https://www.youtube.com/watch?v=AtqhBsh3Hzk
Adorama TV, great Mark Wallace YouTube video explaining the difference between raw and jpeg. Use this with my university photo students.

https://www.youtube.com/watch?v=N0j8QMmglvw
And Tony and Chelsea Northrup YouTube video on raw vs jpeg (great side by side comparisons)

FalconChase you are correct about Capture One, Adobe LR and PS. New Sony camera purchasers can get Capture One (for Sony cameras) for free.

Cheers Spirit Hawk, great choice, you will love it.

That Sony would be a great choice. My friend bought one on my recommendation and is loving it. Cheers

Yes the A7R3 has some features that the D850 just can't physically ever have, so that gives the A7R3 an edge. Both produce fine quality images, but I love the features of mirrorless and will never go back to any DSLR, at any price. Loving my A7R3 and sticking with Sony's innovation leadership. There are other fine mirrorless cameras out there, but for my money Sony has the best system, and getting bigger and better all the time. Cheers

You are right the Sigma 150-600m is over your budget, sorry.

Check out these Sony superzoom compacts with 30X and 50X zooms. Many under ,around $400 or slightly more.
https://www.sony.com/electronics/compact-cameras/t/cyber-shot-digital-cameras?bestfor=big-zoom

Cheers

A7R3 is the best one for you. You don't need the extra cost of the A9 just for its 20 frames per second (20 fps w/electronic shutter, only 5 frames per second w/mechanical shutter). . The 10 frames per second of the A7R3 (both mechanical and electronic shutter) are more than enough for sports, wildlife or anything. The A9 is a great camera and is being used right now at the Winter Olympics with both current Sony telephoto lenses and prototypes of Sony's new 400mm f2.8 lens. A7R3s are also being used at the Olympics for greater resolution needs.

I am a professional Sony user (switched to Sony in Jan, 2017 after 40 years a high-end Canon DSLR user, yes 40 yrs.) and I have the A7R3 and it is the best digital camera on the market in my opinion (sorry D850 folks). I agree get the A7R3 and the Sony G 24-105mm f4 lens, all for the cost of the A9 body alone. And despite what others here say, you can use the great Sigma Art and Contemporary series lenses on this A7R3 with the great Sigma MC-11 adapter and get great autofocus and superior image quality, at less cost than some Sony lenses. I have a mix of both, six Sony FE, G, and G-Master lenses and three Sigma lenses which I love. Cheers

The best choice may be the great Sigma Contemporary series 150-600mm f5-6.3 . Jared Polin of FRO-Knows reviews (YouTube) tested and rated this the best performance/value in super telephoto zooms, as have other reviewers. https://www.youtube.com/watch?v=F6tCUFXiDws&list=PLWbUbVp5Rk9RDX38jDn2Tyu-6B2NQ1oEh

I am a pro and personally have this lens and love it. The Contemporary version is hand-holdable with practice. It is sharp, focuses quickly and also you can get a 1.4X extender if you want. It goes new for $899 to $989 depending on the camera supplier you buy it from. A truly amazing bargain. Now don't confuse this one with its more-expensive heavy-metal sibling the 150-600mm Sport series version. The Sport version is TWICE the price ,and it is too heavy to handhold, a tripod or monopod is a must. In testing, the Contemporary version actually tested slightly sharper throughout the zoom range than the Sport version, for half the price, go figure !

I highly recommend this lens. i have gotten extraordinary sports and wildlife shots with it (adapted to my Sony Cameras with the MC-11 adapter). Sigma makes these in Nikon, Canon and sigma mounts (same price). https://www.sigmaphoto.com/lenses/telephoto-lenses/150-600mm-f5-63-dg-os-hsm-c

The larger individual pixels in a full-frame shot (comparing 24 megapixel full-frame sensor vs 24 megapixel APS-C sensor) tend to have better noise characteristics, and greater dynamic range. So in comparing two separate prints from a 24 megapixel Full frame camera, and from a 24 megapixel APS-C, blown up to the same image size, the full-frame print should also exhibit better "pixel-peeped" resolution (both shot with same full-frame lens, with exposure settings and lighting the same). I have done this test myself in the past as an exercise with my students in my university digital photography class that I teach. That better "pixel-peeped" advantage should continue as the print size is further enlarged.

