Carl Zeiss Distagon T* 25 mm f/2.0 ZE/ZF.2
4. Image resolution
A graph below shows how the tested lens fared in the frame centre, on the edge of an APS-C/DX sensor and on the edge of full frame.
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The performance in the frame centre is simply splendid. Already at the maximum relative aperture the lens gets to 38 lpmm which ensures very good sharpness of photos. On stopping down by 1 EV we can reach a great level of almost 44 lpmm which is maintained also by f/4 and f/5.6 apertures. The resolution decrease, which can be observed later, is due to the limiting influence of diffraction.
On the edge of the frame the situation is not so good anymore. It is obvious the Zeiss constructor followed the steps of the Canon EF 24 mm f/1.4L II USM constructors, focusing mainly on the results in the frame centre at the expense of the edge.
The edge of the APS-C sensor, even at the maximum relative aperture, remains still acceptable as it can provide useful images. It is a pity, though that the lens improves so slowly on stopping down. On the edge of full frame the performance is very much alike but the MTF50 values are by about 2-4 lpmm lower. It makes the area near the maximum relative aperture get below the decency level and the results rebound only on stopping down by about 1.5 EV.
To sum up the lens fares exceedingly well in the frame centre and should be praised for it. When it comes to the edge of the frame the results are average.
Below we present our test chart crops, taken from JPEG files.
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