The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 17-70mm F2.8-4 DC OS HSM C and the Sigma 17-50mm F2.8 EX 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 17-50mm F2.8 EX DC OS HSM is the longer of the two lenses at 92mm. The Sigma 17-70mm F2.8-4 DC OS HSM C with a length of 82mm, is 10mm shorter. Besides being longer, the Sigma 17-50mm F2.8 EX DC OS HSM also has a larger diameter of 84mm compared to the Sigma 17-70mm F2.8-4 DC OS HSM C's 79mm 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 17-70mm F2.8-4 DC OS HSM C weighs 465g, which means it is 100g (17%) lighter than the Sigma 17-50mm F2.8 EX DC OS HSM which has a weight of 565g.
Below you can find the dimension and weight table of the Standard Zoom Lenses in Canon EF Mount for size comparison purposes.
Standard Zoom Lenses in Canon EF Mount for Size Comparison
Lens Mounts
Both the Sigma 17-70mm F2.8-4 DC OS HSM C and the Sigma 17-50mm F2.8 EX 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 T8i, Canon 90D and Canon SL3.
Sigma 17-70mm F2.8-4 DC OS HSM C is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Sigma 17-50mm F2.8 EX DC OS HSM is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
Sigma 17-70mm F2.8-4 DC OS HSM C has a focal range of 17-70mm and 4.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27.2-112mm when used on a APS-C / DX format camera. Sigma 17-50mm F2.8 EX DC OS HSM has a focal range of 17-50mm and 2.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27.2-80mm when used on a APS-C / DX format camera.