Originally Posted by Timmyjoe
I love using manual focus prime lenses, especially on my Nikon Df camera. Unfortunately, when covering events, the changing of the primes, and having to go into the menu and change the non-CPU data for each lens when I swap them, just takes too much time. Have had to go back to the 24-70 on a regular DSLR for event coverage, which is not nearly as much fun, but I don't miss shots.
If I'm shooting an event, I have one camera with a good 24-70 equivalent zoom, and a second body with a fast prime, usually a 50 or 85-90 equivalent for portraiture and shots where I want really shallow depth of field. Alternatively, one body has a fast prime and the second has a 70-200 equivalent. No need to change lenses, and if you use bodies with the same sensor or even the same body, your images will match perfectly.
But as for the original question, as to whether a 27-85 f2.8 can replace a 27 and 85 with fast aperture, the answer would be maybe, depending on light situation, ISO capability of the body, and just how shallow depth of field you want.