So you may ask...why would a professional choose a small mirrorless camera over a full frame Nikon, in this day and age...which I have been using since the 70's, with a shot jaunt with digital Canon 30D to a 40D to a 5D Mark ll! Otherwise I have been with SLR's and DSLR's from Nikon, Canon and Olympus for 40 years. However about 6 years ago right here in my blog I reviewed a new camera I purchased as a walk around and for video, the Sony NEX 6. After using it for a few years I was very impressed with it's auto focus and IQ...I predicted then that mirrorless cameras would progress technologically in the near future! Well zoom ahead 3 years. Sony has continued to evolve their mirrorless line and in 2014 came out with the first full frame, small bodied, mirrorless, inter-changable lens camera, (milc or ILC for short). Sure Leica had a ICL rangefinder back in 2009 or so, but it was $5000.00+ and was lacking in "many, many" areas compared to DSLR's.
Sony's A7-A7r upset the entire full frame DSLR world by all major brands including Nikon and Canon. BTW Sony makes all Nikon sensors! There are now 6 versions of the A7 and many professionals have started using the A7r Mark ll for weddings, studio and even some sports events. Sony's video 4K is spectacular and all the A7's are feature laden, handle well, half the weight and size of any other full frame camera with spectacular glass available to use. As a side note you can use any...I say "any lens" on the A7's with an adapter...you can also use any flash as well!
These cameras take stunning noiseless pictures at super high ISO's. The have WIFI and NFC, focus peaking, Zebra, Eye Focus, and full customization control. Electronic viewfinders that see in the dark and allow you to see what you compose realtime, unlike mirrored reflex cameras! They are quicker and more accurate to focus, video competes with high end strictly video cameras and certainly as good or better than Canon or Nikon's DSLR's.
And to top it all off, are much less expensive compared to the big boys toys. Some reviews are surmising the latest A7 approaches medium format image quality!
Bottom line....The Sony A7r Mark ll competes and betters the Nikon's D5 in many areas, Sony $2500.00 Nikon $6500.00, do the math! Anything below that there is no competition...Sony rates better in every area!
Other makers have mirrorless camera's; Fujifilm, Olympus, Pentax, Nikon, Canon...but most fall short of the Sony A7's due to sensor size!
Well, I could go on and on raving about the Sony A7's, but I think you get the picture. I could enclose a list of bullet points here, but have instead included a link to pro Jason Lanier's reasoning as to why he switched for what it's worth. Also a link to Flickr Sony A7 photo's from around the world....enjoy and happy shooting!