Pentax-D FA 100mm F2.8 Macro WR

Sharpness

When browsing online forums where people discuss the merits of various lenses, you sometimes get the feeling that sharpness is everything. The ability to resolve small details is certainly 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 lower the perceived quality of the picture.

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 100mm lens, this means a distance of 10 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 dependant 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 100mm

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

Center
Edge
Corner
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
f32 Center f32 Edge f32 Corner f32

Images are excellent in the center even at f2.8. They remain this way until f11 where sharpness starts to decrease, up to f32 where images are very soft. The differences between wide open and f8 are very small. The lens can be used comfortably at any of these apertures. Resolution peaks at around f5.6. There is a small amount of purple fringing visible at f2.8 and f4.

The results follow a similar thread on the edge. Even wide open, resolution is very high, and remains so until f11. F32 should be avoided since it is quite soft.

The lens shows almost no differences between the results from f4 to f11 in the corners. There is some slight purple fringing visible at f2.8, but it's gone when the aperture closes. The widest setting also shows decreased sharpness compared to f4. Smaller apertures (f16 and beyond) show a diminishing contrast caused by diffraction.

Test Setup, Still Life

Test charts never tell the whole story. Because of this, we include  here a scene with a lot of detail and colors, in order to evaluate  performances with an actual subject. We used a small wooden clock that is about 10 cm in diameter, and was positioned at a slight angle in order to show depth. The focus was locked on the Sun near the  center of the frame.

Test Results at 100mm

The following images showcase the results at all apertures. Click on any thumbnail to load a full resolution image (caution: large files!).

f2.8 f2.8
f4 f4
f5.6 f5.6
f8 f8
f11 f11
f16 f16
f22 f22
f32 f32

Verdict

It is a rare pleasure to test a lens offering this level of performance. First, the level of details in the images we tested is very high right from the widest aperture. Second, performances are very even across the whole frame. Results might be different with a full frame sensor, but for APS-C it would be extremely difficult to find a lens offering better sharpness.


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