The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 18-50mm F2.8 DC DN C and the Tamron 17-70 F2.8 Di III-A VC 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. Tamron 17-70 F2.8 Di III-A VC is the longer of the two lenses at 119mm. The Sigma 18-50mm F2.8 DC DN C with a length of 77mm, is 42mm shorter. Besides being longer, the Tamron 17-70 F2.8 Di III-A VC also has a larger diameter of 75mm compared to the Sigma 18-50mm F2.8 DC DN C's 62mm diameter.
Below you can find a scaled side-by-side image of Sigma 18-50mm F2.8 DC DN C and Tamron 17-70 F2.8 Di III-A VC to get a better idea of how their sizes compare in real life.
Comparison image of the Sigma 18-50mm F2.8 DC DN C and the Tamron 17-70 F2.8 Di III-A VC Size and Weight 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 18-50mm F2.8 DC DN C weighs 285g, which means it is 240g (45%) lighter than the Tamron 17-70 F2.8 Di III-A VC which has a weight of 525g.
Below you can find the dimension and weight table of the Standard Zoom Lenses in Fujifilm X Mount for size comparison purposes.
Standard Zoom Lenses in Fujifilm X Mount for Size Comparison
Lens Mounts
Both the Sigma 18-50mm F2.8 DC DN C and the Tamron 17-70 F2.8 Di III-A VC has the same Fujifilm X lens mount. Some of the latest released cameras that are compatible with the Fujifilm X lenses are Fujifilm X-T5, Fujifilm X-H2 and Fujifilm X-H2S.
Sigma 18-50mm F2.8 DC DN C is also available in Leica L and Sony E mounts.
Tamron 17-70 F2.8 Di III-A VC is also available in Sony E mount.
Focal Range
Sigma 18-50mm F2.8 DC DN C has a focal range of 18-50mm and 2.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-75mm when used on a APS-C / DX format camera. Tamron 17-70 F2.8 Di III-A VC has a focal range of 17-70mm and 4.1X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 25.5-105mm when used on a APS-C / DX format camera.