Modern 50mm Shootout: Pentax D FA* 50mm, Samyang 50mm, Pentax DA* 55mm F1.4

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 DA* 55mm uses Pentax's well-regarded SMC coatings. While not as advanced as the newer HD coatings used on the DFA* 50mm, SMC has always been considered as a market leader. The Rokinon 50mm uses a modern design, however Rokinon's reputation with coatings is not as stellar as Pentax's.

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.

DA* 55mm Rokinon 50mm D FA* 50mm
F1.4
F2
F2.8
F4
F5.6
F8
F11
F16
F22

At F1.4, the Rokinon shows a wide flare circle around the frame, somethings the other two lenses don't show. It is still present at F5.6. At F8 the DA* 55mm also shows the same effect, almost gone at F11. At F16 all three lenses show ghosts, which are more visible for the Rokinon as they are more defined.

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

DA* 55mm Rokinon 50mm DFA* 50mm
F1.4
F2
F2.8
F4
F5.6
F8
F11
F16
F22

With the sun in the corner the DA* 55mm shows a wide ring of flare at F1.4. For its part the Rokinon shows several ghosts instead. The DA*'s flare diminishes but is still present by F11. At F16 severe ghosting appears. The Rokinon looses some ghosts but gains others, never getting free of the effect. Meanwhile, the D FA* shows minimal ghosting at wider apertures, but the effect is mostly gone by F4 and never as bothersome as with the other two lenses.

APS-C

DA* 55mm Rokinon 50mm DFA* 50mm
F1.4
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, the same observations apply.

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. We present results separately for each lens.

DA* 55mm

Full Frame

Center
Top
Bottom
F1.4 F1.4 Center F1.4 Top F1.4 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

CA is obvious at the top of the frame, but also present in the center at apertures wide than F5.6. PF is also visible a the bottom for wider apertures.

APS-C

Center
Top
Bottom
F1.4 F1.4 Center F1.4 Top F1.4 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 same observations apply with the APS-C crop. The DA* 55mm is quite sensitive to CA.

Rokinon 50mm

Full Frame

Center
Top
Bottom
F1.4 F1.4 Center F1.4 Top F1.4 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 F5.6. Average results for the Rokinon.

APS-C

Center
Top
Bottom
F1.4 F1.4 Center F1.4 Top F1.4 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

Again, the same observations apply to the APS-C crop. CA is not limited to the edges of the frame.

D FA* 50mm

Full Frame

Center
Top
Bottom
F1.4 F1.4 Center F1.4 Top F1.4 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

While the center is free from chromatic aberrations, the top and bottom areas show visible CA up to F5.6 and hints at F8. It is lucky that modern cameras can automatically remove this effect. Purple fringing is present wide open.

APS-C

Center
Top
Bottom
F1.4 F1.4 Center F1.4 Top F1.4 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

The APS-C crop helps with CA. The effect is visible up to F5.6 but almost gone at F4, one stop less than on full frame.

Verdict

None of the three lenses tested are free from CA. The DA* 55mm is the worst offender, since it shows CA even in the center at wider settings. The Rokinon is second, with more controlled CA but a visible fringing up to medium apertures. While not perfect, the D FA* 50mm is the best of the three, with the effect mostly gone beyond F2.8.

Flare is more of a mixed bag for the DA* and Rokinon. In the center, both lenses perform well enough, but things get worse with the light source in the corner, where flare and ghosting take their toll. The Rokinon is arguably the worst offender. The D FA* performs best, even though it's not completely free of ghosting.


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