I moved from the a850 (24.6mp) to the D800 and couldn't be happier.
The high ISO performance is at least two stops better. I shoot ISO6400 on the d800 with less hesitation than ISO1600 on the a850. ISO12,800 is usable if you are stuck for options and you could still print an 8x10 with a bit of processing.
Dynamic range is WAY better. If a shot is 2 stops underexposed, just boost it and it stays looking good instead of getting filled with noise, colour weirdness and banding.
Autofocus is brilliant. Now I have 51 useable points instead of just the centre point. The side points of the a850 weren't ever brilliant they didn't cover much of the frame either, whereas the d800 points are relatively widely spread and focus quickly and decisively.
The jump in resolution is more noticeable than I thought it would be. It's never a bad thing to have more resolution - it just gives you more flexibility.
File size isn't that bad really. 45mb for a compressed raw file. You can buy a 32gb card for £30 now, and a 3tb hard drive for £70. There's really no excuse for the "file size" argument any more.
The downsides: I dislike "Nikon" colour and auto white balance in general. Sony was much more pleasing IMO. But of course you can edit your raw files however you like.
Also the d800 viewfinder is about 15% smaller than the a850 and less bright, which is a shame.
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