[Don Schaeffer]

I have the Sigma 10-20 mm lens. At 10 mm the image is rectangular and has the look of a wide angle image not a fisheye image. Why does a 10.5 mm fisheye lens produce a circular image that covers a wider view angle than my lens which nominally has a shorter focal length?

[matthew]

The short answer is that because the fisheye lens is allowed to distort the image, there's no direct connection between its focal length and field of view. While most rectilinear lenses have barrel distortion, and some can be quite pronounced, it's different from the squishing effect of the fisheye.

The long answer is here: http://www.bobatkins.com/photography/tec..._view.html
Interesting stuff, but it's a little too early in the morning for me to make much sense of it.

[Don Schaeffer]

In other words, the semi-fisheye is a corrected fisheye? I did note that the fisheye lens covers aprox 180 (or 190 degrees) the 10 mm semi-siheye covers 109 degrees.

[matthew]

There is no semi-fisheye - it's just wide.

'Focal length' is just the distance from the centre of the lens to the point where it focuses when it's set to infinity.

A wide-angle lens has its centre closer to the focus point, giving it a wider field view.

It's the design of the fisheye lens that sets it apart, since its optics are intended to distort perspective and decouple the focal length from the field of view.

In the Olympus system, there's an 8mm fisheye and a 7-14mm rectilinear zoom. That just means that the optical centre of the lens is further away from the sensor in the fish than it can be in the rectilinear lens.

[Don Schaeffer]

Thanks Matt. Brilliant answer.