Fisheye lenes

What is a diagonal Fisheye? link

The fish isn't yet quite cross-eyed. Just wait a day or so.

I think he means that it fills the whole sensor so not an circular fisheye.

In the context of the auction description, the vendor do doubt means a full frame fisheye - one that has a diagonal view of 180 degrees. In other words, the image fills the frame, unlike a circular fisheye of up to 220 degrees where there is a circular image within the frame, surrounded by black.

thank you. So are all the pentax fisheyes circular

No most aren't actually so far as i know.

This an image taken with a circular fisheye.

Pentax have always made full frame fisheye lenses. We have to look to other makers for circular fisheye lenses.

So much for humor.

The AOV of any lens is typically measured on the diagonal of the frame its image is projected upon. A full-circle projection on an arbitrary frame of course does not reach the full diagonal. Any frame-filling projection on an arbitrary frame would be trivially 'diagonal'. Even a full-circle fisheye is 'diagonal' on a small enough frame. I find the term "diagonal fisheye" misleading, more marketing crud on par with "35mm equivalent" and crap.factor. It does not describe a characteristic of the lens, just how the lens behaves on a given frame.

Don't worry the humor was noted.

So if I pick a focal length, say 10mm, will a normal wide angle and a diagonal fisheye have the same AOV?

No,

Rectalinear lenses loose some degrees due to correction.
F.e. Pentax DA10-17 has diagonal FOV of 180 degrees os APSC sensor at 10mm. At 17mm it's FOV is around 100 degrees, in comparison Sigma 10-20 at 10mm has just above 100 degrees FOV but obviously much less distortions than the DA at 17mm

Precisely what I thought when I read your joke!

There's plenty of humour about, but if someone asks a sensible question it helps if they get a sensible answer. Otherwise, they might just decide not to bother asking again.

Like all humour, it's all about timimg.