Originally posted by osv that wasn't directed at you specifically.
i provided test results for the o.p. because no one was able to post photos that quantified the sensor differences for 645z vs. d810.
You still don't fully get what the OP is saying. These are images, not about quality, but about format. Because there is more to photography than empirically measurable IQ like resolution and measured DR. In any case the quality difference between the lenses used is quite large. The Samyang 14mm is actually a reasonable lens, but not in the same ballpark as the DA645 25mm.
I wanted to shoot nightscapes last night, and I knew where I wanted to do it, and what my composition was going to be. I only carried my smaller tripod with me so while the 645Z and DA 25 went on the tripod, the A7RII went on top of a reasonably flat fence pole. I wanted a long star trail - up to 2 hours in length, and I wanted the North Star in the frame. The 25mm is equivalent to 19mm on 35mm. So the Samyang is 5mm wider. That's
A LOT wider.
Here are the Pentax images:
Pentax 645Z DA 25 f/4
341 seconds, f/8
5323 seconds f/13 (I was going to shoot for another 30 minutes, but temperature hit 0 C and condensation was forming and then freezing.
Here is the Sony image - I did crop a little off the bottom to make it 16:9 rather than 3:2 to get rid of the fencing in front of it.
There are several rows of stars above the limit of the Sony frame. It comes nowhere near capturing the North Star. On a tripod I could have got it in by pointing the camera further up, but that would have led to more distortion at the edges, something the Pentax image (2nd one, longer time) suffers from. If the Pentax had been 16mm equivalent, I am confident that I would have got the North Star in the frame in the first Pentax composition without raising it much if at all.
The Sony image has width I don't need, and cropping it to 4:3 won't help as I don't get more height, I just get less image. It's not always practical to move further back to get this extra width. This road has a bend behind this point, and the view changes.
There's a whole lot more to photography than charts, even in the practical rather than purely artistic sense.
The OP should definitely hold on to his D810. I wouldn't advocate having one camera as main with no decent back up. But if the format is holding him back, which he says it is, then he needs to have a different format in his armoury and as I have demonstrated, you can't always get this by stepping back and then cropping.
I'm not afraid of post processing, but like to get as much right in-camera as I can. That includes the amount of image in the frame.