HD PENTAX-DA 21mm f/3.2 AL Limited Lens Review

Pentax HD PENTAX-DA 21mm f/3.2 AL Limited Performance

Sharpness is already very good in the centre of the frame at maximum aperture, although clarity towards the edges of the frame falls just short of good levels at this aperture. Stopping down improves sharpness across the frame, with peak clarity being achieved at f/8. Here sharpness is outstanding across the frame.

MTF @ 21mm

How to read our charts The blue column represents readings from the centre of the picture frame at the various apertures and the green is from the edges. Averaging them out gives the red weighted column. The scale on the left side is an indication of actual image resolution. The taller the column, the better the lens performance. Simple. For this review, the lens was tested on a Pentax K-IIs using Imatest.

Chromatic aberrations are well controlled, only just exceeding a quarter of a pixel width at most apertures. This low level of fringing will be difficult to spot, even along high contrast edges.

CA @ 21mm

How to read our charts Chromatic aberration is the lens' inability to focus on the sensor or film all colours of visible light at the same point. Severe chromatic aberration gives a noticeable fringing or a halo effect around sharp edges within the picture. It can be cured in software. Apochromatic lenses have special lens elements (aspheric, extra-low dispersion etc) to minimize the problem, hence they usually cost more. For this review, the lens was tested on a Pentax K-IIs using Imatest.

Falloff of illumination is reasonably controlled for a wide angle lens. At f/3.2 the corners of the frame are 1.48 stops darker than the image centre and visually uniform illumination is achieved with the lens stopped down to f/5.6 or beyond.

Distortion is reasonably high for a lens of this type. Imatest detected 2.04% barrel distortion, which may become apparent with straight lines placed parallel to the edges of the frame. Fortunately the distortion pattern is uniform across the frame, which should make it relatively straightforward to apply corrections in image editing software afterwards.

During testing, this lens proved itself very resistant to flare and contrast levels remain good, even when shooting into the light. The rectangular shade does a decent job of shielding the lens from extraneous light that may cause issues.

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