Pentax SMC DA 18-250mm f/3.5-6.3 ED AL [IF] - Review / Lab Test Report - Analysis
Lens Reviews -
Monday, 04 February 2008 05:09
Page 2 of 2
Typical for extreme range zooms the Pentax lens exhibits a fairly hefty amount of barrel
distortions at 18mm (~4.3%). However, beyond the wide end the problem is actually
very well controlled with only slight to moderate pincushion distortions.
Move the mouse cursor over the focal length text marks below to observe the respective distortions
The chart above has a real-world size of about 120x80cm.
As mentioned the Pentax lens has been designed to cover the APS-C image circle only which
makes perfect sense for Pentax DSLRs anyway. This characteristic combined with the extreme zoom
ratio is a bit of a troublesome situation when it comes to vignetting specifically at
the wide end of the zoom range. At 18mm @ f/3.5 the amount of vignetting is fairly extreme
(~1.7EV) - obviously a setting which should be generally avoided. However, this is the
worst spot - it's much better at other focal lengths and the situation is generally acceptable
by f/5.6 and decent around f/8.
Generally the Pentax 18-250mm produced comparatively decent resolution figures in the MTF lab.
The sweet range of the lens is clearly below 100mm. At 18mm up to 70mm the results are very good
and even comparable to conventional standard zoom lenses which is fairly amazing for such a lens.
At 18mm and 35mm you should avoid the max. aperture setting in order to overcome some border
softness here but from f/5.6 onwards there isn't really much to complain in terms of resolution.
The situation changes when looking at longer focal lengths. At 135mm and more so at 250mm there's a
quite a drop in quality. The center performance remains fairly intact with good to very good results
but the border quality is soft at max. aperture - the contrast level is also generally reduced here.
Stopping down to f/8 or better f/11 boosts the border quality. The 250mm remains the weakest setting
though but this can't really come as a surprise.
The results are slightly worse compared to the Tamron tests in the Nikon/Canon section. Partially
this has to do with the different environments (due to the different AA-filter characteristic
the Pentax charts emphasize higher/lower resolutions) but the manufacturing quality of the tested
sample was also slightly worse compared to the two Tamrons. Such extreme zoom lenses are generally
suffering from quality variations anyway so this is not necessarily a trend here.
Below is a simplified summary of the formal findings. The chart shows in line widths per picture height (LW/PH)
which can be taken as a measure for sharpness. If you want to know more about it you may check out the corresponding
Chromatic Aberrations (CAs)
Lateral CAs (color shadows at harsh contrast transitions) are already an issue for conventional
standard zoom lenses so it doesn't come as a surprise that the Pentax 18-250mm has its fair
share of problems here. That said the results are comparatively well controlled for most of the
range with an average CA pixel width between 1-1.5px at the image borders. The worst spot is reached
at 250mm and about 1.75px of CAs which is fairly hefty for this setting.