shutterfiend wrote in post #7684538
What does it really mean?
If this means, that if I were to stand at the door and take a photograph of the room with my camera tilted, I'd be able to see edges of the door frame at two
corners of the photograph, that isn't happening. At least not with my Canon 15mm f/2.8 fisheye.
Perhaps the 5D sensor is slightly slightly smaller than 35mm film.
A fisheye lens makes a circular image that (by definition) provides 180 degrees of coverage edge to edge of the circle. Within a small range of tolerance, a fisheye lens will have a focal length of about one third the diameter of the circle at infinity focus. Thus, an 8mm fisheye makes a circle about 24mm in diameter, and a 15mm fisheye makes an circle about 45mm in diameter.
If we include the circle entirely within the frame, the diameter of the circle can be no larger than the narrow dimension of the frame. That's why an 8mm fisheye makes a full circle on a 24x36 frame.
If we include the frame entirely within the circle, the diagonal of the frame can't be any larger than the diameter of the circle, or we'll cut off the corners. The diagonal dimension of a 24x36 frame is 43mm, which fits entirely within the image circle of a 15mm fisheye. Because the corners of the frame very nearly reach the edges of that circle, the 180-degree coverage extends very nearly from corner to corner diagonally across the frame.
We call a fisheye whose image circle fits within the frame a "circular fisheye", and the fisheye whose circle is large enough for the frame to fit within it a "full-frame fisheye", but really they are the same. The "full-frame fisheye" of 15mm focal length would create a 45mm image circle, which would (very nearly) fit within the narrow dimension of a 6x4.5 format camera. So, the same 15mm fisheye is a full-frame fisheye on 24x36, but a circular fisheye on 6x4.5. The diagonal of 6x6 is about 80mm, and thus it takes about a 30mm fisheye to serve as a full-frame fisheye.
But the lenses are designed with a little slop, because the very edges are often degraded in performance. A true 180-degree full-frame fisheye on the 24x36 format would be about 14.5mm focal length. Given how much of that coverage is crammed into the very edges of the circle, that half millimeter becomes significant. Most lenses nominally provide 180-degrees of coverage, but in practice provide a hair less in order to provide good sharpness in the corners.
As I recall, the 5D sensor is only about a tenth of a millimeter smaller in both dimensions than 35mm film's nominal size.
Rick "working with approximations" Denney