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Lens review

Carl Zeiss Distagon T* 25 mm f/2.0 ZE/ZF.2

21 February 2012
Szymon Starczewski

7. Coma, astigmatism and bokeh

On a smaller sensor of the Canon 50D the coma is not bothersome at all. Even at the maximum relative aperture this aberration is hardly noticeable. More problems you can encounter on full frame but even there the coma level can be described as medium. It is a pity that, on stopping down the lens by 1 EV, this aberration decreases just slightly.

Carl Zeiss Distagon T* 25 mm f/2.0 ZE/ZF.2 - Coma, astigmatism and bokeh

The astigmatism is not corrected in a perfect way because the result of 6% doesn’t deserve to be described so but it can be certainly considered good. Overall, no major reservations.

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Let’s look at the defocused images of a point-like diode which was put in different parts of the frame and photographed by different apertures.

Carl Zeiss Distagon T* 25 mm f/2.0 ZE/ZF.2 - Coma, astigmatism and bokeh

Also in this case the lens’s performance falls a bit short of perfection. Many images show diversities in the form of concentric rings which signal some problems with the spherical aberration correction. These problems are real- they can be noticed in the resolution measurements as well. When you set the resolution by f/2.0 and reach the best possible result and then reset the aperture to f/2.8 the resolution level doesn’t improve sharply. If you want to see that improvement you must to move the focus ring a bit. This effect might be small but it remains measurable.