Can a screw on solid ND filter work with a Sigma 10-20 ultra wide at 10mm?

Same focal length? Not necessarily so, actually . There are differences between actual and reported focal length across and even within brands. They are normally not large, but you are not necessarily right in assuming that  they will be the same.

A worse problem, though, is another. Increased vignetting from filters depends on the actual construction of the lens: how close does the front casing (the filter thread in this case) of the lens approach the  cone of the field of view? The closer, the worse the possible vignetting effect from filters.

And since there is a  chance that Canon and Sigma lenses differ in physical construction, it is risky to argue that that since the Canon does not vignette at 10mm, neither will the Sigma.

If you say so, I shall not contradict you.

Regards, Mike

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