HD Pentax-D FA* 70-200mm F2.8 ED DC AW

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.

This section presents how the Pentax and Tamron 70-200mm perform in regards to vignetting.

Vignetting Test

The test was performed by pointing the lens at a blank wall. In order to avoid the bias of the in-camera corrections, we shot each image both in JPEG and in DNG RAW, then used Lightroom to develop the RAW files (using defaults settings to develop the files). JPEGs and RAWs were scaled down, converted to grayscale for improved visibility, then exported. The following settings were used:

The following chart summarizes the findings at three focal lengths, for the RAW files. The blue lines represent the Pentax, and the red the Tamron.

There is an obvious, and quite dramatic, difference between the two lenses. It is illustrated by the following images, showing the vignetting test shots at 200mm for the two lenses:

	Pentax	Tamron
F2.8

The next images show the vignetting for the Pentax lens at other focal lengths and apertures.  Click on any thumbnail to compare the vignetting at the given focal length.

	Pentax 70mm	Pentax 135mm	Pentax 200mm
F2.8
F3.5
F4
F5.6
F8

The Pentax's even results (across varying focal lengths) and general performance is commendable. There are many prime lenses with worse vignetting results. In most cases, vignetting will not be field-relevant.

The story is different for the Tamron lens. Wide open, vignetting is quite dramatic except maybe at 70mm. It decreases rapidly, at least, becoming mostly irrelevant beyond F5.6. Luckily, vignetting is easy to correct, especially by using lens profiles in post-processing software.

The JPEG engine in-camera took care of vignetting for the Pentax lens, making it all but disappear. The Tamron of course does not benefit from in-camera corrections. As such, JPEG files from the Tamron will show the same vignetting patterns as the RAW files.

Verdict

As can be expected, Pentax JPEG files with in-camera correction show no visible vignetting. RAW files, allowing a more accurate evaluation of the lenses themselves, show excellent results at all focal lengths, even wide open. Superb. The Tamron is another story, with strong vignetting that may be noticeable if left uncorrected.