canon rumors FORUM

Gear Talk => Lenses => Topic started by: mackguyver on April 17, 2014, 11:02:32 AM

Title: DxO Review of Sigma 50mm 1.4 DG HSM Art
Post by: mackguyver on April 17, 2014, 11:02:32 AM
Review (http://www.dxomark.com/Reviews/Sigma-50mm-F1.4-DG-HSM-A-Canon-lens-review-Art-for-Art-s-sake)

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 (http://www.dxomark.com/Lenses/Compare/Side-by-side/EF50mm-f-1.2L-USM-on-Canon-EOS-5D-Mark-III-versus-Sigma-50mm-F14-DG-HSM-A-Canon-on-Canon-EOS-5D-Mark-III-versus-EF50-mm-f-1.4-USM-on-Canon-EOS-5D-Mark-III___197_795_1306_795_196_795)
Title: Re: DxO Review of Sigma 50mm 1.4 DG HSM Art
Post by: Rudeofus on April 17, 2014, 11:20:29 AM
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?
Title: Re: DxO Review of Sigma 50mm 1.4 DG HSM Art
Post by: mackguyver on April 17, 2014, 11:24:44 AM
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 (http://www.dxomark.com/About/In-depth-measurements/DxOMark-testing-protocols/Light-transmission)
Title: Re: DxO Review of Sigma 50mm 1.4 DG HSM Art
Post by: Rudeofus on April 18, 2014, 04:34:05 AM
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 (http://www.dxomark.com/Lenses/Canon/Canon-EF-24-70mm-F4L-IS-USM) 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 ...