Pentax 50mm F1.4 comparison: M, FA and DFA
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 perform 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:
| || |
The following charts summarize the findings for APS-C and full frame.
On APS-C the three lenses are hard to distinguish from one another. They all show excellent results, with vignetting below 1 stop even wide open.
On full frame the difference is more dramatic. The older M lens shows significant vignetting at wider apertures, although it gets under control starting at F2.8. The FA shows better results than what we expected: the improvement between the M and FA lenses is obvious. The D FA* is well ahead, with low vignetting even at F1.4.
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
On APS-C the three lenses perform well and are mostly similar.
On full frame the D FA* is well ahead of the other two lenses, with the M lens showing the poorest results.