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

Distortions

Like vignetting, geometric distortion is an optical flaw to be expected of most camera lenses.  Distortion comes in two flavors and of course varying degrees: pincushion if straight lines bend outward, and barrel if straight lines bend inward toward the center of the image.  Zoom lenses often exhibit a different type of distortion at each end of the zoom range.

Pentax cameras offer in-camera JPEG distortion corrections for modern lenses, and like vignetting, distortion can also be corrected through desktop post-processing software. (Note that in-camera distortion correction takes about 1 second to process per image and locks up the camera in burst mode while files are being saved, so we do not recommend that you use this feature outside of single frame mode.)

Below is a sample photo taken at 28mm, along with its uncorrected/unedited counterpart:

While correcting distortion can be highly advantageous for reproductions and in photos that contain parallel lines, in everyday shots it can be best to leave distortion untouched.  Not only can this give your photos a more "natural" feel, but applying corrections also slightly reduces the field of view.

Studio Test: 28mm

Back in the studio, we tested the 28-105mm's distortion throughout the zoom range.  All images were shot in full-frame mode with a Pentax K-1.  To get a sense of APS-C performance, only consider the central two-thirds of each image.

We observed barrel distortion at the wide end of the zoom range. 28mm is the focal length at which this lens exhibits the most distortion, though for a wide-angle lens the severity is moderate at worst.

35mm

After zooming in to 35mm, only minor barrel distortion remains.

50mm

The lens switches over to pincushion distortion just prior to the 50mm mark.  Like at 35mm, distortion is negligible at 50mm.

70mm

Things worsen slightly at 70mm, though we would still consider the degree of distortion to be mild and not of practical significance.  Since only the edges of the frame are seriously affected, in APS-C mode distortion at this focal length is negligible.

90mm

Performance at 90mm is very similar to what we saw at 70mm.

105mm

To our surprise, distortion does not worsen at the long end of the zoom range and is mild overall.

Verdict

The D FA 28-105mm lens exhibits moderate barrel distortion at 28mm but only slight pincushion distortion at telephoto zoom settings.  The "sweet spot" lies between 35mm and 50mm, where distortion is negligible.  On APS-C cameras or in crop mode, distortion generally negligible and only discernible at 28mm.

In practical terms, the lens fares well overall and its distortion won't have a significant negative impact on photos, except possibly at 28mm.  When necessary, software corrections can effectively compensate for distortion.