Mileagemaker wrote:
Saw some birds in flight pictures taken with the Sony and was impressed with the sharpness and fast focusing at 600mm.
Maybe what you need is a better lens.
AFAIK, the only 18-400mm on the market is the Tamron and it's not available in Z-mount. So I am guessing you are adapting one in F-mount.
If that's the case, you might want to consider a lens upgrade, rather than a camera upgrade.
There is a Nikkor Z 100-400mm lens that would probably be ideal, but is rather pricey at $2400.
An alternative would be to adapt one of the earlier F-mount Nikkor 80-400mm lenses. (IMPORTANT: I believe you would need the newer "AF-S" version. I don't think the older "AF" version of the 80-400mm would be able to autofocus on your mirrorless camera.)
Here's a comparison of the image quality of the Tamron 18-400mm versus that of the Nikon AF-S 80-400mm VR:
https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=1145&Camera=963&Sample=0&FLI=3&API=1&LensComp=915&CameraComp=614&SampleComp=0&FLIComp=3&APIComp=1You can change the focal length of each lens there to compare them. It appears to me the Nikkor lens is quite superior at all the focal lengths they share. Makes sense, in that the Nikkor is a much less extreme zoom and a pro-quality lens costing upwards of $2000 new, while the Tamron is about $650.
Assuming the price of a new AF-S 80-400mm is more than you want to spend, I found several used AF-S 80-400mm ranging from $900 to $1100, depending upon condition (at Adorama and B&H Photo... there are none at MPB.com or KEH.com right now).
By the way, the Sony RX10 IV doesn't actually have a 600mm lens. It's zoom is 9mm to 220mm... but it uses a smaller 1" sensor that has a multiplier effect that makes 220mm "seem like 600mm on full frame". The same is true of your Z50... it's APS-C size sensor has a 1.5X multiplier effect. So on it's 400mm "acts like 600mm on full frame".
FWIW, the Nikkor 80-400mm lens weighs about 3.5 lb. Your Z50 is under 1 lb., so the total will be approx. 4.5 lb. This is more than the weight of the Sony RX10 IV: approx. 2.5 lb. But that's fairly heavy for a "bridge" camera. The Z50 body is smaller than the "body" of the Sony, too. But the Nikkor 80-400mm lens is larger than what's fitted to the Sony.