The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 50-500mm F4.5-6.3 DG OS HSM and the Sigma 18-125mm F3.8-5.6 DC OS HSM for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is an important deciding factor when comparing two lenses. Sigma 50-500mm F4.5-6.3 DG OS HSM is the longer of the two lenses at 219mm. The Sigma 18-125mm F3.8-5.6 DC OS HSM with a length of 89mm, is 130mm shorter. Besides being longer, the Sigma 50-500mm F4.5-6.3 DG OS HSM also has a larger diameter of 104mm compared to the Sigma 18-125mm F3.8-5.6 DC OS HSM's 74mm diameter.
The weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Sigma 18-125mm F3.8-5.6 DC OS HSM weighs 490g, 75% (1480g) lighter than the Sigma 50-500mm F4.5-6.3 DG OS HSM's weight of 1970g.
Below you can find the dimension and weight table of the SuperZoom Lenses in Canon EF Mount for size comparison purposes.
SuperZoom Lenses in Canon EF Mount for Size Comparison
Lens Mounts
Both the Sigma 50-500mm F4.5-6.3 DG OS HSM and the Sigma 18-125mm F3.8-5.6 DC OS HSM has the same Canon EF lens mount. Some of the latest released cameras that are compatible with the Canon EF lenses are Canon 1D X III, Canon 6D MII and Canon 5D MIV.
Sigma 50-500mm F4.5-6.3 DG OS HSM is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (FX) mounts.
Sigma 18-125mm F3.8-5.6 DC OS HSM is also available in Sigma SA and Nikon F (DX) mounts.
Focal Range
Sigma 50-500mm F4.5-6.3 DG OS HSM has a focal range of 50-500mm and 10.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.6x crop, it provides a 35mm (FF) equivalent of 80 - 800mm.
Sigma 18-125mm F3.8-5.6 DC OS HSM has a focal range of 18-125mm and 6.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-200mm when used on a APS-C / DX format camera.