HD Pentax-D FA 24-70mm F2.8 ED SDM WR
Vignetting
In simple terms, vignetting is the darkening of the corners of an image that occurs at wider apertures. Every lens, wide open, will exhibit some. Most of the time, it need not be a problem for modern photographers. First, it is always possible to shoot at smaller apertures, mitigating the effect. Second, cameras released in the last few years often have built-in tools to remove vignetting in a way that's invisible to the user. Advanced software also often incorporates lens profiles which can seamlessly correct vignetting. Last, it is always possible to purchase higher-quality lenses that will show milder vignetting.
A fast lens is somewhat more likely to show vignetting than a slow one, although this is not a hard rule. A full frame lens also benefits from a sweet spot on APS-C, but shows its flaws more visibly on full frame cameras.
This section presents how the Pentax 24-70mm performs in regards to vignetting.
Vignetting Test
The test was performed by pointing the lens at a blank wall. In order to avoid the bias of the in-camera corrections, it was deactivated prior to the test. Resulting files were scaled down, converted to grayscale for improved visibility, then exported. The following settings were used:
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The following chart summarizes the findings at three focal lengths.
The next images show the vignetting at various focal lengths and apertures. Click on any thumbnail to compare the vignetting at the given focal length.
Wide open, at all three tested focal lengths, the 24-70mm exhibits strong vignetting which will be visible in some shooting scenarios. While not unexpected, this result is far from ideal.
At the wide end, closing down the aperture never completely removes the effect. The best results are observed at 50mm, where vignetting is milder to begin with, and quickly drops to undetectable levels.70mm stand between 24 and 50mm. It is obvious that the lens was optimized for the middle of its range, and decreased performances at the extremes were accepted at the design phase.
Luckily, vignetting is easily corrected, either in-camera or via software. The K-1's vignetting removal tool is particularly effective at removing any lingering effect.
Verdict
Vignetting results are not particularly remarkable, especially at 24mm where it climbs to 1.9 EV wide open. 50mm shows the best results. Closing down the aperture to F5.6 or smaller will bring vignetting to an unnoticeable level. Post-processing and in-camera correction will also remove almost all traces of the effect on pictures.