Pentax SMC-DA 70mm f/2.4 Limited - Review / Test Report - Analysis
Lens Reviews -
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Typical for moderate tele fix-focal length lenses the Pentax DA 70mm produces only
marginal (pincushion-)distortions (0.5%) - nothing to worry about under field conditions.
The chart above has a real-world size of about 120x80cm.
Regarding its APS-C image circle it doesn't really surprise that the DA 70mm f/2.4
produces some vignetting at wide-open aperture (0.86EV). At f/2.8 (just a third f-stop
from the max. aperture) the problem is already a bit reduced and from f/4 onwards
the vignetting is no longer field relevant.
As mentioned in the other pancake lens reviews this kind of 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 the quality is exceptionally high across the whole image frame and the peak is reached at around f/5.6. At f/8 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
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 a bit starting at f/5.6 to an average width of ~0.5px at the image borders
but this is nothing to worry about in field conditions.
As you may/will see in the sample image section the lens can also produce a slight
degree of purple fringing (a color blooming effect) in extreme contrast situations.