Decent Optical Performance and Questionable Quality Control

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twomonger Regular Member • Posts: 157
Decent Optical Performance and Questionable Quality Control
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Pentax's lens releases have slowed to a glacial pace in recent years, particularly with regards to full frame lenses. According to DPReview, the last lens announced was the D FA 15-30mm f/2.8, in Feb 2016, over 2 years ago.

Consequently, the D FA 50mm f/1.4 is a much welcomed addition to Pentax's sparse FF prime lens lineup. While rumors have been swirling since early 2017 and the lens has slipped from a 2017 release, to "fiscal year 2017", to spring 2018, to summer 2018, a top-performing lens would absolutely be worth any wait.

This lens was tested on the 36 MP Pentax K-1.

At $1200, the lens is moderately more expensive than the excellent Sigma 50mm Art f/1.4 ($950) and on a similar price level with the outstanding Sony 50mm ZA f/1.4. The D FA 50mm matches the large size of the Sigma and Sony lenses. Thus, from size and price alone, the user is led to expect similar levels of performance. Pentax fans have fueled the rumor that this lens is "Otus-level," a high bar indeed to live up to.

The lens itself is very solidly built. The manual focus ring is wide, grippy, and easy to use. It has a long focus throw that makes precision adjustments extremely simple. The lens is large compared to previous 50mm f/1.4 designs, but comparable to the modern Sigma Art/Sony. It is well balanced on the 50mm and smaller than the D FA 24-70mm f/2.8. Handling is excellent.

The autofocus speed is excellent. The motor is very fast and doesn't have the spool-up time of other D FA lenses - a major improvement. Viewfinder AF, unfortunately, is unreliable and inaccurate on K-1, particularly in low light or low contrast situations - a long standing Pentax issue. Front and back focusing is common, though in good light focus is acceptable. Fortunately, the fast and high-torque motor mean live view contrast AF is quite fast and usable for snapshots and portraits. The PDAF inaccuracy issues are largely the fault of the poor performance of Pentax bodies as opposed to any flaws with the lens itself.

Sharpness-wise, the lens is very sharp in the center @ f/1.4, visually on par with the Sigma Art 50mm f/1.4 on the 5D IV, or maybe slightly less sharp. Corner performance @ f/1.4 is less impressive, though. While it is a significant improvement from the old 50mm f/1.4 designs, it is simply not as sharp as the 50mm f/1.4 Art on the AA-filtered 30 MP 5D IV. Without sophisticated equipment it is tough to quantify lens performance, but having repeated my own tests the softer corners are very consistent. The corners do sharpen up a bit by f/2.0 and reach near peak sharpness by f/2.8.

Some Pentax forum users have said the D FA 50mm is sharper than the Sigma 35mm Art. This is not an appropriate comparison, as achieving high sharpness (or high optical performance in general) is much easier in a 50mm than 35mm. Even Sigma's own 50mm Art is sharper than its 35mm.

Out of focus areas are very pleasing. This is a largely subjective assessment, but I very much liked the bokeh and it does not show any of the "busy-ness" that can be problematic.

The D FA 50mm's CA performance is acceptable to decent. As DPReview's own hands-on testing has showed, the lens exhibits a significant amount of LoCA, especially near wide-open. It is a common problem in very fast lenses, so it is understandable that Pentax could not eliminate it, but it is worse than the 50mm Art and the apochromatic Zeiss 55mm Otus.

Vignetting is heavy at f/1.4. Light falloff improves a lot by f/2.0 and is minimal by f/2.8 and going to f/4.0 does not improve things further. But all fast 50mm lenses have similarly high vignetting, and the D FA 50mm is not notably worse.

"Coma" (though more accurately termed astigmatism) aberration is moderate at f/1.4 to f/2.0. It decreases a lot by f/2.8 and is gone by f/4.0. The performance is in line with other lenses in the category.

My lens copy had a soft corner that is visible at 25% magnification, which did not disappear even when stopped down to f/4.0. This is unacceptable quality control for such an expensive lens. While some copy variation is unavoidable, Sigma's Art or Canon L series lenses I have owned or tried have not had issues on this scale.

In conclusion, this lens is offers decent optical performance relative to the other standard bearers in the segment, while excelling in a number of other areas. Build quality, handling, AF speed, and bokeh are all excellent. The lens will produce some excellent portraits, even when used wide open. While center sharpness and vignetting performance are comparable with the best, the corner sharpness wide open and LoCA is worse. It is certainly not on par with the 55mm Otus, a lens which is nearly flawless in terms of sharpness and chromatic aberration performance. There is also serious issue with quality control on this lens as well, which may be tolerable in a $400 budget lens, but not in a $1200 flagship lens.

For a bigger manufacturer, a decent 50mm f/1.4 might not be a problem. 50mm lenses are often used in portaits where corner sharpness or LoCA is less important. Canon and Nikon have plenty of lenses are not optically perfect but end up selling very well. While Pentax users, who are almost fanatical in their dedication to the brand, will buy the lens regardless of its performance, Pentax itself is in a precarious position due to a tiny userbase and minimal brand awareness. A non-existent marketing and advertising presence and neglible professional use leaves only reviews and word of mouth as the means of growing the brand.

To win over new users, Pentax needed a smash hit. By that standard, the D FA 50mm f/1.4 may be a bit of a letdown. I personally will be returning this lens – the quality control is a serious issue. Yet QA issues aside, I still have a hard time justifying the cost given the performance. In other mounts there are a number of better options.

Below are some astrophotography examples of the lens at f/1.4, f/1.8, f/2.0, f/2.8, and f/4.0. Exposure might not be perfectly matched across frames. No lens correction was applied. Daylight white balance was used. Astigmatism, chromatic aberration, and vignetting performance can be determined from these images, and the clearly evident soft corner can be seen as well. The mount was adjusted between some frames (due to wind) so the field moves a bit. The great Ha sensitivity of the K-1 can be seen in the California nebula. It is a shame the lens is pretty much unusuable at f/1.4. It is acceptable at f/2.0 and excellent by f/2.8. At $1200, I expect a lens to be usuable at f/1.4.

I cannot recommend this lens for astrophotography, given its current price. Those with only K-mount are better off making do with the 35mm f/1.4 Art, which is a different focal length and isn’t as sharp as a 50mm but can serve a similar purpose. Users in other mounts are better off using the 50mm Art.

f/1.4 30 sec ISO 1600

f/1.8 50 sec ISO 1600

f/2.0 70 sec ISO 1600

f/2.8 130 sec ISO 1600

f/4.0 240 sec ISO 1600

 twomonger's gear list:twomonger's gear list
Sony Alpha a7R II Pentax K-1 Sigma 35mm F1.4 DG HSM Art Sigma 18-35mm F1.8 DC HSM Art HD Pentax D FA 24-70mm F2.8 ED SDM WR +2 more
HD Pentax-D FA* 50mm F1.4 SDM AW
Prime lens • Pentax KAF4
Announced: Oct 26, 2017
twomonger's score
3.0
Average community score
4.0
Canon EOS 5D Mark IV Pentax FA* 50mm F1.4 SDM AW Pentax K-1
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