Pentax-D FA 100mm F2.8 Macro WR
Aberrations
Even the best designed lens, created with the highest quality glass and fine-tuned as thoroughly as possible, will present flaws in some circumstances. The following list describes the main optical flaws 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. Results in 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 and the center of the lens do not have the same focus point. 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 generally better controlled by design.
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.
Flare Test One - Center-of-Frame Sunlit Flare
For this test, we placed the sun in the center of the frame, directly illuminating the sensor. Pictures were captured at various apertures, with and without both of the hoods presented in the Construction and Handling section. You can click on the thumbnails for larger views.
f2.8 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
f5.6 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
f9 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
f16 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
The presence of a hood (whatever the type) changes nothing in the results obtained with the sun centered in the frame. There is no ghosting visible (nor would we expect any). Contrast is on the low side, but that's a matter of exposure as much as flare (with a 100mm lens, the sun fills a large part of the frame). a halo appears around the sun at f16, caused by diffraction.
All in all, a commendable performance.
Flare Test Two - Edge of Frame
For this test, we placed the sun in the top left corner of the frame, directly illuminating the sensor. You can click on the thumbnails for larger views.
f2.8 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
f5.6 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
f9 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
f16 | |
| Without hood |  |
| Dedicated hood |  |
| Step-down ring |  |
Wide open results are excellent. There is no veil or contrast decrease to speak of. At smaller apertures, one single internal reflection element creates one ghosting spot in the lower right quadrant. The aperture needs to be closed to f16 to make it become a double spot. The lens does a superb job of controlling flare, a testament to the quality of Pentax coatings.
Another interesting observation is that the presence of a hood, dedicated or custom, does not visibly improve the performances when the light source is within the frame, even with a source as large as the sun. The story could be different with a light source just outside the frame but the recessed front element would probably help protect the lens against flare in that case.
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. Various apertures were used. The following scene was used (the image is tilted at approximately 45° of the focal plane of the camera):
 |
We then looked at three parts of the image: the focus point, the top and bottom (beyond and before the focus point). You can click on the images to see larger views, and navigate each aperture image set by using the left-right arrows.
Focus point | Top | Bottom | |
| f2.8 |  |  |  |
| f4 |  |  |  |
| f5.6 |  |  |  |
| f8 |  |  |  |
Performance in the central, in-focus area are excellent, showing the tiniest of purple fringing on the black-to-white transitions at f2.8, and visible only when reviewing images at the pixel level. This fringing is almost gone at f4, and invisible at smaller apertures. There is no chromatic aberration to speak of.
Things are different in the out-of-focus areas, however. The lens shows some green chromatic aberration beyond the focus point and red fringing before the focus point. This effect is sometimes referred to as "Lateral CA". Again the effect is gone beyond f4, but in this case it can be observed without fully zooming, though even then it's subtle.
Purple Fringing
Here we cropped a part of the flare test image at f5.6 to illustrate the worst case of purple fringing we observed in real-life use. It is a very extreme situation by all accounts, including over-exposure. Purple fringing here is quite possibly caused mainly by the sensor, not the lens. Still, purple fringing is probably the weakest link in this lens' image quality evaluation, occurring from time to time in images, mostly in out-of-focus areas and along overexposed edges. Easily corrected in post-processing, it must still be taken into account when evaluating the lens' performance.
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Verdict
Although not perfect, the lens is excellent regarding aberrations. Flare control is surprisingly good, even in the extreme conditions we used for testing. Chromatic aberrations are controlled well, although there is some CA visible at the pixel level in the out-of-focus areas at wide apertures. Purple fringing is present in some shooting scenario (high contrast out-of-focus areas).
An outstanding performance.
