What makes a fisheye a "fisheye"?

If both of these lenses have the same focal lengths, how is one a "fisheye" and one one not? I'm just kinda confused by this one...

Sigma 8-16mm

Canon 8-15mm

Thanks!

Coz the sigma is designed for crop sensor as a ultra wide angle lens, not fisheye! On the other hand, the canon is a full frame format lens!

My understanding is that fisheyes are just ultra wides that aren't corrected for barrel distortion.

A fisheye lens will not keep straight lines straight in a photo unless they are dead center through the frame. The closer to the edge of the frame a straight line gets, the more curved it will appear in the photo if taken with a fisheye lens (hence the "fisheye" effect). A rectiliniear lens will always keep straight lines straight no matter where they fall in the frame.
http://en.wikipedia.or​g/wiki/Rectilinear_len​s
http://en.wikipedia.or​g/wiki/Fisheye_lens

Thanks guys; I'd thought it was just a side effect of having such a wild angle of view.

The tradeoff is in field of view. Canon's 14mm rectilinear lens has a 114 degree FOV while the 15mm fisheye has a 180 degree FOV.

wow...that's a significant difference there. But, everything not dead center of the 15mm shot would look all 'gumby-fied" vs the 14, where everything would still look, nominally, normal; right?

Snydremark wrote in post #13393436
wow...that's a significant difference there. But, everything not dead center of the 15mm shot would look all 'gumby-fied" vs the 14, where everything would still look, nominally, normal; right?

Any straight line not through the dead center of the frame will be curved with the fisheye. The effect gets progressively more pronounced as you move toward the edge of frame. Its not always as crazy looking or as obvious as you'd think. Composition and perspective makes a huge difference. Take a look in the image archive to see what I mean.

The 14mm would indeed keep straight lines straight even at the edge of frame. Perspective is still critical with any ultrawide angle lens. Rectilinear lenses tend to have a stretched appearance in the corners so you have to know how to deal with it.

interesting; thanks, Mike!

I don't know if you use Lightroom but it has lens profiles for most WA lenses that removes a lot of the distortion. I have a Tokina 10-17 that is very popular for underwater photography and was able to find a profile that someone made for it that I loaded into LR which really improved the images. I found it subsequent to the ones on my site so it is not in evidence there.

To understand fisheye lenses better, read this:

http://www.unc.edu/~jo​hnsonc/Fisheye_csj.pdf

Anyone got a side by side of an 8mm fisheye and 8mm rectilinear?

Interesting read; thanks for that