Pentax SMC DA 17-70mm f/4 AL [IF] SDM - Review / Test Report - Analysis
Lens Reviews - Pentax
Tuesday, 16 December 2008 21:00

Distortion

The DA 17-70mm f/4 is a fairly typical representative of its class. It shows relatively heavy barrel distortion (~3.1%) at the wide end of the range (17mm). The 24mm setting provides distortion-free images whereas you can observe slight to medium pincushion distortions at 40mm and 70mm.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
17mm 24mm 40mm 70mm

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

Vignetting

The Pentax lens is a dedicated APS-C lens and like most of its kind it shows a fairly pronounced degree of vignetting at large aperture settings. Unsurprisingly the weakest spot is at 17mm @ f/4 (~1.1EV). Stopping down to about f/5.6 resolves the most of the critical vignetting.

MTF (resolution)

The Pentax lens was capable of delivering very impressive resolution figures in the MTF lab. At 17mm and 24mm the center performance is superb and the border quality is easily on a very good level. At f/4 the extreme borders are a little softer at 17mm but stopping down resolves the problem. At 40mm we've an evenly high center to border resolution distribution. There's a little performance drop at the extreme tele end of the zoom range. The center performance is still very good here whereas the border quality suffers a bit although it's still perfectly Ok even at max. aperture. The global sweet spot of the lens is reached around f/8.

The tested sample showed a slight centering defect at 70mm - a perfect sample may even be slightly better at this setting.

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)

Many of the recent Pentax zoom lenses struggle somewhat regarding the amount of lateral CAs (color shadows at harsh contrast transitions) at the image borders. The DA 17-70mm f/4 is far from being perfect here as well but it manages to do a better job. As expected the 17mm setting produces the most pronounced CAs peaking beyond 2px at f/4. However, stopping down reduces the problem to acceptable levels here. You may also spot some CAs at 24mm but it's not really objectionable anymore and beyond it's a non-issue in most situations.




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