Studio Tests

The little Pentax kit lens is a distinctly competent performer, and tends to be very sharp indeed at apertures of F8-F11. Like all kit lenses, though, there’s a trade-off, and sharpness is nothing much to write home about wide open at the telephoto end; there are also some issues with chromatic aberration and falloff at the wider end of the range.

Sharpness This lens is at is best at wideangle (at 18mm it matches the more expensive 16-45mm at 20mm), but somewhat indifferent at telephoto. As might be expected, performance improves on stopping the lens down a bit, and is generally best at F8-F11, prior to the onset of diffraction. As usual F22 and smaller should generally be avoided, and don’t even think about stopping down to F40 at 55mm; the results make us wonder why Pentax even bother offering this aperture.
Chromatic Aberration Chromatic aberration is overall quite well controlled; as usual it’s at its worst in the 18-24mm region, where the overlay of the red and blue lines indicates visually-intrusive green/magenta fringing. However the situation improves at longer focal lengths, and CA is all but invisible at 35mm and longer.
Falloff We consider falloff to start becoming perceptible when the corner illumination falls to more than 1 stop below the centre. Falloff is pretty pronounced at 18mm, perhaps reflecting an element of under-design to the small-diameter front element. We measure a maximum of 1.3 stops wide open, which slowly but steadily decreases with stopping down all the way to F11. The situation improves at 24mm, and 1 stop wide open; unusually however we also see some vignetting creeping wide open at the telephoto end. Overall this is probably the 18-55mm’s weakest point.
Distortion As usual distortion is rather pronounced at wideangle, with 1.4% barrel at 18mm. This changes to pincushion beyond 24mm, reaching a maximum at 55mm of -0.86%. This is more or less normal performance for a kit lens.

Macro Focus

Coverage is very good here, with a measured magnification of 0.37x. This is at a focus distance of around 25cm, with a working distance of about 11.5cm between the front element and the subject.

The image is extremely soft and 'dreamy' wide open at F5.6, with a pronounced soft focus effect (presumably from the effects of spherical aberration). However it improves dramatically on stopping down to F8 and beyond. Distortion and chromatic aberration are both minimal. Not a bad performance at all.
Macro - 64x43 mm coverage
Distortion:Very mild pincushion
Corner softness: Low
Focal length: 55 mm (82mm equivalent)

Specific image quality issues

Overall this lens is a very consistent performer in the field, and searching for flaws in its performance feels slightly churlish. Notably it seems much better than most kit lenses at handling flare, presumably due to higher quality internal construction, and assisted in no small part by the well-designed hood.

'Soft Focus Macro' at 55mm

This lens's only slight Achilles’ heel in terms of image quality is a distinct softness wide open at 55mm, with the lens set to shorter focus distances. Technically this is due to under-correction for spherical aberration, which results in a characteristic soft-focus, slightly 'dreamy' feel to the images, with highlights ‘smearing’ into shadows. It's not unusual to see this kind of effect in a lens of this type at close focus distances - both the Canon 18-55 IS and the Pentax 16-45mm do something similar - but the Pentax 18-55mm shows it to an unusually high degree. It's worth noting that a slight soft focus effect wide open at 55mm might even offer some benefits; it can give a flattering softness to head and shoulders portraits, coupled with a more attractive background blur.  Unfortunately it does rather wreak havoc on the lens’s closeup performance at wider apertures, unless you like a soft focus effect to your macro shots. Having said that, it can be essentially eliminated by stopping down to F8, so is easy enough to work around.