Resolution at f5.6
(Nikon on FotoDiox Pro Adaptor)

Nikon 15mm f3.5 AIS v Sigma 12-24mm f3.5-4.5

 

100% Crops (f5.6) Centre image circle

Nikon 15mm / f5.6 / FotoDiox adaptor	Sigma 12-24mm / 15mm / f5.6

It's immediately obvious that the thicker adaptor makes a big difference to the performance of the Nikon 15mm. The focal plane is predictably flat, and centre frame sharpness is now better than the Sigma, reversing the previous result.

 

100% Crops (f5.6) near corner

Nikon 15mm / f5.6 / FotoDiox adaptor	Sigma 12-24mm / 15mm / f5.6

Out near the corner, in this 100% crop showing both near and far field objects, the Nikon 15mm behaves like an entirely different lens: outperforming the Sigma 12-24mm by almost as great a margin as that by which it was outclassed in the previous test.

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Related links:

Sigma 12-24mm v X