Studio Tests

NOTE the line marked 'Nyquist Frequency' indicates the maximum theoretical resolution of the camera body used for testing. Whenever the measured numbers exceed this value, this simply indicates that the lens out-resolves the sensor at this point - the calculated MTF values themselves become meaningless.

We’ve been quite impressed with this lens when using it in the field, and the studio tests show why. The resolution results may not appear wonderful - the lens isn't all that sharp at 16mm, and the corners are soft wide open at all focal lengths - but this is offset by excellent characteristics in other areas, with little in the way of chromatic aberration, falloff or barrel distortion. This lens is, in fact, an excellent example of why absolute sharpness is not always the Holy Grail of optical design.

Macro Focus

A very creditable performance here, with an image area of 78mm x 52mm, equating to a maximum magnification of 0.31x, unexpectedly rather better than Pentax’s specified 0.26x. This is achieved at 45mm focal length and a focus distance of 25cm, giving a working distance of about 10cm between the subject and the front of the lens.  Image quality is very good indeed; slightly soft at F4, the centre is very sharp at F5.6, with corner performance close behind. There's just mild pincushion distortion, plus a little blue/yellow chromatic aberration. Impressive stuff.

Macro - 78x52 mm coverage Distortion: Mild pincushion Corner softness: Very low Focal length: 45 mm (68mm equiv)

Specific image quality issues

Chromatic Aberration

Overall chromatic aberration is fairly well controlled, but is (as usual) most problematic at wide angle. At 16-20mm, green/magenta CA can become intrusive towards the edge of the frame (see example below). At longer focal lengths, our studio tests show clear blue/yellow fringing, but in real-world shooting this turns out to be essentially a non-issue; it certainly doesn't have the destructive visual impact of red/cyan or green/magenta fringing.

100% crop	16mm, F8.0

Portait Format Softness

One clear issue with our sample was extreme softness at the edge of the frame when shooting at wide angle and in portrait format, especially with the lens angled sharply upwards (perhaps not a common shooting situation, but distinctly noticeable in some architectural shots). We can only speculate that this could be a decentring phenomenon due to the play in the lens barrel when fully extended at 16mm.

100% crop	16mm, F5.6
100% crop	16mm, F5.6

Flash Shadowing

With its unusual ‘reverse zoom’ design, this lens shows severe shadowing of the built-in flash at wide angle settings. The effect will be dependent upon the specific camera body used, and how far the flash lifts above the lens axis; with the K10D, the manual states that ‘vignetting may occur when the focal length is shorter than 28mm, or when the focal length is 28mm and the shooting distance less than 1m’. Our own testing shows these to be reasonable, although somewhat conservative guidelines.

Here's a picture of the tasteful DPR office carpet, showing the flash shadowing using our K10D test body with the lens set to 16mm. This is close to being as bad as it gets, but most modern lenses show no shadowing at all, and clearly a real-world image showing this effect would be pretty unusable.