Pentax-DA 50mm F1.8

Sharpness

Sharpness, or the ability to resolve small details, is an important measure of a lens' performance. 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.

Historically, 50mm lenses have long been regarded as reference points for sharpness. The classic 50mm design is fairly simple, requiring few optical elements. Especially when closed down, a 50mm lens is expected to deliver sharp images.

There are many ways to evaluate sharpness. Some are quantitative, such as counting 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 use 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 50mm lens, this means a distance of 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 lens' field of view, testing all apertures each time.

Resolution is obviously dependent on the sensor used. For this test, we used Pentax's current top-of-the-line sensor found in the K-3.

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 means 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 simulator of the K-3 was activated, but set at the "Low" setting.

Test Results at 50mm

The following images showcase the results at all apertures. You can click on images to see lightboxes arranged in columns.

Center
Edge
Corner
F1.8
Center f1.8 Edge f1.8 Corner f1.8
F2
Center f2 Edge f2 Corner f2
F2.8
Center f2.8 Edge f2.8 Corner f2.8
F4
Center f4 Edge f4 Corner f4
F5.6
Center f5.6 Edge f5.6 Corner f5.6
F8
Center f8 Edge f8 Corner f8
F11
Center f11 Edge f11 Corner f11
F16
Center f16 Edge f16 Corner f16
F22
Center f22 Edge f22 Corner f22

Center sharpness is very good, even wide open. It increases up to approximately F2.8, beyond which there is no further gain. F16 shows a slight decrease in sharpness due to diffraction, and F22 is visibly softer, while remaining usable.

Edge sharpness is acceptable wide open, and improves up to F5.6. F16 is again the point where diffraction begins to affect sharpness, and F22 is again visibly softer.

As expected, corners are the weakest link. Sharpness is again acceptable wide open, but improvements are seen up to F8. F11 already shows the first signs of decreased resolution, F16 is visibly softer and F22 even more so.

Real-life test

Test charts never tell the whole story. The following images illustrate the performances of the lens with a real-life, colored and detailed test subject.

F1.8
F1.8
F2
F2
F2.8
F2.8
F4
F4
F5.6
F5.6
F8
F8
F11
F11
F16
F16
F22
F22

Summary

If sharpness across the frame is important, the lens is best used at F5.6, F8 or F11. If only center sharpness is important (for portraits or other such applications), then the lens is usable at most apertures, and will deliver tack-sharp results at F2.8 and smaller.

These results are impressive, especially for a lens of this price. It is easy to recommend the DA 50mm F1.8 if sharpness is an important aspect.


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