The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 17-50mm F2.8 EX DC OS HSM and the Sigma 18-50mm F2.8-4.5 DC OS HSM 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 17-50mm F2.8 EX DC OS HSM is the longer of the two lenses at 92mm. The Sigma 18-50mm F2.8-4.5 DC OS HSM with a length of 89mm, is 3mm 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 18-50mm F2.8-4.5 DC OS 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 18-50mm F2.8-4.5 DC OS HSM weighs 395g, 30% (170g) lighter than the Sigma 17-50mm F2.8 EX DC OS HSM's 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 Lenses in Canon EF Mount for Size Comparison
Filter Threads
The Sigma 17-50mm F2.8 EX DC OS HSM has a filter size of 77mm whereas the Sigma 18-50mm F2.8-4.5 DC OS HSM 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 77mm filters that we recommend you to consider for the Sigma 17-50mm F2.8 EX DC OS HSM:
And these are the 67mm filters that we recommend for the Sigma 18-50mm F2.8-4.5 DC OS HSM:
Lens Mounts
Both the Sigma 17-50mm F2.8 EX DC OS HSM and the Sigma 18-50mm F2.8-4.5 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 17-50mm F2.8 EX DC OS HSM is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Sigma 18-50mm F2.8-4.5 DC OS HSM is also available in Pentax KAF3 , Sony Alpha, Sigma SA and Nikon F (DX) mounts.
Focal Range
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.
On the other hand, the Sigma 18-50mm F2.8-4.5 DC OS HSM has a focal range of 18-50mm and 2.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-80mm when used on a APS-C / DX format camera.