Easy answer, 20 vs 36 MP, full frame vs APS, ease of use, output, robustness, control, configuration, flexibility - almost everything that can be judged makes the D810 the better camera. Only size and weight make the Z50 a winner and potentially access to a more future-proofed development stream.

So why did I sell my D810 to buy a Z50? Read on..

I loved my D810 and it was my primary camera until about 18 months ago. Having used and enjoyed the Fuji X-T series I had been convinced that mirrorless was the future for quite some time. Even in early implementations, I preferred an EVF to an optical viewfinder. I liked the ability to see any chosen aspect ratio through the evf, histogram in evf and focus peaking in evf. As soon as the Z7 was announced, I pre-ordered and received it in Oct 2018. From the moment it arrived, it was very close to the perfect camera for me. My thoughts can be found here and here. The D810 was relegated to reserve camera, a term which perhaps warrants a little explanation. I’ve not used two cameras simultaneously since the days of film and the desire to have both colour and mono available at the same instant. I never carry two cameras but always, always, take two on trips away. I’ve never had a camera fail during a trip but the possibility of spending a week in the far North West or on the islands without a functioning camera fills me with horror. I know one wonderful landscape photographer whose camera was written off on the first day of his trip to Harris and numerous others who have faced equipment failure when on the road.

Therefore I always take a backup or reserve camera away on trips. For those who use two cameras at the same time, I’d always advise having two identical models each set up identically. However I’m advocating owning a camera that, all being well, will never get used. I can’t afford to buy a Z7 and leave it in a box doing nothing except silently accompanying me on trips away. Even a Z6 would represent £1000 of redundancy. I’d been relatively comfortable with the D810 filling this role. It did however mean taking some additional lenses to fill gaps from the Z set. After some reflection, I decided that the price achievable for S/H D810 was about as good as it was ever going to get and a decline was likely. I’d no wish to buy a camera that would remain boxed up but the launch of the Z50 gave another alternative possibility. The Z50, while providing only 20MP, offered an exceptionally light and compact carry everywhere camera. A complete kit of camera and two lenses fits easily into a ThinkTank Hubba bag or a Billingham Hadley with loads of room for other stuff. This offered options for a number of uses:

Probably won’t need a camera but just in case
Holiday walk around camera
Long distance walks, high ground, wild camping.
High Fells ( though potentially less easy to manage with winter gloves)
Quick access grab camera for the front seat while driving between locations.

If the worst comes to worst on a trip away, I’ve still got a 20MP Z mount camera with focal lengths from (equivalent) 24mm - 375mm. The kit lenses are decent, the wide zoom especially so. I can get wider with the 14-30mm and both the 50mm and 85mm f1.8’s offer good options. Yes I’d rather have 46MP but if the camera gets used in an emergency, I suspect I’ll just be glad to have had a way of making photographs. Financially, I obtained £1220 for the D810 on Ebay and I've just taken delivery of a Z50 Kit with both lenses for £859. The difference covers a few camera specific accessories and a new photo backpack on pre-order.

So of course a Z50 isn’t ‘better’ than a D810 but there are specific circumstances when it sems to make a better choice.

Initial feelings are very positive but a full review will follow when I've gained enough experience with it. for now, the sensor is a pretty established piece of tech and the camera is both tiny and a pleasure to use.

Following images to illustrate relative size between camera/kit lenses and the smallest bags I have to hand. There’s probably an even smaller solution that I’ve not found so far.

Kit with ThiinkTank Speed Demon belt bag

Inside ThinkTank Hubba Hubba Hiney. Still room for a spare battery, a couple of filters and lens hoods.