Pentax SMC-DA 15mm f/4 AL ED Limited - Review / Test Report - Analysis
Lens Reviews -
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The DA 15mm f/4 Ltd produces moderate barrel distortion (~1.5%) - this is comparatively low although you may spot the problem in very critical situations.
The chart above has a real-world size of about 120x80cm.
Pentax DA lenses are optimized to cover the APS-C image circle only. These lenses tend to produce a comparatively higher degree of vignetting than full format lenses in this scope. At f/4 the problem is indeed very pronounced at 1.3EV - this is clearly visible in most situations. The vignetting is already much better at f/5.6 but it takes f/8 till the problem is no longer field relevant.
The DA 15mm f/4 is capable of producing very sharp images. This is certainly true for the center resolution which is already stunningly high at f/4. The border quality is only "good" here and also affected by some field curvature problems (a curved focus field rather than a flat one). The border situation improves only slightly at f/5.6 and the really boost in quality occurs at f/8 and f/11 where you can experience a very good resolution quality across the whole image frame.
Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Chromatic Aberrations (CAs)
Lateral chromatic aberrations (color shadows at harsh contrast transitions) are fairly well controlled for a lens in this focal length class. The average CA pixel width at the image borders stays at and below 0.9px - this may be visible here and there but usually not to an annoying degree.
(Note: CAs can be reduced via imaging tools)