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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?

clipped bokeh

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up vote 18 down vote accepted

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:

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would this happen on full-frame bodies also? – Paul Cezanne Feb 1 '13 at 0:18
Thanks! Yeah, I knew to expect cat-eyes based on reviews, but the mirror box clipping I was not aware of. – JohannesD Feb 1 '13 at 10:02
@PaulCezanne yes it happens on full frame bodies to (see the example I posted). It's to do with how close to the sensor the exit pupil is, so it can happen on any camera but is most likely with wider f/1.4 lenses. – Matt Grum Feb 1 '13 at 10:37

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