I did some research on the subject, and so far, every source I’ve seen says that Canon lenses are wide-open until you actually trip the shutter. So a 50mm f/1.8 will be brighter than a 50mm f/2.8.
I’m sorry, but I’m really getting mixed up. I’ve tried my 50mm 1.8 and played with my DOF preview button. I can’t hear nothing at f/1.8, but when I rise the aperture, I can hear the aperture engaging in my lens. Also, I can see the viewfinder only darkening at f/3.2. So I guess that f/2.8 is the limit for the viewfinder for focusing. It’s quite obvious that my lens is wide-open, but at the same time, I’m pretty sure that my viewfinder isn’t at f/1.8, because my pictures at f/1.8 are a lot more softer than what I see in my viewfinder.
My technical knowledge is very low, but I’ll have a lot to study with all the links you’ve attached. The link on Canon’s AF looks really interesting 🙂 Thank you so much for your help!