Sigma 50-500 mm f/4.5-6.3 APO DG OS HSM
7. Coma and astigmatism
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As you can notice looking at the crops from the centre and the corner of an APS-C sensor, the Sigma 50-500 mm OS corrects the coma very well. The diode image is not deformed at any focal length which should make the potential users very happy.
Similarly positive opinion can be expressed concerning the astigmatism correction. The average difference between the vertical and horizontal MTF50 function values amounted to only 3.7% which is a very good result. It places the Sigma in a very favourable light - in the case of a 10x zoom ratio lens, boasting a 500 mm focal length to boot, every inaccuracy concerning the production and setting of the whole movable optical system, even very slight, might often cause an increase of this aberration to a very high level.
