The Pentax smc FA 50mm F1.4 is soft wide open on the APS-C format, especially in comparison to the Sigma 50mm F1.4 EX DG HSM, but sharpens up rapidly on stopping down, matching the Sigma by F2.2 and giving impressive results right across the frame at normal working apertures. Distortion is especially low, and as usual for a full-frame lens used on APS-C, falloff is nothing to worry about.
|Sharpness||The lens is distinctly soft wide open, and the checkerboard crops reveal extremely low contrast with blacks rendered as blue-grey. Sharpness improves rapidly on stopping down; it's very good at F2, and excellent right across the frame in the lens's 'sweet spot' around F2.8-5.6. (The 'doughnut' profile at these apertures, with the maximum sharpness no longer in the centre, is indicative of a slight focus shift as a result of spherical aberration.) Stopping down further results in softer images due to diffraction, with F18-22 probably best avoided.|
|Chromatic Aberration||At wide apertures we see blue colour blur across the whole frame, typical of lenses of this class and a symptom of spherical aberration. This disappears on stopping down to F2, from which point onwards the lens shows measurable, but scarcely relevant lateral CA.|
|Falloff||We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the centre. As usual for a full-frame lens used on APS-C, falloff isn't really an issue; there's just a stop at the extreme corners wide open, which clears on stopping down.|
|Distortion||The lens is extremely well-corrected, with just 0.3% barrel distortion measurable; this is very low even by the standards of FF lenses on APS sensors, and certainly too low to impact on actual use.|
Specific image quality issuesAs always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests.
Softness wide open
It's an F1.4 lens, and therefore the Pentax 50mm is soft wide open, a characteristic exacerbated by its usage on a high-resolution 1.5x crop APS-C sensor. It's also worth noting that depth of field is so shallow at F1.4 that real-world results are highly dependant upon focus accuracy, and this lens will tax the abilities of any focusing system, either auto or manual (not to mention the fact that the slightest relative movement between photographer and subject will result in a misfocused image). We found the K20D to be slightly inconsistent at focusing this lens, with several shots often necessary to achieve perfect focus when shooting wide open.
As usual we like to provide an indication of the results which can be expected in real-world use, but in this case have decided to eschew our usual brick-wall approach in favour of a marginally more interesting subject. At F1.4, the lens does little more than provide an impression of fine detail at low contrast, at either the centre or the corner of the frame. However it improves dramatically on stopping down to F2.8, yielding a much more detailed rendition of the texture on the surface of the metalwork.
|Pentax K20D, ISO 400||Pentax K20D, ISO 400|
|100% crop, centre of frame||100% crop, centre of frame|
|100% crop, top left corner||100% crop, top left corner|
The Pentax 50mm F1.4 seems to be somewhat more susceptible to flare than its Canon and Nikon counterparts, presumably due to that compact barrel design with the front element rather more exposed to stray side light. The flare patterns produced by this lens can also look quite spectacular, in both size and colour, although this is partially due to the fact that we can only test it on an APS-C sensor (comparative flare shots on the other 50mm primes used full frame cameras).
The flare patterns seen at various apertures with the sun placed in the corner of the frame are shown below; clearly the best result is seen at F8, where the lens does a much better job than at smaller or larger apertures. If you regularly shoot under strong side-lit conditions, you'll also frequently see flare especially at wider apertures; use of a lens hood is strongly recommended.
|F1.8, Pentax K20D||F8, Pentax K20D|
|F16, Pentax K20D||F2.2, Pentax K20D (sun out of frame)|
Background Blur ('bokeh')
One genuinely desirable, but difficult to measure aspect of a lens's performance is the ability to deliver smoothly blurred out-of-focus regions when trying to isolate a subject from the background, generally when using a long focal length and large aperture. 50mm F1.4 lenses can generally be made to blur even relatively close backgrounds into oblivion at wide apertures, a huge advantage for portrait shooting especially on DX.
The bokeh produced by this lens can certainly be described as full of character, but at larger apertures (F1.4-F2) it's distinctly hard-edged and 'busy'. This means that while the fast maximum aperture in principle allows excellent subject isolation, the background can still end up competing for the viewer's attention, which arguably negates the point. Stopping down to F2.8 results in a much smoother overall effect.
|F2, Pentax K20D||50% crop|
|F1.4, Pentax K20D||50% crop|
|F2, Pentax K20D||50% crop|
|F2.8, Pentax K20D||50% crop|
Lateral chromatic aberration is negligible in our studio tests, and is equally near-impossible to find in real-world shots; quite simply it's not an issue when using this lens. However intense green/magenta bokeh chromatic aberration is visible at wider apertures. When combined with that hard-edged bokeh rendition, this can yield some quite problematic results, such as colour shifts in background detail. Again however the effect diminishes on stopping down, and has almost disappeared by F2.8.
|F1.4, Pentax K20D||100% crop|
|F1.4, Pentax K20D||50% crop|