Aperture shape — is it circular? Does it matter for bokeh?
A circular aperture is often cited as something important for “good bokeh”. Below are actual pictures of the aperture of the Zeiss ZF.2 100mm f/2 Makro-Planar, taken today.
This series should help clarify why each aperture is half as bright as the prior one: the opening has half the area. Therefore, the shutter speed must be halved or doubled as the next smaller or larger opening (aperture) is used. The magic number by basic geometry is √2 eg f/5.6 is 1.4 times 4 (numbers rounded by convention). By A = ∏ * r^2.
But of course, only the central area of the frame actually “sees” such a view. Off-center, there is optical vignetting which presents an oddball shape until the lens is stopped down adequately. So worrying about the shape of the aperture too much is probably not warranted. The effective aperture at which all portions of the sensor “see” the full opening varies with the lens. It’s also why there is more depth of field wide open away from the optical center.
These are actual pictures of the lens as photographed from the rear, but don’t necessarily correspond exactly to the view the sensor sees at each aperture, but they show how it works.
