4.5mm? Does that really say 4.5mm?

Yes it does. No, it isn't a typo. This is Sigma's amazing circular fisheye lens for digital cameras with cropped sensors. It produces a circular images with an 180-degree field of view. (You can also use it on full-frame cameras such as the 1Ds-series, 5D-series and film cameras; but the Sigma 8mm would be a better choice because it will produce a larger circular image.)

It's a great fun lens to use, but not the easiest to get the hang of!