Pentax SMC-DA 18-55mm f/3.5-5.6 AL - Review / Test Report - Analysis
Lens Reviews - Pentax

Distortion

Typical for many kit zooms the Pentax lens produces a pronounced degree of barrel distortion at 18mm (2.5%) changing to slight pincushion distortion when zooming at 55mm (0.5%). At 35mm the lens is basically distortion-free.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
18mm 35mm 55mm

The chart above has a real-world size of about 120x80cm.

Vignetting

Lenses with a reduced image circle tend to vignette quite a bit specifically when it comes to wide-angles. This is also true for the Pentax DA 18-55mm AL. At 18mm the vignetting is very hefty with 1.4EV at f/3.5. Stopping down reduces the problem slightly but not to a really desirable degree leading to the conclusion that the lens is slightly under-designed here (front element too small). At 35mm the vignetting is still strong at wide-open aperture but from f/5.6 it is no real problem anymore. Same goes for the 55mm setting.

MTF (resolution)

The resolution characteristic of the Pentax lens is a little bit of a mixed bag. At 18mm the center performance is excellent, the borders are also fine but the extreme borders are somewhat soft and stopping down doesn't really resolve the problems (in the focus plane). At large aperture settings the lens suffers from low contrast but this is resolved by f/5.6. At 35mm and 55mm the resolution is very good to excellent even at large apertures.

Below is a simplified summary of the formal findings. The chart shows line widths per picture height (LW/PH) which can be taken as a measure for sharpness. If you want to know more about the MTF50 figures you may check out the corresponding Imatest Explanations

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) surprisingly well controlled with an average pixel width around 1px at the image borders at 18mm and less so beyond. Very few standard zooms show such a moderate CA characteristic.




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