Why are out-of-focus highlights clipped with a fast lens wide open?
Asked 1/31/2013
4 views
2 answers
0
Using a Sigma 30mm f/1.4 on a Canon 450D, I notice that wide open, out-of-focus highlight circles near the lower half of the frame have their top edge cut off. The effect is strongest at maximum aperture. I first suspected an aperture blade not fully retracting, but the aperture looks normal on inspection. Is this caused by the camera’s mirror box, the lens design, or something else?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
21
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:
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
0
Generated from our catalog & community — verify before relying on it.
This is normal wide-open clipping/vignetting of off-axis light, not usually a faulty aperture blade.
With very fast lenses, light from out-of-focus points near the edge of the frame can be partially blocked before it reaches the sensor. Depending on the lens and camera, that clipping can come from the mirror box, mount throat, rear element placement, or the lens barrel itself. The result is cut-off or “cat-eye” bokeh highlights, especially at wide apertures and certain focus distances.
So yes: the mirror box can be responsible, but it is not always the only cause. Lens design plays a big role too, particularly with large rear elements or an exit pupil close to the sensor/film plane.
In practice, this is generally unavoidable with some fast lenses when used wide open. Stopping down usually reduces the effect.
Recommended products
UniqueBot
AI13y ago
Your Answer
Related Questions
Do beveled or rounded aperture blade edges improve image sharpness?
Why does a manual Praktica B 50mm f/1.8 look sharp in the viewfinder but blurry in the photo on a Canon 450D?
Dark specks drifting in Live View bokeh on a Canon Rebel T7 — sensor dust or dust inside the lens?
Why do out-of-focus highlights turn into partial circles near the edge of the frame?
Why does bokeh look weaker in the optical viewfinder than in the final photo?