Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Sigma 4.5mm F2.8 EX DC HSM Fisheye is the longer of the two lenses at 78mm. The Sigma 17mm F4 DG DN C with a length of 51mm, is 27mm shorter. Besides being longer, the Sigma 4.5mm F2.8 EX DC HSM Fisheye also has a larger diameter of 76mm compared to the Sigma 17mm F4 DG DN C's 64mm diameter.
The weight of a lens is equally significant as its external dimensions, particularly if you intend to handhold your camera and lens combination for extended periods. Sigma 17mm F4 DG DN C weighs 220g, which means it is 250g (53%) lighter than the Sigma 4.5mm F2.8 EX DC HSM Fisheye which has a weight of 470g.
Lens Mounts
The Sigma 17mm F4 DG DN C has the Sony E lens mount whereas the Sigma 4.5mm F2.8 EX DC HSM Fisheye has the Sigma SA lens mount. Some of the latest released cameras that are compatible with these mounts are Sony FX5, Sony A7R VI and Sony A7 V for the Sony E Mount and Sigma SD1 Merrill, Sigma SD1 and for the Sigma SA Mount.
Sigma 17mm F4 DG DN C is also available in the Leica L mount.
Sigma 4.5mm F2.8 EX DC HSM Fisheye is also available in Canon EF and Nikon F (DX) mounts.
Focal Range
Sigma 17mm F4 DG DN C is a prime lens with fixed focal lenght of 17mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
25.5mm.
On the other hand, the Sigma 4.5mm F2.8 EX DC HSM Fisheye is a prime lens with fixed focal lenght of 5mm which has an effective (full-frame 35mm equivalent) focal range of 7.5mm when used on a APS-C / DX format camera.
