The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron 20-40mm F2.8 VXD and the Sigma 18-50mm F2.8 DC DN C 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. Tamron 20-40mm F2.8 VXD is the longer of the two lenses at 86mm. The Sigma 18-50mm F2.8 DC DN C with a length of 75mm, is 11mm shorter. Besides being longer, the Tamron 20-40mm F2.8 VXD also has a larger diameter of 74mm compared to the Sigma 18-50mm F2.8 DC DN C's 65mm diameter.
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Below you can find a scaled side-by-side image of Tamron 20-40mm F2.8 VXD and Sigma 18-50mm F2.8 DC DN C to get a better idea of how their sizes compare in real life.
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 DC DN C weighs 290g, 20% (75g) lighter than the Tamron 20-40mm F2.8 VXD's weight of 365g.
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Filter Threads
The Tamron 20-40mm F2.8 VXD has a filter size of 67mm whereas the Sigma 18-50mm F2.8 DC DN C has a 55mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 67mm filters that we recommend you to consider for the Tamron 20-40mm F2.8 VXD:
And these are the 55mm filters that we recommend for the Sigma 18-50mm F2.8 DC DN C:
Lens Mounts
Both the Tamron 20-40mm F2.8 VXD and the Sigma 18-50mm F2.8 DC DN C has the same Sony E lens mount. Some of the latest released cameras that are compatible with the Sony E lenses are Sony A9 III, Sony A7C Mark II and Sony A7CR.
Sigma 18-50mm F2.8 DC DN C is also available in Leica L and Fujifilm X mounts.
Focal Range
Tamron 20-40mm F2.8 VXD has a focal range of 20-40mm and
2.0X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
30 - 60mm.
On the other hand, the 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.