Sigma 70-200 mm f/2.8 EX DG APO OS HSM
8. Vignetting
The photos of evenly lit surface show that on the smaller sensor of the Canon 50D there is still no cause for worry.
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For the combination of 70 mm focal length and f/2.8 aperture we lose only 18% of light in the corners (-0.59 EV) so also nothing to bother yourself with. Especially that on stopping down to f/4.0 the vignetting decreases to 5%.
At 135 mm the situation is even better – at the maximum relative aperture the vignetting amounts to 16% (-0.52 EV) and on stopping down to f/4.0 it decreases to 4%. Only the 200 mm focal length gives us a chance to notice the brightness loss in the corners in real photos. At the maximum relative aperture the vignetting is 27% (-0.93 EV), but it becomes imperceptible again by f/4.0, where it decreases to 10%.
Let’s see how the lens fares on full frame. Appropriate photos are presented below.
In this category the standards were set very high by the Nikkor 70-200 mm VR II. In its case the vignetting at the maximum relative aperture increased with the rise of the focal length from 22% at 70 mm to 34% at 200 mm. How does the Sigma perform here? Unfortunately it faces huge problems. At the shortest focal length by f/2.8 the light fall-off in the frame corners amounts to 44% (-1.68 EV). On stopping down to f/4.0 the level of this aberration decreases to 22% and by f/5.6 it is still 16%. Only by f/8.0 the vignetting becomes imperceptible (10%).
Almost identical situation can be observed at 135 mm. The vignetting is 45%, 23%, 16% and 9% there by f/2.8, f/4.0,f/5.6 and f/8.0 apertures respectively. It becomes even worse for the longer end of focal lengths range. At the maximum aperture the brightness loss in the frame corners amounts to 49% (1.93 EV) there. On stopping down the aperture by 1 EV we see it reduced to 26% and on stopping down by 2 EV it decreases to 18%. By f/8.0 the vignetting is just 11%.
It’s not the end of the story. The original Sigma converter seems to be a true bottleneck. Apparently it doesn’t manage to transfer the whole light, gathered by the lens, and at the maximum aperture (after using the converter it amounts already to f/4.0) the vignetting reaches a huge level of 67% (-3.2 EV). Even on stopping down to f/5.6 the light fall-off in the corners remains significant, getting to 43%. You can see it still by f/8.0 where its value amounts to 25%. It stops bothering us only by f/11 and f/16 where it is 17% and 11% respectively.
A huge light loss in the frame corners of photos taken with the lens-plus-converter set explains why we have to deal with such resolution problems there.
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