[toni-a]

I don't see your point either. As mentioned, a sharper lens will always give sharper images provided the circumstances stay the same, so you do benefit from sharper glass. Quantity of light has nothing to do with it. It si about the same circumstances.

 

The real question is whether you personally will benefit enough from it to make it worthwhile to you. For me personally it isn't, to be very honest. I own a 135L, and that lens is very very sharp from wide open, it renders beautifully, and is not terribly heavy, and according to lab tests the Sigma is only marginally better. In short, I will not "upgrade".

 

Kind regards, Wim

 

While I didn't want to go into the details, my point is that handheld you won't in most cases exploit the full potential of a lens, unless you are in ideal conditions. The influence of poor lighting on image quality handheld, is greater  than lens quality  itself.

 

Aside that I totally agree with you "marginally better in lab tests" doesn't really mean any difference in quality would be noticeable on prints.