (not demonstrated in the image, sorry.)

At first I thought that maybe there's an aperture blade that does not retract completely, but based on visual inspection that does not seem to be the case. Might it be the mirror box itself that's the culprit?

Answer 1

Yes you are looking at clipping by the mirror box, I'm afraid it's unavoidable when using a lens with large aperture and exit pupil close to the film plane at certain focusing distance. Even if the mirror box doesn't clip the light cone the lens barrel will (for off axis points of light) leading to cat eye bokeh.

Here's an example from a full frame camera that clearly demonstrates both effects. It was shot with the Canon 85 f/1.2L at a close focus distance:

Answer 2

Yes it is due clipping. But I'm not sure it always is the mirrorbox that is to blame. For example the EF 85mm F1.2L has a rear lens element that actually goes into the mirrorbox. So here I suspect that it is the lens design itself that is to blame - see example of bad clipping using the EF 85mm F1.2L II here:

https://petapixel.com/2017/12/01/shooting-portraits-christmas-lights-ordinary-bedroom/

And Nikon F mount lenses are more suspectible to clipping than Canon EF mount which seems to be due to the size of the mount (F mount is smaller than EF mount).

So IMO I think that clipping happens because of the rear lens placement in combination with its diameter. The mirrorbox may be the reason for clipping in some lenses but not all of them.