Will most people be able to see the difference? No ,not all will see much of a difference as APS-C shots have been blown up to large sizes and are quite satisfactory. But physics is physics, and pixels are pixels (yes there is some variance, as in-camera sensors and image processors vary in their performance from camera model to camera model), but yes it does make a difference. I own both APS-C and Full-frame mirrorless Sony cameras and use the full-frame when I need the better image resolution for larger display and printing of my images. Still love the image quality from my 24 megapixel APS-C A6500, as a companion to my 42 megapixel Full-frame A7R3. Cheers

No if you put a APS-C lens on the full frame A7R3 ,the camera defaults to 18 megapixel capture. The camera is figuring the APS-C lens can't resolve at 42 megapixels. But if you just crop a 42 megapixel A7R3 shot to the 1.5x crop of an APS-C you still have 42 megapixel resolution and dynamic range, just enlarged.

I have both a Sony A6500 (APS-C) and a Sony A7R3 (full frame). The difference is 24 megapixels (APS-C) vs 42 megapixels (Full frame). The APS-C costs much less and makes great photos, but the Full-frame has greater dynamic range and can make images that will blow up/or crop with less image quality loss. If I could only afford the A6500 it could take great photos for a lifetime, but I am also a professional shooter and for most of my client work I use the full-frame to give them the absolute best final images that could hold up to being poster-sized. Cheers, use what you can afford. There are great APS-C cameras out there that will satisfy most folks still and video needs.

For example, the newest Fuji X-H1 has all the features of the best full-frame cameras and more, but has APS-C sensor. https://www.youtube.com/watch?v=uO18zQFsx-sSo ; https://www.youtube.com/watch?v=Aw3K0Bp2D74. No longer do APS-C take a back seat to the still and video features of a full-frame camera. Buy the best you can afford and be happy. Image quality of the best APS-C cameras is excellent.

I have both a Sony A6500 (APS-C) and a Sony A7R3 (full frame). The difference is 24 megapixels (APS-C) vs 42 megapixels (Full frame). The APS-C costs much less and makes great photos, but the Full-frame has greater dynamic range and can make images that will blow up/or crop with less image quality loss. If I could only afford the A6500 it could take great photos for a lifetime, but I am also a professional shooter and for most of my client work I use the full-frame to give them the absolute best final images that could hold up to being poster-sized. Cheers, use what you can afford. There are great APS-C cameras out there that will satisfy most folks still and video needs.

For example, the newest Fuji X-H1 has all the features of the best full-frame cameras and more, but has APS-C sensor. https://www.youtube.com/watch?v=uO18zQFsx-sSo ; https://www.youtube.com/watch?v=Aw3K0Bp2D74. No longer do APS-C take a back seat to the still and video features of a full-frame camera. Buy the best you can afford and be happy. Image quality of the best APS-C cameras is excellent.

Good choice, you will happy with this lens and its versatility. I certainly am enjoying making great shots with mine. Just checked B & H and they say 7-14 business days for availability. I would call them and get on the list here or at Adorama or Allens Camera ,or your local dealer. It is worth the wait. This lens' popularity should speak to its desirability,good reviews and good word-of mouth.
Cheers

Yes but doesn't the OP say "What lens to start with for Sony A7R2" How many subjects fit using only a 90mm lens ? I would be very limited if that was my only or first lens, even if excellent quality. Not likely the OP will buy any 90mm lens as a lens to start with, sorry. It would be an interesting exercise for my university photo students to only use a 90mm lens for all subjects, but not too easy a task. Happy you love this lens, though. Cheers