HD Pentax-D FA* 85mm F1.4 ED SDM AW
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. A zoom is more likely to show vignetting at its wider focal lengths.
This section presents how the D FA* 85mm performs in regards to vignetting.
The test was performed by pointing the lens at a blank wall. In-camera correction was de-activated; results with lens correction active will be significantly better. 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 with the D FA* 85mm.
Vignetting is of course more important when using a full frame sensor. Wide open, it is approximately 1.9 EV, which will be noticeable if no correction is applied. This result is, frankly, unimpressive. Closing the aperture slightly, from F1.4 to F2, reduces vignetting tremendously, to about 0.9. It further drops at F2.8, and will be hard to notice starting from there.
On APS-C, there is still a drop between F1.4 and F2, but even wide open vignetting is below 1 EV and will not be distracting in regular scenarios. Starting at F2, vignetting is almost completely absent.
Luckily vignetting is the easiest optical flaw to correct via software. Activating auto-corrections in-camera will make the effect near-absent in the end.
The next images show the vignetting at various apertures for both sensor sizes. Click on any thumbnail to compare the vignetting at the given focal length.
Vignetting figures are excellent for APS-C sensors. On full frame sensors, wider apertures show strong vignetting, which improves significantly at F2.8 and below. Nowadays, this is not a big problem since software correction is easily accessible. Still, vignetting is one aspect where it is obvious that the Pentax designers decided to rely on software corrections.