Lens review

Sigma 28 mm f/1.8 EX DG Aspherical Macro

7. Coma and astigmatism

The Sigma’s weak resolution at 1.8-2.8 apertures for the frame center and frame edge as well doesn’t appear out of thin air. The coma and astigmatism are the major culprits in this case. When it comes to the first optical aberration mentioned, its volume can be assessed by comparing a spot image of a diode in the centre and at the edge of the frame – on an APS-C/DX sensor and on full frame. Such a comparison can be seen in the picture below.

As you see the coma is bothersome even on the smaller sensor – you can notice it even at the maximum aperture but on stopping down by one stop it disappears completely. A situation in the full frame corner is completely different though. The coma is huge there; even if you stop down it decreases slowly, remaining on a significantly high level.

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Astigmatism is clearly visible up to f/5.6 aperture level. Its average value for apertures up to f/4.0 amounts to as much as 16%.