HD Pentax-DA* 16-50mm F2.8 ED PLM AW
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. In simple terms, lateral CA occurs in the in-focus zones, while longitudinal CA occurs in out-of-focus zones. |
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 reflections 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, and will be tested together. Flare and ghosting will also be measured as a pair. Distortion will get its own page later in this review. The other aberrations will not be formally tested as their effects are both harder to isolate and less prominent in everyday shooting.
Flare
Flare will affect images in which a bright light source, such as the sun, is present in the frame or near its border. The use of a lens hood helps reduce the effect for side lighting, as does a recessed front element. High-quality lens coatings play a very important role in minimizing flare, by improving light transmission and minimizing internal reflections.
The DA* 16-50mm PLM uses Pentax's most recent HD coatings, which are an improvement on the already market-leading SMC coatings used in the past
Flare Test One - Center-of-Frame Sunlit Flare
As usual, we used a bright light source which covers a wider range of wavelengths. We offset the light slightly in order to see eventual ghosting which could be hidden if there was a straight line between the light source and the sensor. You can click on the thumbnails for larger views.
35mm
F2.8 | |
F4 | |
F5.6 | |
F8 | |
F11 | |
F16 | |
F22 |
With the source in the center, there is some flare at wider apertures, and a small number of ghosts close to the light source.
At smaller apertures, flare decreases but the number of ghosts increases markedly. Starting at F11, several artifacts appear around the light.
While not dramatic, these results are not the best we have seen from Pentax.
Flare Test Two - Edge of Frame
For this test, we placed the source in the top right corner of the frame, directly illuminating the sensor. You can click on the thumbnails for larger views.
35mm
F2.8 | |
F4 | |
F5.6 | |
F8 | |
F11 | |
F16 | |
F22 |
With the source in the corner, we see a few ghosts along the diagonal at wider apertures. Some ghosts appear closer to the light as the aperture closes, and becomes dramatic when the F-number gets really small.
Again, these results are not particularly impressive coming from Pentax .
Chromatic Aberration Test
For this test we used a well-lit, sharp transition from dark to bright, in order to make manifest any chromatic aberration present in the image.
We then looked at three parts of the image: the focus point, the top and the bottom (beyond and before the focus point). The center point is the same. You can click on the images to see 100% crops, and navigate by using the left-right arrows.
Center | Top | Bottom | |
F2.8 | |||
F4 | |||
F5.6 | |||
F8 | |||
F11 | |||
F16 | |||
F22 |
There is no CA at all in the center of the frame. Not surprising, but great results nonetheless.
At the top and bottom, there is the barest hint of CA only at F2.8. It is gone by F4. Without being perfect, this is also excellent.
Purple Fringing
Purple fringing occurences are few with the 16-50mm PLM. The image below shows a 100% crop of the worst occurences that we observed.
Verdict
Chromatic aberration is well controlled, with only F2.8 showing discrete occurrences wide open. Purple fringing is not completely absent but quite mild.
Flare and ghosting isn't as good, sadly. Pentax lenses generally perform better. In particular, ghosting is present in many cases with the source in the center or in a corner.