Pentax SMC DA 40mm f/2.8 Limited Pancake Lens Review

Pentax SMC DA 40mm F/2.8 Limited Pancake Lens Performance

At maximum aperture, sharpness in the centre of the frame is already excellent, although the clarity towards the edges of the frame lags behind somewhat. Stopping down the aperture improves sharpness across the frame, with the largest gains being seen towards the edges of the frame. Good sharpness across the frame is achieved at f/4 and excellent sharpness is realised at f/8.

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-5 using Imatest.

Chromatic aberrations are well controlled, with fringing remaining around half a pixel width between f/2.8 and f/8. Stopping down further increases CA levels towards the edges of the frame, but as it only just exceeds 0.75 pixel widths, this level should pose few issues, even in large prints, or harsh crops from the edges of the frame.

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-5 using Imatest.

Falloff of illumination is reasonably controlled for a lens with an f/2.8 maximum aperture. At f/2.8 the corners of the frame are 1.11 stops darker than the image centre and visually uniform illumination is achieved with the lens stopped down to f/5.6 or beyond.

Only a very slight amount of barrel distortion is present in images taken with this lens. Imatest detected 0.586% barrel distortion, which will be very hard to spot, unless straight lines very near the edges of the frame run absolutely parallel with the frame border. The distortion pattern is uniform across the frame, so if you require absolutely straight lines, this low level of distortion should be relatively easy to correct.

During testing, this lens proved itself quite resistant to flare and contrast levels remain good, even when shooting into the light.

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