Most of the answers in this thread are at least partial nonsense.
For example, professionals do not usually open lenses up to f / 1.2 to shoot in the head with full-frame cameras, because the depth of field in that aperture and range is so short that you will have a focused eye and nothing more. Trust me, I wear this type of lens! Oh, and the image quality is often relatively poor until it closes a couple of stops. And the commitments to obtain large apertures mean that an expensive f / 1.2 lens may not be as sharp as a lens with a narrower maximum aperture, once closed; possibly the sharpest standard lens that exists is the equivalent f / 2.8 46 mm in Sigma Foveon sensor compactors. That f / 1.2 85 mm lens that someone from above thinks is very professional has a rather poor edge quality until it closed in 2 to 3 stops (for example, see the review of the lens on photozone.de). Now, a true professional would know, and then buy the f / 1.8 version much cheaper and lighter, or buy the f / 1.2 if he needed the focus boost in low light.
On the other hand, the effect of moving from f / 1.8 to f / 1.2 at headshot distances is probably crazy, or at least something you would rarely want to do (DoF chart for 85 mm f / 1.2). At 2.5 m, it is the difference between a sharp area of 10 cm and one of 8 cm. Would anyone sensible really care about the 2 cm difference? You are more likely to want to reduce the aperture and increase the DoF, instead of doubling the cost of a lens that can reduce it by 2 cm!
The reason for the great lenses in YouTube videos is because they are often for guys with cameras, gullible middle-aged men who want to become glamor photographers. The big zooms and the huge DSLR impress them because they are what they see the paparazzi using on television and because of obvious reason. Real photographers are likely to use something much more compact, such as a Fuji X with a 56 mm lens, or a Sony A7 or Nikon, whatever with a 50–85 mm lens.
Possibly, the main use of VERY wide aperture lenses by professionals is not to shoot but Attention. Focus systems hate dim light, especially phase detection systems in DSLR. Fans, on the other hand, often spend hours taking photos of a tree with only a focused leaf thanks to that incredibly neat depth of field, and if it makes them happy, why not?
Another point to understand about lenses such as the 85mm Canon f / 1.2 is that its DoF is small at f / 1.2 and a head shot range that often cannot be focused with sufficient accuracy for a competent image. This is not a problem for mirrorless cameras, which focus much more precisely because they do so from the image sensor.
Above all, the Canon f / 1.2 85 mm is not sharper than the cheapest and lightest f / 1.8, it is LESS sharp. This is almost always the case with very bright lenses in full frame: they are special purpose tools designed to sell to serious shooters with very unusual needs (20% of the market?) And to people who really do not know what they are & # 39 ; buy again (80%?)
Again, this does not translate into other systems: f / 1.2 is the equivalent of f / 1.4 or f / 1.8 for Fuji and Micro 4/3 systems. f / 0.95 is the specialized / crazy aperture for standard and short telephoto lenses.