SMC Pentax A 50mm f/2.8 Macro

SMC Pentax A 50mm f/2.8 Macro

Another old standard focal length macro (short telephoto on APS-C); pretty much every manufacturer had a standard focal length macro in the f/2.8-3.5 range.

Build quality of this lens is good, and typical for the Pentax A series lenses. Not quite as much metal as the earlier K mount lenses, but nice looking and still well made. The extending barrel is metal and so are the filter threads. The rubber grip has maintained its form, and manual focus feel is nice and smooth.

This macro only focuses to 1:2, so it requires extension or teleconverter to get to 1:1 or greater. It has Pentax standard 49mm filter threads. It is a 6 element in 4 group design, and it weighs 220g (7.76oz). Diaphragm has 6 blades, that have a very slight curve at wide apertures, and straight stepped down to smaller apertures. Minimum focus distance is 0.24m (9.44 in). Remember that I am photographing a normal subject chart at approximately 1:54 magnification, not a 1:1 chart so performance could differ for macro subjects.

NEX 3 Optical Characteristics

NEX 3 MTF 50 (Sharpness)

NEX 3 MTF 20 (Resolution)

NEX 3 Lateral Chromatic Aberration

The lens has minimal distortion (0.08%)

All data gathered using Imatest.


The lens is sharp from the center from f/2.8, but the corners are weaker throughout the range than some other 50mm macro lenses.

The lens has noticeable lateral CA at all apertures; weak for a macro lens. Longitudinal CA in the bokeh is almost negligible as well.

Purple fringing is negligible.

Astigmatism is minimal, point light sources in corner maintain shape well.

Flare resistance will be updated if a field shoot is done.

Distortion is negligible.

Bokeh: if I have a chance I will take some bokeh shots.

– Sharp Center at f/2.8

– Only focuses 1:2 without extension or teleconvert
– Could have better corner performance

Uses (sample):
– General Macro
– General Short Telephoto

This lens could have better corrected CA and corner sharpness for a macro, but still performs well. For general photography I would use a 50mm f/1.8 or larger aperture lens instead, but this is nice when you need close focus. The lens performs well enough in the center at all apertures and the corners improve stopping down. However, I should point out that the lens doesn’t perform any better over the same aperture range as most standard 50mm f/1.4 or f/1.8 lenses, other than for distortion.

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The gallery images are © 2011 Eric Tastad, and may only be used for personal evaluation of the lenses. The images were exported straight from camera with my default LR3 settings for the NEX 3, which have some sharpening set since it is a fairly high resolution sensor and LR tends to under sharpen.