Modern 50mm Shootout: Pentax D FA* 50mm, Samyang 50mm, Pentax DA* 55mm F1.4
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 three lenses 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 charts summarize the findings on full frame and APS-C.
On APS-C, the three lenses are nearly identical. At around three-quarters of a stop wide open, the effect will not be bothersome. As the aperture closes, vignetting becomes negligible. The D FA* has a theoretical advantage but it will be nearly impossible to notice in most cases.
On full frame, the differences are more visible. At wider apertures, the DA* shows heavier vignetting than the other two, something to be expected given its APS-C roots. It doesn't do much worse than the Rokinon, however. The D FA* is the star of the show, with low vignetting even wide open. At F2.8 and below, the effect stops being bothersome and the three lenses mostly even out.
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 view larger sizes.
D FA* 50mm
Vignetting figures are excellent for all three lenses on APS-C. On full frame sensors, the D FA* 50mm produces by far the best results. The Rokinon's results are still better than average and easy to correct. While the DA* 55mm's results are the poorest of the three, they exceed expectations for what is officially an APS-C lens.