HD Pentax-D FA* 85mm F1.4 ED SDM AW

Sharpness

Sharpness, or the ability to resolve small details, is an important measure of the performance of a lens. Sharp images let the viewer concentrate on the composition, color, and light. On the other hand, soft images (except when looking that way on purpose) distract the viewer and decrease the perceived quality of the picture.

Like every STAR lens, the 85mm F1.4 is regarded as a top-tier lens. This comes with high expectations for sharpness, especially for a short tele focal length.

There are many ways to evaluate sharpness. Some are quantitative, such as the number of lines per millimeter that can be resolved, while others are comparative, such as using a standardized scene to pit lenses against one another. The latter is the favored method at Pentax Forums.

Test Setup, Star Chart

In order to evaluate sharpness, we photograph a standard test chart (or "star chart") that can be used to compare lenses to one another. The general rule is that the distance from the lens to the test chart must be 100 times the focal length of the lens. For our lens, this means a distance of 8.5 meters.

The test is not designed to show how good a lens can be. Quite the contrary: it is a stress test designed to put the lens at its limits to show where it falls short. The test shows the apertures where the lens performs best, and makes it possible to compare with another lens.

Our test has the star chart put successively at the center, edge, and corner of the field of view of the lens, testing all apertures each time.

Resolution is obviously dependent on the sensor used. For this test, we used both of Pentax's current top-of-the-line sensors: the K-1's full frame 36 MP and the K-3 's APS-C 24 MP.

New Pentax DSLR bodies include the ability to disable anti-aliasing, a very useful feature allowing higher resolutions to be reached. However, the nature of our sharpness test and the pattern of the test chart mean that, with anti-aliasing totally disabled, moiré is likely to appear. In order to strike the best balance between sharpness and moiré reduction, the anti-aliasing simulators were activated and set at the "Low" setting.

Test Results at 85mm on Full Frame

The following images showcase the results at all apertures using a full frame camera. Focusing was performed by using focus peaking in live view. Focus was corrected after each lens movement. You can click on images to see full resolution crops.

Center
Edge
Corner
F1.4
FF Center F1.4 FF Edge F1.4 FF Corner F1.4
F2
FF Center F2 FF Edge F2 FF Corner F2
F2.8
FF Center F2.8 FF Edge F2.8 FF Corner F2.8
F4
FF Center F4 FF Edge F4 FF Corner F4
F5.6
FF Center F5.6 FF Edge F5.6 FF Corner F5.6
F8
FF Center F8 FF Edge F8 FF Corner F8
F11
FF Center F11 FF Edge F11 FF Corner F11
F16
FF Center F16 FF Edge F16 FF Corner F16

The D FA* 85mm delivers center sharpness results that are close to flawlessness. Even at F1.4, the center is impressive, with marginal improvements until F8. F11 is close still, and F16 is visibly softer. We can see why the designers did not offer an F22 aperture.

It is almost unheard of to see such resolving power at F1.4. This is quite probably the highest sharpness capabilities of any lens currently available on the K-mount at F1.4.

Edges and corners are a different story. They are adequate, but much softer, at wider apertures. At F5.6 and F8, the results are significantly improved, without reaching the quality of the center. This is in part because the center has such a high resolution.

These results show that the lens has been optimized with portraits in mind: it is ideally suited to isolate a subject from its surroundings. It is not, however, designed for edge-to-edge sharpness.

Test Results at 85mm on APS-C

The following images showcase the results at all apertures using an APS-C camera. Focusing was performed by using focus peaking in live view. Focus was corrected after each lens movement. You can click on images to see full resolution crops.

Center
Edge
Corner
F1.4
APS-C Center F1.4 APS-C Edge F1.4 APS-C Corner F1.4
F2
APS-C Center F2 APS-C Edge F2 APS-C Corner F2
F2.8
APS-C Center F2.8 APS-C Edge F2.8 APS-C Corner F2.8
F4
APS-C Center F4 APS-C Edge F4 APS-C Corner F4
F5.6
APS-C Center F5.6 APS-C Edge F5.6 APS-C Corner F5.6
F8
APS-C Center F8 APS-C Edge F8 APS-C Corner F8
F11
APS-C Center F11 APS-C Edge F11 APS-C Corner F11
F16
APS-C Center F16 APS-C Edge F16 APS-C Corner F16

Even with the higher resolving power of the K-3's smaller pixel pitch, it is hard to tell apart center results at F1.4 and F8. Again, this is nothing short of amazing. F11 and F16 show the same tendency as on full frame, of course.

The edges and corners show results similar to those on full frame. This indicates that the stellar center sharpness does not extend very far from the center.

Real-Life Test

Test charts never tell the whole story. The following images illustrate the performance of the lens with a real-life test subject with a lot of detail. We used a close distance to differentiate with the star chart tests, which are performed at a significant range. We moved our camera to place the subject in the center, the edge and then corner, refocusing between each shot.

Test Results at 85mm on Full Frame

The following images showcase the results at all apertures on full frame. Findings will be applicable to APS-C, following the conclusions of the previous section. Focusing was performed by using focus peaking in live view. Focus was corrected after each lens movement. You can click on the images to see full resolution crops.

Center
Edge
Corner
F1.4
FF Center F1.4 FF Edge F1.4 FF Corner F1.4
F2
FF Center F2 FF Edge F2 FF Corner F2
F2.8
FF Center F2.8 FF Edge F2.8 FF Corner F2.8
F4
FF Center F4 FF Edge F4 FF Corner F4
F5.6
FF Center F5.6 FF Edge F5.6 FF Corner F5.6
F8
FF Center F8 FF Edge F8 FF Corner F8
F11
FF Center F11 FF Edge F11 FF Corner F11
F16
FF Center F16 FF Edge F16 FF Corner F16

The results observed with the test chart are repeated here. Center sharpness is more than impressive at all apertures except F16, and egdes/corners need medium apertures to reach their peak.

Summary

The D FA* 85mm's resolution figures are superlative in the center, even wide open. This is certainly class-leading. The edges and corners are also excellent at F5.6 and F8, where they come close to the center, and outperform a few of the other lenses that we tested in the past. At wider apertures, the edges and corners are visibly softer; while perfectly usable, they certainly are no match compared to the center. This highlights that the lens was designed with portraiture in mind.


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