LensTip.com

Lens review

Carl Zeiss Touit 32 mm f/1.8

29 September 2013
Arkadiusz Olech

5. Chromatic and spherical aberration


Chromatic aberration

The longitudinal chromatic aberration makes itself felt a bit at the maximum relative aperture – it would be difficult not to notice green-yellow cast of images behind the focus and a bit reddish hue of images before the focus. Fortunately the slight stopping down by one exposure value limits the extent of that problem significantly.

Carl Zeiss Touit 32 mm f/1.8 - Chromatic and spherical aberration



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The lateral chromatic aberration shouldn’t be an issue for the users of the Zeiss 1.8/32. Near the maximum relative aperture it is low and on stopping down it increases to medium values. What’s interesting, we noticed a slight difference between the aberration values you get when you develop RAW files employing different demosaicing processes. In the files processed by the old VNG algorithm the aberration seems to be a bit lower – it might be caused by the fact that it is masked by colourful artifacts, non-existent in the 3-pass process.

Carl Zeiss Touit 32 mm f/1.8 - Chromatic and spherical aberration

Carl Zeiss Touit 32 mm f/1.8 - Chromatic and spherical aberration


Spherical aberration

Circles of light you get by defocusing light points show an almost textbook example of spherical aberration.

Carl Zeiss Touit 32 mm f/1.8 - Chromatic and spherical aberration


A cross section of the circle in front of the focus features noticeably lighter edges and a bit darker centre. The circle you get behind the focus, for a change, has a lighter centre and darker edges. As you deal here with really a textbook example of this aberration we decided to enliven the text, adding an illustration with the detailed analysis of the problem.

Carl Zeiss Touit 32 mm f/1.8 - Chromatic and spherical aberration


It is clear that, when you position the plane in front of the focus, more light beams are focused near the edge of the circle so in the cross section there are distinct peaks at the ends. Behind the focus, for a change, more light beams intersect in the centre so it is lighter there; the shape of the intensity distribution is similar to the Gaussian distribution (the bell curve).

The spherical aberration manifests itself distinctly also at the maximum relative aperture where you cannot get point-like images of sources of light – every single one of them is surrounded by a weak but noticeable halo like in the photo below.

Carl Zeiss Touit 32 mm f/1.8 - Chromatic and spherical aberration


Fortunately the spherical aberration decreases very quickly on stopping down; by f/2.2-2.5 it is practically invisible, so there is no “focus shift” effect.