SMC Pentax-FA 77mm F1.8 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 77mm 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 77mm.

Vignetting is of course more important with a full frame sensor. Wide open, it rises up to 1.4 EV, which could be visible in some cases if no correction is applied. F2 is not much better, but F2.8 and smaller apertures bring the amount of vignetting to unnoticeable levels.

The same observations apply to APS-C, but as expected with milder levels of vignetting at all apertures.

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.

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

Verdict

Vignetting figures are generally excellent for APS-C sensors, and will have no measurable impact for apertures smaller than F2. On full frame sensors, wider apertures show stronger vignetting, which improves significantly at F2.8 and below. Nowadays, this is not a big problem since software correction is easily accessible. Just like with the other FA Limited, the 77mm shows its age in this regard.