Stupid questions about fisheyes

Hey guys, my friend asked me the other day about why do fisheye lenses (he was talking the fish eye adapters that you buy) when zoomed out to a really short focal length like 18mm, become circular?

i.e.

All lenses produce an image circle. When zoomed all the out, the complete image circle is small enough to be contained on your sensor or film.....thus the circle

Actually, it has to do with the kind of fisheye lens involved: circular fisheye or diagonal fisheye.

A circular fisheye lens, when used with a sensor of the sized for which it was designed, will (by definition) capture 180° in all three optical planes (diagonal, horizontal [longer], and vertical [Shorter]). This results in a circular image.

A diagonal fisheye will (by definition) capture 180° in the diagonal optical plane, but not in the horizontal and vertical planes. This results in a rectangular image.

In the case of zoom fisheyes, it can be a trure circular or diagonal fisheye only at the shortest focal length (if at all). In the case of a circular zoom fisheye lens, this results in a circular image at the shortest focal length, severely to mildly vignetted images at longer focal lengths, and (usually) a rectangular image at the longest focal lengths.

A good discussion of how fisheye lenses do this may be found in the instructions for the Lens Tables sticky in the EF and EF-S Lenses forum in the Equipment Section.

You can fix that in PS CS5

hahahahaha!

for the low low price of $1700