Firstly the option is only visible when you are assigning it to a custom button. I hope you understand that part.
In the custom button assignment menus
Pick the top "camera icon",
Then #10 sub-category - "Shooting Display"
It's the last option under this category.
If you still can't find it...
Thank you, no shot at night time with long exposure. the area is light polluted too which kinda helps the foreground!
--
Focus on what you have, not on what you don't.
https://www.flickr.com/photos/nandbytes/
Dpreview article says "The a7R V offers Medium and Small sized Raw files taken from the full area of the sensor."
It's pretty clear they are downsampled RAW files. I'm inclined to believe dpreview and Sony's own manual both which claim they are RAW files over you.
Edit:
Even the camera says...
you can get scaling with lossless compression on A7RV. you can pick between L, M & S for lossless RAWs.
https://helpguide.sony.net/ilc/2230/v1/en/contents/TP0003057049.html?search=lossless%20compression
Also detailed in A7RV review on dpreview
https://www.dpreview.com/reviews/sony-a7rv-review...
it does take longer to stack higher res files, but its not a massive issue. We have faster and faster compute day by day. And its not something he'll be doing day in day out, its a one time thing. So I wouldn't say that should be a consideration for OP.
I literally just got the A7RV today! So can't comment much on it directly.
But had A1 before and A7RIV before it.
I wasn't mentioning generic facts but those that can benefit you for astro.
more pixels gives you the flexibility to crop and compose. you have a 20mm prime and 35mm, unless you...
captures more pixels and gives you more flexibility with compositions if needed.
Better EVF and LCD screen.
also has focus bracketing which can be helpful if you are shooting wide open with your lens and want more of the foreground in focus.
with single exposure you can just take one shot for both the sky and foreground. And this is the only case where single exposure works. To get really good shots with lots of details this way you need to have bright wide lenses (sigma 14mm f1.4 comes to mind) with very dark skies with no light...
IBIS off
4-5s exposure if you want point stars and stack multiple exposures if you need to get more detail out of the milkyway
Its a great lens - 6 images stack - 5s each - A1+20G/1.8
--
Focus on what you have, not on what you don't.
https://www.flickr.com/photos/nandbytes/
I can't believe this thread still gets viewers every so often.
I had a Minolta 100-300mm APO D and I have some wonderful pictures with it. But it's not as good as any of the recent native telephoto lenses on e-mount.
I have personally purchased a used tamron 70-300mm RXD for only sightly more...