Fujifilm Fujinon XF 35 mm f/1.4 R
7. Coma, astigmatism and bokeh
The crops above show one more interesting thing. Look closely at the image of the diode in the frame centre. By f/2.0 it is nice and point-like but at the maximum relative aperture that point-like shape turns into a small patch. It is not a result of bad focus but the influence of spherical aberration which is felt the keenest exactly near f/1.4 and disappears almost completely on slight stopping down. We mentioned it in chapter 4.
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The tested lens corrects the astigmatism in a perfect way for a change – a nice difference compared to the Sigma 1.4/30 which had a lot of problems with that off-axis aberration. In the case of the Fujinon an average difference between horizontal and vertical MTF50 function values was just 1.2% which is an excellent result.
Below you can see defocused images of a diode which was put in the centre and in the corner of the frame. The problems with spherical aberration, noticed before, are distinct. On stopping down the light spread is quite even in the rings, though, without noticeable concentric rims, which suggest out-of-focus areas easy on the eye.
