HD Pentax-D FA 28-105mm F3.5-5.6 ED DC WR

Vignetting

Vignetting is characterized by the darkening of corners and edges of an image caused by non-uniform light distribution across the frame.  Nearly every camera lens suffers from some degree of vignetting.  As the aperture of the lens is stopped down, the degree of vignetting diminishes.

Being a compact lens with a relatively high zoom ratio, the D FA 28-105mm is more prone to vignetting than the average lens.  In fact, we see vignetting as its biggest weakness.  Fortunately, Pentax DSLRs offer in-camera corrections for JPEG files.  Similarly, desktop post-processing software such as the Pentax Digital Camera Utility, Adobe Photoshop, or Adobe Lightroom can correct vignetting for both JPEG and RAW files.  Since modern cameras offer plenty of shadow recovery potential, these corrections can effectively negate the effect of vignetting.  

Outdoor Vignetting Test

Let's take a look at a worst-case scenario: a landscape photo shot wide-open at 28mm, the focal length with the strongest vignetting.  Use the slider to compare the original and corrected photos:

In the original version, vignetting noticeably darkens the details in the corners of the frame and makes the sky appear strange.  Following software correction, however, the vignetting all but vanishes.

Now let's take a look at 105mm, the longest zoom setting:

Here vignetting is not as severe as at 28mm, but it's still noticeable without corrections.  Again, after being corrected the vignetting has a negligible impact on the final image.

Studio Test: Full-Frame Mode

We put the 28-105mm through its paces in our studio, analyzing the vignetting at varying aperture settings through the zoom range.  Click on any thumbnail for a scrollable gallery of test photos for the given focal length.

	28mm	35mm	50mm	70mm	90mm	105mm
F/max
-0.5 stop
-1 stop
-1.5 stop
-2 stops
-3 stops

We consider the vignetting to be quite severe at 28mm, especially at wide aperture settings.  By F8 vignetting becomes well-controlled, and for wider aperture settings an effective remedy is simply to zoom in a little past 30mm.

The lens performs the best around the 50mm mark, where vignetting is hard to spot after stopping down by one stop to F6.7.

At the remaining zoom settings we recommend stopping down to F8, though F9.5 is needed at 105mm to fully eliminate dark corners.

Studio Test: Crop Mode

APS-C cameras such as the Pentax K-3 only see the central portion of the frame and are thus much less susceptible to light falloff when a full-frame lens is mounted.

	28mm	35mm	50mm	70mm	90mm	105mm
F/max
-0.5 stop
-1 stop
-1.5 stop
-2 stops
-3 stops

On crop cameras or in APS-C crop mode, stopping down by one stop at 28mm and half a stop at other zoom settings is sufficient to negate the effects of vignetting.

Verdict

Although the D FA 28-105mm exhibits a substantial amount of vignetting at wide aperture settings— especially at 28mm— you might never notice it if you make use of in-camera vignetting corrections or RAW correction profiles.

The bigger implication here is that the 28-105mm's vignetting noticeably darkens a full-frame viewfinder (such as the one in the K-1).  In addition, the falloff is easy to discern while composing.  Users accustomed to shooting with primes or wide-aperture zooms thus might find this to be a turn-off.