The Tamron 17-70mm f/2.8 vs. Sigma 17-40mm f/1.8: One Wins on Paper, the Other Wins in Practice

Choosing between a wider aperture and a longer zoom range is one of the most common trade-offs in APS-C lens selection, and few comparisons make that tension as concrete as the Tamron 17-70mm f/2.8 versus the Sigma 17-40mm f/1.8. These two lenses cost $200 apart, share the same weight, start at the same focal length, and yet produce noticeably different results depending on what you're shooting.

Coming to you from Curtis Padley, this detailed video puts both lenses through 13 head-to-head rounds covering build quality, sharpness, low-light performance, autofocus, stabilization, focus breathing, chromatic aberration, minimum focus distance, and price. Padley scores each round and crowns an overall winner, which he admits surprised him. His initial expectation was that the Sigma would take it, but the Tamron edges ahead by a single point, largely due to its focal range advantage, built-in optical stabilization, and stronger value proposition. The Sigma has no stabilization at all, which shows clearly in the video footage comparisons, especially on cameras without in-body image stabilization. Even on bodies that do have IBIS, the Tamron's lens-based stabilization working in tandem produces visibly smoother results.

Where the Sigma fights back hard is in image quality and low light. It's the only APS-C mirrorless zoom currently available with a constant f/1.8 aperture, and that gap over the Tamron's f/2.8 amounts to roughly one and one-third stops of light. In practice, that translates to lower ISO, faster shutter speeds, or both. Padley's sharpness tests show the Sigma consistently outperforming the Tamron in center sharpness at 17mm across nearly every aperture, though the Tamron actually pulls ahead in corner sharpness at longer focal lengths. The bokeh comparison is also striking: the Sigma produces smoother, cleaner out-of-focus highlights, while the Tamron shows more pronounced onion-ring patterns in its bokeh balls, which Padley finds less visually refined.

The price difference sharpens the decision in a real way. At $699, the Tamron gives you a 70mm reach, optical stabilization, strong close-focusing capability (down to 19 cm at 17mm), and solid all-around image quality. At $919, the Sigma delivers a more optically refined image, a unique constant f/1.8 aperture, and a rendering quality that Padley describes as closer to a prime lens. He personally shoots with the Sigma despite it losing the overall comparison, specifically because the f/1.8 aperture lets him leave fast primes like a 16mm, 23mm, or 30mm at home when traveling. That's a legitimate cost-saving argument depending on how your kit is built.

Check out the video above for the full breakdown, including the round-by-round scoring and Padley's complete sharpness test results at every focal length and aperture combination.