Pentax SMC DA 40mm f/2.8 Limited - Review / Lab Test - Analysis
Lens Reviews - Pentax
Article Index
Introduction
Analysis

Distortion

Typical for standard/normal lenses the Pentax DA 40mm exhibits only a minor degree of barrel distortion (~0.6%) which is usually not an issue.

The chart above has been taken from the initial review. This aspect cannot change on a new DSLR.

Vignetting

Regarding its APS-C image circle it doesn't really surprise that the DA 40mm f/2.8 produces a some vignetting at wide-open aperture (0.8EV). At f/4 the problem is already greatly reduced and it's negligible from f/4 onwards.

MTF (resolution)

The Pentax lens delivered an impressive performance in the MTF lab. The lens is already capable of delivering very good results at f/2.8 across the frame. The excellent peak is reached at f/5.6. Typical for all lenses diffraction reduces the quality from f/11 onwards.

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) are a non-issue of the Pentax. A rare but very welcome characteristic.

Bokeh

The bokeh (the quality of the out-of-focus blur) is an important feature for a lens which is typically used for shallow depth-of-field situations. The DA 40mm f/2.8 Limited isn't really perfect here unfortunately.

Out-of-focus highlights show a outlining effect at f/2.8 so it's better to stop down a little. The highlight discs are perfectly circular throughout most of the image field. The inner highlight field is quite smooth albeit not perfect.

During a field session we spotted a rather odd highlight rendition beyond the outlining effect. Have a look at the following sample crop - see these inner spotlights in the highlight discs ? We've only seen them in this special situation though so they may relate to the rather extreme contrast.

The quality of the general back- and foreground blur is slightly nervous.

This nervous background blur is also visible in the folllowing sample crop:

Bokeh Fringing

Bokeh fringing is a common problem on large aperture lenses and the Pentax lens isn't flawless here either. If you look at the provided sample crops below you should be able to spot a purple halo in front of the focus zone and a green one beyond. This is very visible at f/2.8, less so at f/4 and the effect is gone by f/5.6.

You may also spot a slight focus shift towards the background when stopping down ("residual spherical aberrations") - if you observe the "17" at the various aperture settings in the chart below you will notice that it doesn't get any sharper and the focus hotspot moves towards the background.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/2.8 f/4 f/5.6

Sample Images

You can find some sample images in our initial review of the lens.

Verdict

The Pentax SMC DA 40mm f/2.8 exhibits a quite harmonious optical performance combined with excellent build quality. It is capable to produce impressively sharp images across the frame straight from the max. aperture. Distortions are marginal and CAs are a non-issue. Vignetting is visible at f/2.8 but not a show-stopper. The bokeh (out-of-focus blur) is slightly sub-average and bokeh fringing can be an issue at very large apertures. The lens does also suffer from a focus shift when stopping down - this can have an impact in close focus scenarios.

The biggest "problem" of the lens is not quality but its application. It is a comparatively slow lens and not faster than many zooms. Classic "normal" lenses are about as good at comparable apertures but with a lot more potential for low light and shallow depth-of-field photography ... at a lower price. Some users may argue with its size and weight but even with the DA 40mm f/2.8 a DSLR is not a pocketable camera. That all said it remains an very good performer and as such it is a welcome addition to the Pentax lens lineup.

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