For people work on 1.6x or 1.5x bodies, the 85mm prime lens is very difficult to work with in many social photography situations.
At the same time, it's ideal for isolating a single person in a social situation.
You know, this actual subject is one of my "pet peeves". The amount of pure misinformation on the 85mm lens is so widespread, I've even done a blog post on how the 85mm lens works on APS-C versus FF.
Isolating a person in a social situation with an 85mm lens on a 1.6x Canon??YEAH--if you're 34 feet away! That's how far you have to be to show a six foot tall person with an 85mm lens on 1.6x bodies--thirty-four feet away. It's almost impossible to put 34 feet between the camera and a subject in many social situations--people come in between you, so it's very difficult to deploy an 85mm lens indoors in any home that's not a mansion. How big is the living room in an average home???? If a social event has people milling around, or the venue is not huge, an 85mm lens on a crop-body is nearly useless in many situations. "isolating them"? What? Here is the field of view calculator URL
Field of View - Rectilinear and Fisheye Lenses
Uh, no, 85mm on 1.6x is almost impossible to use in a social situation, unless the room is HUGE. And as we know, at longer shooting distances, depth of field on APS-C increases very,very quickly. With the aperture set to say f/8, DOF is from 28 feet to 43 feet, with a depth of field band of 15.4 feet in depth--there is virtually NO "isolation" whatsoever; that much depth of field will render the foreground AND the background quite recognizable. The visual effect is worse than the numbers might indicate.
For a person who hopes to shoot family portraits/events/news/documentary work with a Canon or Nikon APS-C body, an 85mm prime lens is USELESS in many situations, but a 24-70mm f/2.8 will allow the photographer to actually have some flexibility at normal distances, in normal sized rooms. Sure, an 85/1.8 is inexpensive compared to a Canon 24-70-L, but it is also a one-trick pony, while the zoom lens offers like 46 different focal length and framing choices. The choice is obvious.
Online Depth of Field Calculator