DxO Review of Sigma 50mm 1.4 DG HSM Art

[mackguyver]

Review

It looks like the only real surprise is the TStop of 1.7. Not a big deal, but a bit unexpected given the TStop of 1.5 for the Sigma 35A.

Comparison to the Canon 50L and 1.4

 

[Rudeofus]

Since all of these fast lenses have severe vignetting wide open, I wonder how they measure T stops. Do they measure average across the whole frame, or only in the center, or somewhere in between these two extremes?

 

[mackguyver]

It looks like they measure it across the frame, and they don't mention vignette compensation:
http://www.dxomark.com/About/In-depth-measurements/DxOMark-testing-protocols/Light-transmission

 

[Rudeofus]

It looks like they measure it across the frame, and they don't mention vignette compensation:

That would explain why a slow IS zoom like the 24-70 F/4 IS has T=4 whereas all fast lenses have T stop significantly above their largest aperture. In other words, this T stop number is a poor substitute for their vignetting number and as such should be considered most useless.

Very confused ...