Sigma 18-35mm F1.8 DC HSM "Art"
Vignetting
For this assessment of the lens, we focused on the amount of vignetting produced by the lens, or the characteristic where the brightness of an image is not uniform across the entire frame, particularly the further one gets toward the edges of the frame.
Before we get to the images, it's important to note that the complete removal of any and all vignetting from a lens' widest apertures, if not entirely impossible, would require an extremely complex design with very large optical elements (and thus big dollars). Even for the most expensive lenses available to consumers, across any brand, vignetting to some degree is expected, especially within a stop of maximum aperture.
One last thing before we get to the comparisons - it's important to note that although the below images may seem severely underexposed, they are that way deliberately to enhance your ability to detect the vignetting.
Minimum Focal Length (18mm)
For our first focal length test, we were at the minimum focal length of the Sigma F1.8 zoom. The following settings were applied across all the sample images during this iteration:
F1.8 |  |
F2.2 |  |
F2.8 |  |
F3.5 |  |
F4.0 |  |
F4.5 |  |
F5.6 |  |
F8.0 |  |
Medium Focal Length (24mm)
For our next focal length, 24mm, the following settings were in effect (same as above):
F1.8 |  |
F2.2 |  |
F2.8 |  |
F3.5 |  |
F4.0 |  |
F4.5 |  |
F5.6 |  |
F8.0 |  |
Maximum Focal Length (35mm)
Finally, we wanted to see how the Sigma managed vignetting when zoomed all the way in. Once again there was no change in the testing methodology:
F1.8 |  |
F2.2 |  |
F2.8 |  |
F3.5 |  |
F4.0 |  |
F4.5 |  |
F5.6 |  |
F8.0 |  |
Verdict
As expected, this Sigma exhibits vignetting at its widest aperture. What caught us by surprise, though, is how well that vignetting was kept under control. At 18mm, all vignetting is effectively eliminated by F2.8, one and a third stops later. In the middle of the focal length range, the lens is effectively vignette free within half a stop of its maximum aperture - truly remarkable!
The strongest case of vignetting happens at full zoom, with the corners a full 20% darker than the center. But just like the first two settings we tested, by F2.2 it is severely minimized, and by F2.8 all but gone.
In real world shooting conditions, vignetting shouldn't be a significant concern, however if you find yourself shooting landscapes or starscapes wide open, it can become problematic.
For some flare and chromatic aberration, click to the next page.
