SMC Pentax-FA 31mm F1.8 AL Limited

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. A zoom is more likely to show vignetting at its wider focal lengths.

This section presents how the FA 31mm Limited performs in regards to vignetting.

Vignetting Test

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 chart summarizes the findings with the FA 31mm.

Vignetting is of course more important with a full frame sensor. Wide open, it is above 2 EV, which will certainly be noticeable. It can be beneficial for portraits and when the subject is close to the center, but in general this is not desirable. APS-C sensors show about 1 EV of vignetting, a much better controlled result. Luckily, in both cases vignetting decreases quickly as the aperture closes. It stabilises at around three quarters of an EV from F4 onward with a full frame sensor, and around one quarter of an EV from F2.8 onward with APS-C sensors.

Closing down the aperture from F1.8 to F2, a minimal exposure difference, yields a visible improvement in vignetting.

These results are not impressive for full frame sensors. Luckily vignetting is easy to correct, either in-camera or via post-processing. With APS-C sensors, vignetting is much less visible and will not be relevant in most shooting scenarios.

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.

	Full frame	APS-C
F1.8
F2
F2.8
F4
F5.6
F8

Verdict

Vignetting figures are very good for APS-C sensors but far from impressive for full frame sensors. Nowadays, this is not a big problem since software correction is easily accessible. It still shows the age of the lens design, as newer lenses are generally better corrected.