Pentax DA 35mm F2.4 vs 40mm XS vs 40mm Limited

Image Quality: Sharpness

In reviewing these lenses for image quality we attempted to test every relevant aspect of the lenses' performance, including sharpness, vignetting, distortion, bokeh, and flare resistance. For sharpness testing, we used both static chart-based evaluation and field tests, since we feel test charts don't always show off the subjective sharpness of the lens—i.e., the combination of resolving power and contrast that results in perceived sharpness.

Studio Sharpness Test

For this test, we placed the test camera on a tripod perpendicular to a test chart affixed to a white wall, evenly illuminated by studio lights. We shot each lens at every full stop from f/2.8 to f/22, as well as f/2.4 for the 35mm.

• Single-point Live View autofocus (on center of chart)
• ISO 80
• JPEG (highest quality)
• White balance set to Auto
• Auto-exposure; Multi-segment / Evaluative metering
• Image mode set to camera default ("Bright")
• Dynamic range extension off; In-camera lens corrections off
• Camera on tripod, 2-second self-timer; SR off

Conclusions

The first thing to note about these results is that all three lenses do quite well, even wide open. The DA 40mm f/2.8 XS, in particular, is sharp across the entire frame, while the DA 40 Limited and DA 35 display some corner softness at the same f-stop. The DA 40 Limited sharpens up nicely in the corners by f/4, while the DA 35 isn't really tack sharp until f/8 or f/11. All three lenses are quite sharp in the center, even from f/2.8. In an ideal world, this should be the expected behavior, since f/2.8 is a pretty slow maximum aperture for a prime lens, but in the real world f/2.8 primes don't always produce such excellent sharpness from the get-go.

Interestingly, the DA 40mm f/2.8 Limited, while not the sharpest of the bunch here wide open, doesn't display the same horrific performance that you'll see from it in our field sharpness tests below. To be honest, we're not sure how to account for this disparity. In general, the lens was quite sharp when shooting at close and mid-range distances at wider apertures; when shooting at infinity at the same f-stop, it was abysmal, every time.

Keep in mind that this test was conducted near minimum focal distance for each lens, while the field sharpness tests below were conducted at or near infinity focusing. Some lenses excel at close focusing and fall apart when shooting landscapes; others, the opposite; and still others manage to master both.

Field Sharpness Tests

For these tests, we placed the test camera on a tripod with a view of a distant or mid-range landscape. We shot each lens at every full stop from f/2.8 to f/22, as well as at f/2.4 for the 35mm.

• Single-point Live View autofocus (on center of scene)
• ISO 80
• JPEG (highest quality)
• White balance set to Auto
• Auto-exposure; Multi-segment / Evaluative metering
• Image mode set to camera default ("Bright")
• Dynamic range extension off; In-camera lens corrections off
• Camera on tripod, 2-second self-timer; SR off

Conclusions

We can see in the first field sharpness test that our copy of the DA 40 Limited has serious problems shooting at infinity with wider aperture settings. In fact, the performance at f/2.8 and f/4 is so bad that we're pretty certain this particular unit had some kind of mechanical defect. (We've owned DA 40 Limiteds in the past that did not exhibit this behavior in the slightest.) In contrast, the DA 40 XS (using what appears to be the same optical configuration) is pretty bitingly sharp from wide open. What's really interesting, however, is that even at f/4 (where it has truly awful border performance) the DA 40 Limited clearly outperforms the DA 40 XS in the center of the image. This might be down to sample variation, focusing behavior, or it might be a side effect of whatever is causing the Limited's awful border performance—we really can't say.

The DA 35mm f/2.4 hangs right in there with the DA 40 XS at f/2.8, and even compares well at f/2.4. It has perhaps the best corner and border sharpness from wide open, though the DA 40 XS has a slight advantage at the center. By f/4, all three lenses are pretty close in center sharpness, and they stay that way throughout the rest of the aperture range.

In the second field sharpness test, the DA 40 Limited once again displays awful performance all over at f/2.8, but, puzzlingly, it's the sharpest in the center from f/4 onward. The borders, unfortunately, don't get reasonably sharp until f/8. Once again the DA 35 is the sharpest at the edges wide open, but both it and the DA 40 XS suffer from substantial purple fringing until about f/8. We didn't do a standalone test for chromatic aberrations, but this second field sharpness test shows that the DA 40 Limited clearly offers the best resistance to fringing of the three.