Sigma 17-70 mm f/2.8-4.0 DC Macro OS HSM
8. Vignetting
For the combination of 17 mm and f/2.8 the brightness loss in the frame corners amounts to 34% (- 1.22 EV). It’s a lot but, what’s interesting, still less than in the case of its predecessor. On stopping down to f/4.0 we see this aberration’s level decreasing to 21% and by f/5.6 it is just 15%. On further stopping down we don’t notice any significant vignetting decrease.
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At 28 mm and by f/3.5 the vignetting level reaches 31% (−1.07 EV) and decreases to 22% by f/4.0; then, by f/5.6, it comes to 14%. At 50 mm the situation is very much alike. At the maximum aperture the light fall-off in the corners is 31% (−1.1 EV) and on stopping down by 1 EV it decreases to just 11%.
Virtually nothing changes at 70 mm aperture. The vignetting amounts to 30% (−1.04 EV) when wide open and by f/5.6 it becomes practically imperceptible because it decreases to 11%.
The Sigma’s performance here is very curious. Zoom lenses with a similar focal lengths range often have very high vignetting at wide angle – it can reach as much as 50% there. In the middle of the focal range this aberration is usually low and it increases at the maximum focal length. The Sigma is an exception to that rule. Its vignetting level is by and large constant, regardless of the focal length used. Perhaps it is a result of a better fastness than in the case of its competitors.
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