I've recently bought a Sigma 8mm f/3.5 EX DG circular fisheye lens.
The image circle's diameter is a bit under 20mm [Calculated as (circle's diameter in pixels / image's width in pixels) * 24], with two circles surrounding it - apparently something inside the lens. I'm attaching an image to illustrate. I expected the image to be 23mm-24mm in diameter.
Is this the lens' correct behavior, or should I send it to be fixed ?
Thanks in advance, Uri.