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I'm currently looking at the Sigma 10-20mm f/3.5 lens for my Canon EOS 80D (APS-C). However, one potential problem I see is that my ND-filter set is for 67mm while the Sigma lens has a diameter of 82mm. Of course I can use a step-down ring, but I'm worried that such a large difference in diameter will lead to heavy vignetting. On the other hand, the Sigma DC lenses work for all crop sensors including Nikon's variant at 1,5 CF, whereas my camera has 1,6 CF, so I'm hoping that the image circle of the lens will be slightly larger than the sensor anyway, reducing the vignetting I will get.

How bad will the vignetting be (for ND and other filters)? If you have any example images where this or a comparable combination of equipment was used, that would be great as well!

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The EOS 80D has a sensor measuring 22.3 × 14.9 mm. The lens, at 10mm, on this sensor, takes in a 96° horizontal angle of view. A step-down ring going from a filter diameter of 82mm to 67mm is quite a jump, and, with such a wide angle of view, is going to result in severe, objectionable vignetting.

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On the other hand, the Sigma DC lenses work for all crop sensors including Nikon's variant at 1,5 CF, whereas my camera has 1,6 CF, so I'm hoping that the image circle of the lens will be slightly larger than the sensor anyway, reducing the vignetting I will get.

The difference between Nikon 1.5X APS-C sensors (actually 1.52-1.53X depending on the exact measurements of various different sensors in different models) and Canon 1.6X APS-C sensors (actually 1.6-1.61X depending on the specific sensor) is only around 5 percent.

The difference between an 82mm opening and a 67mm opening is around 20 percent.

Can you see where this is going?

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