Helios 40-2 85mm f/1.5 (M42 to Pentax) - Review / Test Report - Analysis
Lens Reviews - Pentax

Distortion

Well, so let's start with some good news - the Helios produces an absolutely negligible degree of barrel distortion (~0.1%) which is only detectable in lab conditions.

The chart above has a real-world size of about 120x80cm.

Vignetting

Thanks to the sweet spot behavior on the K10D vignetting is well controlled. However, regarding the very large aperture vignetting is still a little stronger at ~0.66V at wide-open aperture - usually this is no problem except in very critical scenes. From f/2 and up the problem is negligible.

MTF (resolution)

As you can see below the Helios shows us a wild ride in terms of resolution. The inner center resolution is good at f/1.5 but the quality collapses beyond. There's only some sort of color soup left at the borders. At f/2 and f/2.8 the usable center portion broadens and the quality reaches very good levels here whereas the borders are still dismal. At f/4 the center quality reaches excellent levels and the borders get finally boosted to good quality. At f/5.6 and more so at f/8 the image quality is very good.

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)

It may be surprising but the CA characteristic (color shadows at harsh contrast transitions) of the Helios is actually superb and nothing to worry about. (Due to the dismal border resolution it did not make any sense to take CA readings at f/1.5 and f/2)

Typical for most large to ultra-large aperture lenses the Helios suffers somewhat from Longitudinal Chromatic Aberrations (LoCA) - reddish to greenish out-of-focus contrast transitions (just in front and behind the focus field).


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