Pentax 50mm F1.4 comparison: M, FA and DFA

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 M 50mm uses one of the first versions of Pentax's SMC coatings. This 1978 version, while class-leading at the time, is certainly not as advanced as the newer HD coatings used on the DFA* 50mm. The FA's coatings are also SMC, dating from 1991 and probably an improvement over the M 50mm'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.

M 50mm FA 50mm DFA* 50mm
F1.4
F2
F2.8
F4
F5.6
F8
F11
F16
F22

The first flare effects begin to appear at F8 for all three lenses, and they are subtle enough to be easy to miss. At F11 and F16, the M lens shows some decreased contrast caused by the ghosts of the starbursts lines. The FA does better, with no detrimental effect to speak about. The D FA*s larger glass elements show their limit, with a hint of ghosting at F11 and F16. The effect is not bothersome but is present nonetheless.

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

M 50mm FA 50mm DFA* 50mm
F1.4
F2
F2.8
F4
F5.6
F8
F11
F16
F22

Even wide open, the M and FA lenses show ghosting in the corner opposed to the sun. The D FA* isn't perfect but the effect is much more subdued. Ghosts are  mostly gone for all three lenses by F5.6, but others come back for the two older lenses at F16.

APS-C

M 50mm FA 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. The magnification offered by the APS-C sensor shows that ghosts are visible for the two older lenses until F11 (F16 for the M). The effect isn't worse than with most other lenses. The D FA* offers the best result, with some subtle ghosting but no contrast decrease.

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.

M 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

Green CA is visible on the top image for apertures wider than F8. The center is remarkably clear of CA. The bottom shows hints of purple fringing at wide settings.

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. CA is not limited to the edges of the frame.

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
F22 F22 Center F22 Top F22 Bottom

The FA 50mm is similar to the M 50mm for CA and PF. This is one area where the updated design did not bring much improvements.

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 APS-C crop does not bring much difference, except that CA goes away at F5.6, showing that it it confined to the edges.

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

There is some PF at apertures wide than F2.8 at the bottom, and a hint of CA at the top. This is a clear improvement over the other two designs.

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 shows the CA in more detail. It's present at F2.8 and wider, and kept under control afterwards.

Verdict

None of the three testes lenses have much problem with flare when the sun is in the center. Some ghosting appears when it is located in the corners, the worst offender being the M, followed by the FA. The D FA* is not devoid of ghosting but the effect is much less obtrusive.

CA is present at wider apertures for all three lenses, being more apparent for the M, followed by the FA and the D FA*. This is one area where we would have expected the newer lens to outshine its siblings. It is likely that some leeway was given to the designers since CA is easy to correct automatically nowadays.


fb.com/PentaxForums PentaxForums +Pentax Forums @PentaxForums News | Reviews | Forum

Support Pentax Forums Donate to Pentax Forums Support Pentax Forums