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 Nikon 18-70mm f3.5-4.5G ED-IF 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 17-70mm F2.8-4 DC OS HSM C is the longer of the two lenses at 82mm. The Nikon 18-70mm f3.5-4.5G ED-IF with a length of 76mm, is 6mm shorter. Besides being longer, the Sigma 17-70mm F2.8-4 DC OS HSM C also has a larger diameter of 79mm compared to the Nikon 18-70mm f3.5-4.5G ED-IF's 73mm 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. Nikon 18-70mm f3.5-4.5G ED-IF weighs 420g, 9% (45g) lighter than the Sigma 17-70mm F2.8-4 DC OS HSM C's weight of 465g.
Below you can find the dimension and weight table of the Standard Zoom Lenses in Nikon F (DX) Mount for size comparison purposes.
Standard Lenses in Nikon F (DX) Mount for Size Comparison
Filter Threads
The Sigma 17-70mm F2.8-4 DC OS HSM C has a filter size of 72mm whereas the Nikon 18-70mm f3.5-4.5G ED-IF has a 67mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 72mm filters that we recommend you to consider for the Sigma 17-70mm F2.8-4 DC OS HSM C:
And these are the 67mm filters that we recommend for the Nikon 18-70mm f3.5-4.5G ED-IF:
Lens Mounts
Both the Sigma 17-70mm F2.8-4 DC OS HSM C and the Nikon 18-70mm f3.5-4.5G ED-IF has the same Nikon F (DX) lens mount. Some of the latest released cameras that are compatible with the Nikon F (DX) lenses are Nikon D3500, Nikon D7500 and Nikon D5600.
Sigma 17-70mm F2.8-4 DC OS HSM C is also available in Canon EF , Pentax KAF3, Sony Alpha and Sigma SA 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
25.5-105mm when used on a APS-C / DX format camera.
On the other hand, the Nikon 18-70mm f3.5-4.5G ED-IF has a focal range of 18-70mm and 3.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-105mm when used on a APS-C / DX format camera.