View Full Version : Image circle at different apertures?

5-Apr-2008, 05:08
Could someone clarify for me...
I have a Fujinon 90mm f8, I understand that the image circle at f22 is 216, how does the image circle change at different apertures? Does the image circle reduce at f16, f11 etc, and is it greater at f32??

Dan Fromm
5-Apr-2008, 05:51
The size of the circle illuminated doesn't change with aperture.

The size of the circle of good definition grows, for a while, as the aperture is made smaller. This because some aberrations, including coma, have a greater effect off axis than on and are reduced by stopping down. Curvature of field is also reduced by stopping down. "for a while" because at some stop -- it varies by lens -- diffraction dominates the aberrations; stopping down farther than that reduced image quality across the field, doesn't improve image quality off axis.

5-Apr-2008, 08:59
Great, thanks for the info Dan