Pentax DA55 and FA50 - a comparison

Technical Specifications:
smc DA* 55mm f/1.4 SDM

Angle of View 28.6 degrees
Construction 9 elements / 8 groups
No. of aperture blades 9
Minimum focussing distance 0.45m
Filter size 58mm
Weight 375g (415g with hood)
Dimensions (Length x Diameter) 66 x 70.5mm
Special Features - Super protect coating of front lens
- Aero Bright Coating
- Circular diaphram
- SDM only
- Weather sealed
- "Quick Shift" focusing

Size: FA50, Nokton, DA55
Pentax FA50, Voigtländer Nokton, Pentax DA55


What follows are some images showing the performance of the Pentax DA55/1.4 compared to the Pentax FA50/1.4.
All tests with the K20D.

Disclaimer: This is a test by a layman, who does not really know what he is doing.


1. Resolution: The resolution of both lenses in the center is the same. The FA shows some weaknesses at f1.4 in the corner.
At infinity the DA is also slightly better at f1.4, mainly due to better contrast.
At f5.6 they are the same.

2. Contrast: The DA55 shows a slightly better contrast.

3. CAs: Not yet fully tested. No big difference though.

4. Flare: The DA shows better flare control.

5. Bokeh: The Bokeh of the DA55 and the FA50 is almost identical. The circle of confusion looks like in most modern lenses today: A smooth Gaussian transition in the foreground and a bright border in the background ("over-corrected spherical aberration"). The bright border is a little stronger in the FA50.

AF Speed: The big disadvantage of the DA55 is that it is much slower compared to the FA50 in AF speed. The DA55 is about 3 times slower than the FA (measured using Audacity)! I am wondering why this is so and what Pentax has to say about it. The argumentation that the AF of the DA55 is slow because it is a portrait lens does not hold. The FA85, which the DA55 should replace, is very fast.
A few quotes on this from the forums:
Frank (pentaxforums): "For a new SDM lens, AF is indeed a bit slow, slower than I hoped."
Mark "Dimo" Dimalanta (pentaxforums): "the FA 50 does lock focus faster than the DA*55."
Heribert (dfn-forum):"It is slower compared to the FA, significantly. I have tested it on the K10 and K20."
Wheatfield (pentaxforums): "glass is very good, but the AF sucketh. AF is noticeably slower than the 70/2.4"
Rainer, TKH (dfn forum):"The DA55 needs significantly longer for moving from closest focussing distance to infinity. The FA135 is like a rocket, comperatively."
Hannes (private com.): "Compared, the DA70 is a rocket!"
Markus, Maweryk (dfn forum):"The FA is faster and also the other results from blende8 correspond to my real life shots."
Pentax (inofficial): "Due to the slim DoF we have abstained from giving it full speed. We chose a compromise to diminish hunting and oscillating." - Lame excuse IMO. It oscillates the same as other lenses. IMO, the real cause is the weak micro-motor. It is just slow compared to the in-camera motor or true ring motors.

Update: DA55 on the K-7
I have tested the DA55 on the K-7 and can say that it is not faster.

Backfocus: So far those copies that needed a focus correction showed backfocus. The reports range from +3 to +8 focus adjustment on the K20D. Be careful and check twice before buying this lens for a camera body without focus adjustment.

Manual focus: It's ok, but not optimized. It is not as good as the DA200 which has a gear ratio of 2:1 for manual focussing. The DA55 has no transformation, it's 1:1.

Throw of the focussing ring:
FA85: ca. 90 degrees (superfast AF)
DA55: ca. 110 degrees (slow AF)
FA50: ca. 120 degrees (fast AF)
Nokton: ca. 210 degrees
DA200: ca. 260 degrees

Full frame? A test on a film body shows no vignetting. I haven't checked corner sharpness though. The rear lens element is larger than that of the FA50. Since the lens has no aperture ring, its use on manual bodies is limited.

The overall optical quality of the DA55 is slightly better compared to the FA50. This was expected. Looking at the images below, the improvements may seem considerable. In my view this is not the case. Note, these are 100% crops from a 14 MP camera! In a 20 x 30 cm print you will see nothing of it. In practice the slower AF speed may play a more dominant role than corner sharpness at f1.4.
At least for me. But each to his own.
Thinking about it, perhaps a useful application of this lens could be in astrophotography, where you are shooting wide open. The good correction will certainly be beneficial here. comes to the same conclusion: "when we look at the price of Pentax classic FA 1.4/50 everything becomes clear. It is optically worse than DA* 1.4/55 but only marginally. In reality, it will be difficult to notice any remarkable differences."

Resolution has been tested using a tripod, MLU and manual focussing, with a focus array of 20-30 images each.
Of each lot the sharpest image has been selected by visual inspection.

All 100% crops.

Resolution at Infinity:
Note that the DA has a 5mm longer focal length.
I haven't corrected for this.

Another one at f1.4:

Contrast has been measured using the method described here, using Image-J.



CA and fringing:
Within the plane of focus CAs are almost non existent.
Out-of-focus fringes: Typical red in the foreground, green in the background
(all images DA55, the FA50 looks the same)

Below is the strongest flare I was able to produce with the two lenses by shooting directly into the sun.
Without hood.
The better flare control of the DA might be, at least in part, due to the recessed front lens.

Circle of confusion in the background:
(below the brightness across the ring is shown)

Pattern bokeh:
This has been produced by turning the focus ring in small steps, starting from infinity (left). The bokeh is virtually identical for both lenses.
Please note that it is impossible to do this stepping 100% identical for two lenses. Slight differences in the focus distance are unavoidable.
top: DA
bottom: FA

Backgrund bokeh using seagulls:

Some sample images:

DA55, f1.4, 1/160 s, ISO 100

DA55, f1.4, 1/60 s, ISO 100

DA55, f1.4, 1/3200 s, ISO 100
100% crop:

DA55, f1.4, 1/1000 s, ISO 100

100% Crop:

DA55, f1.4, 1/1250 s, ISO 100

100% Crop:

DA55, f1.4, 1/4000 s, ISO 100