Pentax SMC DA 70mm f/2.4 Limited - Review / Lab Test - Analysis
Lens Reviews - Pentax
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Introduction
Analysis

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Distortions

Typical for moderate tele fix-focal length lenses the Pentax DA 70mm produces only a marginal (pincushion-)distortion (0.5%) - this is nothing to worry about in field conditions.

Note: the chart has been reused from the old review. This aspect cannot change on a new base-DSLR.

Vignetting

The Pentax lens covers the APS-C image circle only but it's vignetting characteristic is still moderate. You may spot traces of vignetting at max. aperture but the issue is pretty much gone once you stop down a little.

The Pentax K5 has a slightly different tone curve compared to the K10D so the values differ a little.

MTF (resolution)

Pancake lenses tend to produce a very even resolution across the frame and the Pentax DA 70mm f/2.4 Limited is no exception in this row. The center resolution is already excellent at f/2.4 whereas the borders and the outer corners follow on a very good level. At f/4-f/5.6 the quality is exceptionally high across the whole image frame. Beyond, the usual diffraction effects start to reduce the resolution again.

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 almost a non-issue for the Pentax specifically at large apertures. The problem increases marginally from f/5.6 onwards but at an average width of ~0.6px at the image borders but this is nothing to worry about.

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 Pentax lens does a good job here.

Out-of-focus highlights show a marginal outlining effect but this is below disturbing levels. At max. aperture the highlight discs are perfectly circular throughout most of the image field but they get a little "edgy" when stopping down beyond f/3.2. The non-circular aperture blades show up here.

The quality of the general blur is pretty smooth but we've also seen fairly hard-edged contrast transitions in field images.

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 visible between f/2.4 and f/4.

You may also spot a slight focus shift towards the background when stopping down ("residual spherical aberrations") - if you observe the "19" in the chart you will notice that it doesn't get any sharper by stopping down whereas the background gets gradually sharper according to the increased depth-of-field.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/2.4 f/3.2 f/4 f/5.6

Sample Images

You can find some sample images in the old K10D-based review.

Verdict

The Pentax SMC DA 70mm f/2.4 Limited is an great lens with no significant technical weakness. A highlight is certainly the resolution characteristic which is generally excellent regardless of the (tested) aperture setting. It's also very even across the image frame. The vignetting is only slightly visible at f/2.4 but no real issue anymore at smaller aperture settings. Neither distortions nor CAs are field-relevant either. An interesting aspect of a large aperture tele lens is quality of the bokeh (the out-of-focus blur). It's pretty good in general but at times you may spot a few hard edges at contrast transitions. There's some bokeh fringing at large aperture settings but this is rather typical in this lens class. The focus shifts a little when stopping down - this might have an impact on focus accuracy at smaller focus distances but then most users will probably prefer to use this lens at max. aperture anyway.

Typical for all Pentax Limited lenses the build quality is excellent and it is a joy to use such a high quality lens in the field. The price of less than 600€/US$) may appear to be very high for such a tiny lens but I think it's very fair in relation to its performance. Therefore - highly recommended!

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