Sigma 18-200 mm f/3.5-6.3 II DC OS HSM
At the shortest focal length and by f/3.5 the brightness loss in the frame corners amounts to as much as 54% (-2.24 EV) – so is huge. On stopping down the aperture the situation still remains rather mediocre. By f/4.0 the vignetting is 48% (-1.87 EV) and by f/5.6 it reaches still a significant value of 33% (-1.18 EV). By f/8.0 the light fall-off gets to 27% (-0.91 EV) and on further stopping down you can’t see any noticeable decrease of this aberration.
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At 30 mm focal length the maximum relative aperture is f/4.0 and the vignetting amounts to 43% (-1.64 EV). On stopping down the aperture to f/5.6 the brightness loss in the frame corners reaches 27% (-0.93 EV) and by f/8.0 it gets to 20% (-0.66 EV). By f/11 and f/16 apertures the vignetting keeps the same level – that of 16-17%.
A very similar situation can be seen at 50 mm focal length. At the maximum relative aperture the light fall-off in the corners of the frame is 44% (-1.68 EV) and by f/5.6 it reaches 36% (-1.28 EV). In the case of f/8.0 and f/11 apertures the vignetting amounts to 23% and 15% respectively.
In the case of the 100 mm focal length and the maximum relative aperture the level of vignetting could be determined as 47% |(-1.86 EV). Applying the f/8.0 aperture allows us to decrease that aberration to the value of 31% (-1.07 EV) and by f/11 it amounts to 20% (-0.64 EV). The vignetting becomes imperceptible only by f/16, where it gets to the level of 10% (-0.32 EV).
The combination of the maximum focal length and the maximum relative aperture is not free of huge vignetting level - it reaches as high as 46% (-1.76 EV) there. By f/8.0 this aberration gets to 31% (-1.07 EV and by f/11 it is 18% (-0.56 EV). The problem disappears only by f/16 where the vignetting is just 9% (-0.26 EV).
To sum up: in this category small dimensions of the tested lens made themselves felt very keenly. The vignetting remains huge practically at every focal length; often even after stopping down you have to tackle its noticeable influence.