There are a few things that I'm impressed with by the Sony system.
First, the EVF. I believe its an amazing feature to see your exposure PRIOR to taking the shot. It sounds simple until you actually use it in action.
It makes visualizing things so much easier and really helps get the creative thinking process started.
2nd, the built in anti-shake reduction on their A99. Why isn't this included on the Sony A7 series?
I've used the built in system on the Pentax platform and its AMAZING to have it for EVERY lens. (Think about having it for all the old manual focus lenses out there)
3rd reason is focus peaking. You either have used it and know how awesome it is or you have never used it.
Its absolutely STUPID that this isn't included on more DSLRs. I can't imagine in the next few years that this feature will not be more popular. Once more and more people see it, they simply will demand it on their cameras. Its a game changer.
Last reason would be face tracking auto focus. This actually works pretty well. It isn't completely flawless and will occasionally be out of focus. But that is exactly how every autofocus mode I've ever used is. Not every shot I take with a DSLR is perfectly in focus. Its just a frustrating fact of owning a DSLR. But the Face Tracking system makes life so much easier as it makes it harder for the camera to focus on the wrong person. Your "keeper rate" is much higher with this.
Otherwise, I don't really feel the mirrorless system was too much smaller. Weight wasn't a consideration either.
The sony system simply has too many features to ignore it as a possible purchase.
The A77 & A99 series are nice and have some nice fast zooms available (70-200 f2.8, 24-70 f2.8 etc) but they don't have much 3rd party support yet (Tamron/Sigma etc)
The a7 has absolutely ZERO fast lenses (fastest they have is an f4 which isn't fast enough for me).
However the a7 allows you to use any Canon or Nikon lens without any noticeable quality or light loss. Also, the Nikon/Canon lenses retain VC, Autofocus, Aperture control, etc.
With that in mind, I was going to buy an A7 as there is virtually a unlimited glass available for it if I can mount any manufacturer's lens to it (with the appropriate adapter).
The deal break with the Sony a7 is that I have not yet found a radio controlled off camera flash that is HSS/TTL compatible. I enjoy manual flash, but there are times I need the HSS. To do this on Sony requires me to buy a Sony branded "master flash" which will stay on camera to control a 2nd off camera flash. Since Sony flashes are over $500 a piece it would require over a $1000 investment just to have 1 TTL/HSS off camera flash. That's just ridiculous to me.
If anyone has figured out a cheaper way to do this let me know.
Otherwise, I'm forced to look into other camera options

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