Olympus M.Zuiko 25mm f/1.8 - Review / Test Report - Analysis
Lens Reviews -
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The M.Zuiko lens produces only a slight degree of barrel distortion (~0.5%) which is barely noticeable in field conditions. This applies to uncorrected RAW images as well.
The lens shows only a fairly high degree of vignetting (1EV/f-stops) at fully open aperture. Stopping down reduces the light falloff but traces remain even at f/5.6. The uncorrected RAW figures are slightly worse but this may simply relate to a different tonal curve compared to JPEGs generated by the camera.
The M.Zuiko delivered quite impressive results in the MTF lab. The image center is excellent straight from the maximum aperture. The borders as well as the corners are on a good to very good level. Stopping down has a marginal effect till you reach diffraction-affected apertures (f/8 and beyond). The peak performance is reached between f/2.2 and f/2.8. The field curvature is quite low.
The tested sample had a very good centering quality.
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 CAs (color shadows at harsh contrast transitions) are just slightly visible with an average pixel width of around 1px at the image borders. This is not disturbing though.
The quality of the bokeh (out-of-focus blur) is an important characteristic for large aperture lenses. The Olympus lens does a good job here but it's not without flaws.
Out-of-focus highlights have a circular shape throughout most of the image field. However, the inner highlight zone shows a rather busy sub-structure.
On the positive side, the rendition of the out-of-focus blur in the focus transition zone is very smooth.