Dumb question time... I recently acquired the fastest lens I've ever owned, a Voigtlander 40mm f/1.2 which I use on full-frame (a7ii). I really enjoy it, but my question is not necessarily lens specific. I just find it very challenging to use wide open indoors. I am in particular thinking of people pictures. Typically I might be 6-12 feet (2-4 meters) to the subject in such a situation, or even close, and the depth of field (DOF) is pretty razor thin at f/1.2. Sure, I can stop down to help that, but then why not just use slower glass to start with? I'll venture a guess to the answer. Most lenses seem to hit a sweet spot stopped down a bit from wide open. So for a 2.8 lens that might be 5.6, 1.4 might be 2.8, and my lens appears to improve at f/2 or so. So the sweet spot at f/2 while still a thin DOF is maybe enough to at least have a chance of collecting enough light with decent image quality, whereas say an f2 lens wide open will have the same DOF due to physics, but not as good image quality because it is wide open. Is that the idea? Not that f/1.2 is useless. It can be very useful in other situations, e.g. outdoors in the evening. Or when razor-thin DOF is what you are after. Yet I have read so many times about people wanting fast lenses for evening social settings, and now having a little experience with a fast lens has made me wonder about this.