Pentax SMC DA* 200mm f/2.8 ED [IF] SDM - Review / Test Report - Analysis
Lens Reviews - Pentax
Saturday, 16 February 2008 15:45

Distortion

Typical a medium tele fix-focal lens the level of distortion is absolutely negligible and nothing to worry about in field conditions.

Vignetting

The DA* 200mm f/2.8 is a full format lens thus enjoying the usual sweet spot advantage when used on APS-C DSLRs. At f/2.8 the lens exhibits only slight vignetting (0.54EV) which is usually not field relevant anymore. From f/4 & up the issue is absolutely negligible.

MTF (resolution)

The Pentax lens produced pretty impressive resolution figures in the MTF lab. At f/2.8 the quality is already very good(+) and exceptionally uniform across the frame. The peak performance is reached around f/5.6. Beyond diffraction is the limiting factor.

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 generally well controlled (around 0.6px at the image borders) and usually nothing to worry about.

Purple Fringing

Unfortunately the DA* 200mm f/2.8 has quite a problem regarding purple fringing. Purple fringing is a blooming effect that occurs at extreme contrast transitions. The camera sensor has surely a few stakes in here but the effect is also dependent on the lens. Here's a 100% crop showing a particularly bad scenario. You will find others in the sample image section although with a lesser severity.

Typical for purple fringing the problem is worst at wide-open aperture so in critical scenes it makes sense to stop down a little.

Longitudinal Chromatic Aberrations (LoCA)

LoCAs (non-coinciding focal planes of the various colors) are a further aspect. LoCAs show up as magenta colored halos in front the focus point and green beyond. Regarding its 2 large ED elements it is a bit disappointing the the DA* 200mm f/2.8 shows fairly strong LoCAs at f/2.8. The problem eases when stopping down but it's still visible at f/4 and (less so) at f/5.6.

Note: please ignore the false colors around the horizontal lines WITHIN the focus zones. These are moirees caused by the extremely weak vertical AA filter of the K10D.

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

Bokeh

The bokeh (the quality of the out-of-focus blur) is a major aspect for all tele lenses because it is often an essential part of a scene. The Pentax DA* lens does a very good job here. The out-of-focus highlights are perfectly circular and very uniform at f/2.8. At f/4 the aspect remains pretty much intact whereas the circular shape deteriorates somewhat more at f/5.6. The general quality of the blur is good but a tiny bit bit nervous at f/2.8 and very smooth beyond. As already mentioned above you will also find a rather significant amount of longitudinal chromatic aberrations (LoCA) in this test scene.

Here're some full size samples taken at f/2.8, f/4 and f/5.6.




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