The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 18-125mm F3.8-5.6 DC HSM and the Sigma 4.5mm F2.8 EX DC HSM Fisheye for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Sigma 18-125mm F3.8-5.6 DC HSM is the longer of the two lenses at 89mm. The Sigma 4.5mm F2.8 EX DC HSM Fisheye with a length of 78mm, is 11mm shorter. On the other hand, the Sigma 4.5mm F2.8 EX DC HSM Fisheye has a larger diameter of 76mm compared to the Sigma 18-125mm F3.8-5.6 DC HSM's 74mm 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 4.5mm F2.8 EX DC HSM Fisheye weighs 470g, 4% (20g) lighter than the Sigma 18-125mm F3.8-5.6 DC HSM's weight of 490g.
Lens Mounts
The Sigma 18-125mm F3.8-5.6 DC HSM has the Sony Alpha 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 A68, Sony A77 II and Sony A58 for the Sony Alpha Mount and Sigma SD1 Merrill, Sigma SD1 and for the Sigma SA Mount.
Sigma 18-125mm F3.8-5.6 DC HSM is also available in the Pentax KAF3 mount.
Sigma 4.5mm F2.8 EX DC HSM Fisheye is also available in Canon EF and Nikon F (DX) mounts.
Focal Range
Sigma 18-125mm F3.8-5.6 DC HSM has a focal range of 18-125mm and
6.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of
27-187.5mm when used on a APS-C / DX format camera.
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.
