HD Pentax-DA* 11-18mm F2.8 ED DC 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 Pentax "star" lens, the 11-18mm F2.8 comes with high expectations for sharpness. Wider angle lenses often struggle at the edges and corners, but the bold claim implied by the F2.8 maximum aperture suggests that the 11-18mm should deliver high resolution figures.

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 distances between 1.1 and 1.8 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 Pentax's current top-of-the-line sensor: 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 11mm on APS-C

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

Center
Edge
Corner
F2.8
APS-C Center F2.8 11mm APS-C Edge F2.8 11mm APS-C Corner F2.8 11mm
F4
APS-C Center F4 11mm APS-C Edge F4 11mm APS-C Corner F4 11mm
F5.6
APS-C Center F5.6 11mm APS-C Edge F5.6 11mm APS-C Corner F5.6 11mm
F8
APS-C Center F8 11mm APS-C Edge F8 11mm APS-C Corner F8 11mm
F11
APS-C Center F11 11mm APS-C Edge F11 11mm APS-C Corner F11 11mm
F16
APS-C Center F16 11mm APS-C Edge F16 11mm APS-C Corner F16 11mm
F22
APS-C Center F22 11mm APS-C Edge F22 11mm APS-C Corner F22 11mm

Sharpness is impressive in the center, even at F2.8. Edges and corners are not too far behind, and mostly catch up around F4. By F5.6, the entire frame is as sharp as can be expected. This will be useful for landscape shooters, who will be able to rely on the DA* 11-18mm to deliver uniform and detailed results across the frame.

Diffraction kicks in at F16, with a visible drop in sharpness. F22 should be avoided as the sharpness decreases even more.

Test Results at 14mm on APS-C

Center
Edge
Corner
F2.8
APS-C Center F2.8 14mm APS-C Edge F2.8 14mm APS-C Corner F2.8 14mm
F4
APS-C Center F4 14mm APS-C Edge F4 14mm APS-C Corner F4 14mm
F5.6
APS-C Center F5.6 14mm APS-C Edge F5.6 14mm APS-C Corner F5.6 14mm
F8
APS-C Center F8 14mm APS-C Edge F8 14mm APS-C Corner F8 14mm
F11
APS-C Center F11 14mm APS-C Edge F11 14mm APS-C Corner F11 14mm
F16
APS-C Center F16 14mm APS-C Edge F16 14mm APS-C Corner F16 14mm
F22
APS-C Center F22 14mm APS-C Edge F22 14mm APS-C Corner F22 14mm

The observations obtained at 11mm mostly transfer to the middle of the zoom range, at 14mm. The center resolution is high even at F2.8, and improves at F4 and F5.6. At this latter value, edges and corners have caught up and the whole frame is again very detailed. Even at F2.8, edges are sharper than corners, closer to the center resolution.

Test Results at 18mm on APS-C

Center
Edge
Corner
F2.8
APS-C Center F2.8 18mm APS-C Edge F2.8 18mm APS-C Corner F2.8 18mm
F4
APS-C Center F4 18mm APS-C Edge F4 18mm APS-C Corner F4 18mm
F5.6
APS-C Center F5.6 18mm APS-C Edge F5.6 18mm APS-C Corner F5.6 18mm
F8
APS-C Center F8 18mm APS-C Edge F8 18mm APS-C Corner F8 18mm
F11
APS-C Center F11 18mm APS-C Edge F11 18mm APS-C Corner F11 18mm
F16
APS-C Center F16 18mm APS-C Edge F16 18mm APS-C Corner F16 18mm
F22
APS-C Center F22 18mm APS-C Edge F22 18mm APS-C Corner F22 18mm

Surprisingly, it is at 18mm that the lens shows the most important differences between center and corners. The center is, yet again, very sharp at all aperture values below F16. Edges and corners improve as the aperture closes but do not compare with the center until F11. They are still perfectly usable, particularly for apertures smaller than F4.

It is apparent that the lens was optimized for usage at wider focal lengths, which makes sense as there are many lenses covering 18mm on APS-C. Still, the compromises are minimal and should not be taken to mean that performances are disappointing at 18mm.

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 11mm on APS-C

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

Center
Edge
Corner
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
F22
APS-C Center F22 APS-C Edge F22 APS-C Corner F22

The results observed with the test chart are repeated here. The center is perfectly usable wide open at all apertures, even the wider ones. The difference between the center and the edges and corners is more apparent here, but the drop in sharpness is not overly dramatic.

Test Results at 14mm on APS-C

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

Center
Edge
Corner
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
F22
APS-C Center F22 APS-C Edge F22 APS-C Corner F22

Again, the center is better, and shows high sharpness even at F2.8. The edges catch up at medium apertures and the corners struggle a bit more.

Test Results at 18mm on APS-C

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

Center
Edge
Corner
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
F22
APS-C Center F22 APS-C Edge F22 APS-C Corner F22

It is at 18mm that the differences are the smallest between the center and the edges and corners. Again, the center shows impressive sharpness results.

Summary

These results show taht the DA* 11-18mm is a worthy addition to the STAR lineup. Center sharpness is excellent at all aperture values, and edges and corners are not too far behind. Uniformity across the frame is thus high, particularly at medium apertures, and the lens will be a good choice for anything involving wide fields of view. Note that results are marginally better at wider focal lengths.


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