[toni-a]

 

Many problems with your thoughts...

• f0.95 only means that the aperture is bigger than the focal length. That is what it means... What the size of aperture is only gets clear when you know the focal length.
  
• The AF system of your DSLR does NOT get more light with an f0.95 lens than with an f2 lens, for instance. It is a misconception that it does. The AF system has its own "aperture" which is the limiting factor. That is why Canon talks about AF points for f5.6 sensitivity and f2.8 sensitivity. It is also why AF points can get (partly) blind with smaller aperture lenses.
  
• F2.8 on APS-C is never "more useful" than f4 on FF. It can be that the more modern APS-C camera has some advantages over the decade old FF camera, of course.
  

 

Well all my three cameras focus more easily in low light with canon 50mm f1.4 than with 17-55f2.8 than 24-105f4L at 50mm.

 

Saturday evening I was shooting portraits with the sunset in the background, using flashes to light my subject, and yes with  Canon 100mm macro  at f2.8  I used much lower flash  output than with 24-105f4,  which resulted on more pleasant skin tones and less contrast and harsh shadows