Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test - Analysis
Lens Reviews - Pentax

Distortion

The distortion characteristic of the Pentax is quite typical for a lens in this class. It shows heavy barrel distortion at 18mm and moderate pincushion distortion at tele settings.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
18mm 24mm 50mm 85mm 135mm

Vignetting

The light-falloff is very heavy (1.5EV) at 18mm @ f/3.5 - this will be visible in most scenes - and it's advisable to stop down to f/5.6 to tame the issue to an acceptable level. The vignetting isn't as pronounced at longer focal lengths at max. aperture but it makes sense to close the aperture by one f-stop in critical scenes.

MTF (resolution)

The resolution characteristic of the Pentax lens is very mixed. The center resolution is excellent at 18mm in the relevant aperture range. The borders are good at f/3.5 but the extreme corners are soft and the very high field curvature doesn't really help here either. Stopping down to f/5.6 boosts the borders to very good levels and the corners well into good territory - technically. However, regarding the field curvature you should stop down to f/11 in order to make sure that the corners are at least somewhat in-focus. THIS DOES NOT SHOW UP IN THE CHART BECAUSE WE REFOCUS THE BORDERS/CORNERS! The 24mm is the sweet spot of the lens. The center performance is exceedingly high here and the border/corner quality is much higher at max. aperture. It even reaches very good to excellent results at medium apertures. The field curvature has eased quite a bit as well. We're seeing a downward trend starting at 50mm. The center quality remains impressive but the borders/corners are only good/fair at f/4.5. Stopping down lifts the outer regions to very good quality again. Unfortunately this is where the (moderate) glory ends - at 85mm we're seeing a sharp decrease in global performance. The center quality remains untouched but the border/corner quality drops dramatically. It's downright poor at f/5.6 and not really stellar at medium apertures either. At 135mm there's no border/corner quality to speak of anymore even at f/11 - this is quite embarrassing although you may argue that the outer region isn't all that important in most scenes at such focal lengths.

We were obviously surprised by the rather poor performance so we send the lens to the local Pentax service in Hamburg/Germany. Result: the tested lens is within factory specifications!

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)

The amount of lateral CAs is Okay for such a lens. It exceeds 1.5px on the average at the image borders at 18mm and 135mm - this is visible but also expected for such a lens. The CAs vary around 1px throughout the middel range so it is quite well controlled here.




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