SMC Pentax-FA 31mm F1.8 AL Limited

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 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.

The FA 31mm Limited uses Pentax's well-regarded SMC coatings. While not as advanced as the newer HD coatings, SMC has always been considered as a market leader.

Flare Test One - Center-of-Frame Sunlit Flare

As usual, we used the sun as our light source for flare testing. It is bright and covers a wider range of wavelengths than most artificial light sources. We offset the sun 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.

F1.8
F2
F2.8
F4
F5.6
F8
F11
F16
F22

With the sun in the center, there is absolutely no flare in the images. An artifact visible to the right at F22 could be a manifestation of flare, but is more likely caused by a dust particle somewhere in the light path, as it lacks the coloring usually associated with flare. We couldn't ask for better results.

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. We looked at both the APS-C and full frame fields of view.

Full frame

F1.8
F2
F2.8
F4
F5.6
F8
F11
F16
F22

A red halo is visible at the bottom right as early as F2.8, joined at F4 by a second, green halo. Both get more defined as the aperture closes. At F22 a small series of dots is visible, if discreet.

While not bothersome, this shows the limitations of the "semi-petal" hood shape. A larger hood would have improved results. Light coming from the top or bottom will probably be almost devoid of ghosts.

APS-C

F1.8
F2
F2.8
F4
F5.6
F8
F11
F16
F22

APS-C is basically a crop of the full frame field of view. As such, it hides most ghosting effects. The barest hint of ghosting is visible at F16 and F22 but is so small as to be hard to notice.

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). We tested the crop size of a full frame and APS-C sensor. The center point is the same. You can click on the images to see 100% crops, and navigate by using the left-right arrows.

Full Frame

Center
Top
Bottom
F1.8 F1.8 Center F1.8 Top F1.8 Bottom
F2 F2 Center F2 Top F2 Bottom
F2.8 F2.8 Center F2.8 Top F2.8 Bottom
F4 F4 Center F4 Top F4 Bottom
F5.6 F5.6 Center F5.6 Top F5.6 Bottom
F8 F8 Center F8 Top F8 Bottom
F11 F11 Center F11 Top F11 Bottom
F16 F16 Center F16 Top F16 Bottom
F22 F22 Center F22 Top F22 Bottom

While the center is free from chromatic aberrations, the top and bottom areas show visible CA up to F8. It is lucky that modern cameras can automatically remove this effect.

APS-C

Center
Top
Bottom
F1.8 F1.8 Center F1.8 Top F1.8 Bottom
F2 F2 Center F2 Top F2 Bottom
F2.8 F2.8 Center F2.8 Top F2.8 Bottom
F4 F4 Center F4 Top F4 Bottom
F5.6 F5.6 Center F5.6 Top F5.6 Bottom
F8 F8 Center F8 Top F8 Bottom
F11 F11 Center F11 Top F11 Bottom
F16 F16 Center F16 Top F16 Bottom
F22 F22 Center F22 Top F22 Bottom

The APS-C crop helps with CA. The effect is visible up to F5.6, one stop less than on full frame.

There is almost no purple fringing in any of these images.

Verdict

The SMC FA 31mm Limited shows astounding resistance to flare when the light source is in the center. Things are not as good in the corners with full frame, but remain better than average, with discreet ghosting patterns that are hard to spot. On APS-C even corner illumination yields almost no ghosting.

Chromatic aberration is important up to F8 on full frame and F5.6 on APS-C. This is a weakness of this older design, easy to fix via software.


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