HD Pentax-D FA 28-105mm F3.5-5.6 ED DC WR
Aberrations
No lens can be totally free of optical flaws. The following list describes the main defects that a lens can suffer from.
Name | Description |
| Chromatic aberration | Different colors do not have the same focus point. The result is colored lines (usually red or green) on edges showing a sharp transition from clear to dark tones, and a general decrease of the sharpness. Occurs mostly at wider apertures. Easy to correct via software. Mitigated by the use of achromatic lens elements. |
| Purple fringing | Sometimes caused by chromatic aberration effects. Can also occur because the RGB color filters in front of pixels create differences in pixel sensitivities. Creates a purple band on edges showing a sharp transition from clear to dark tones. Occurs mostly at wider apertures. Easy to correct via software. |
| Flare | Internal reflection on the various lens elements cause a decrease of contrast, the apparition of a bright veil, or ghosting. Occurs if an image includes bright light sources, especially if the light source is near the edge. Using a lens hood helps to control flare. Better lens coatings greatly reduce the effect. |
| Ghosting | A type of flare causing artifacts (orbs) to appear on an image including bright light sources, especially if the light source is near the edge. Can be used artistically. |
| Coma | Flaws in the optical design cause point sources (such as stars) located on the sides of the frame to appear elongated. Dependent on the lens design. |
| Distortion | Straight lines appear curved. Dependent on the lens design. Tested in another page. |
| Spherical aberration | Light rays hitting the sides of the lens do not have the same focus point as those passing through the center. Mitigated by the use of aspherical elements. |
Not all of those optical effects are easy to test independently. Purple fringing and chromatic aberrations are almost always coupled. Flare and ghosting will be measured as a pair, while distortion is discussed on the following page. The other aberrations will not be formally tested as their effects are both harder to isolate and generally well-controlled by modern lens designs.
Chromatic Aberrations
For the purpose of evaluating chromatic aberrations (mainly red/green fringing in this case), we supply three sets of full-size photos. Each set consists of an uncorrected version developed from RAW and a corrected version with Adobe Camera Raw's "remove chromatic aberrations" setting enabled. Click on any thumbnail for the full-resolution file.
The thumbnails are followed by full-size crops of a specific area that we will discuss.
Scene A - 28mm F6.7
The first scene is a landscape taken at the widest zoom setting. The edges of the frame— which are also affected by distortion— exhibit chromatic aberrations the most prominently.
 Corrected |  Uncorrected |
Overall, the fringing is still fairly insignificant in the grand scheme of things.
Following software corrections, it's as good as gone without any real trade-off.
Scene B - 28mm F9.5
Our second scene was also taken at 28mm, though with a smaller aperture.
 Corrected |  Uncorrected |
Again, you can observe fringing closer to the edges of the frame. The tower toward the top, however, shows that fringing is well-controlled straight out of camera:
Following corrections, it vanishes entirely.
Scene C - 105mm F8
The next scene examines the telephoto end of the zoom range.
 Corrected |  Uncorrected |
Here, fringing isn't quite as discernible, but it affects a larger portion of the frame. It can also be see in out-of-focus areas, namely the water in the foreground.
Once again, though, a quick round of corrections completely eliminates any signs of the red and green fringes.
Also, did you spot the moire next to the hook? This is a testament to the general sharpness of the lens.
Purple Fringing
Purple fringing can be provoked in harsh lighting scenarios such as the test scene below. Click on the image for a 2000px version of the original:
Purple fringing test scene taken at 28mm, F5.6
Even at this size, which exceeds what one would use when sharing on the web, uncorrected purple fringing can hardly be observed.
Upon zooming in to 100%, we observe purple fringing around the edges of the leaves toward the left side of the frame:
We applied a purple fringing correction of "1" in Adobe Camera Raw and almost completely eliminated the fringing at the pixel level. A setting of "2" or higher can fully eliminate the fringing, possibly at the expense of color detail.
Stopping down also greatly helps to reduce the fringing, and it can likewise be eliminated through in-camera lens corrections for JPEGs (though we recommend shooting in RAW for better image quality).
All-in-all, purple fringing isn't a big issue with this lens.
Ghosting and Flare
The D FA 28-105mm has a complex optical design with 15 elements in 11 groups. It also offers angles of view from wide to telephoto, and thus going in to our tests, we expected to easily be able to provoke flare. However, since this lens is treated with Pentax's premium HD coating, we expected ghosting to be better-controlled than in earlier lenses.
The actual severity of these aberrations will depend on both the intensity and the location of a the point light source in your frame. Also, using a hood can reduce or eliminate flare for light sources just outside of the frame. We used a hood for all our tests.
28mm Tests
Click on any thumbnail in this section to get a larger version of the image. We'll start with photos taken at 28mm.
First test: well-controlled flare
In the first photo with the Sun in the center of the frame and partially shaded by leaves, flare is minimal and thus very well-controlled overall.
Test 2: isolated flare
With a stronger Sun near the edge of the frame, however, we observe more flare along with a purple halo.
 Mild |  Moderate |  Severe |
In this next set of photos, we show just how big of a different the positioning of your light source can make. In the best case, there is hardly any flare, while in the worst case, you get a lot of everything.
Test 3: well-controlled flare with faint halo
In our last 28mm test shot, we show the possibility of getting a faint halo even when flare is otherwise well-controlled. Again, it all depends on the location of the light source.
105mm Tests
While both flare and ghosting are easy to provoke at 28mm, things get much better as we zoom in.
First test: well-controlled flare
This first test photo shows virtually no aberrations with a shaded sun in the corner of the frame.
Test 2: isolated ghosting
If we turn up the brightness, we may get a sizable reddish-purple halo around the sun, but little flare. This is the HD coating in action.
Test 3: some flare and internal reflections
With a more elevated Sun, we spot flare, more purple, and some internal reflections— but this is about as bad as it gets at 105mm.
Test 4: shading the Sun
If we add a little bit of shade, the effect is diminished and we only see a small bit of purple and a small internal reflection. Overall, things look good at 105mm.
Verdict
The D FA 28-105mm generally controls chromatic aberrations and purple fringing quite well, especially in the center of the frame. At wider apertures and in the extremes, you're more likely to encounter fringing, so for peace of mind we recommend regularly applying fringing corrections while developing RAW files.
Flare can be easy to provoke at 28mm, and when it happens, it is often accompanied by internal reflections and ghosting. On the other hand, at 105mm the lens controls both much more effectively. Ultimately, we don't recommend this lens for shooting landscapes containing the Sun, but a combination of camera settings, positioning, and experimentation can of course help reduce or even eliminate flare in practice. We've certainly shown the extremes in our tests.
The flare generated by this lens tends to have a smooth, less geometric appearance, and thus some might even choose to provoke it for an artistic effect.
