Android doesn't provide any native way of mirroring one screen on another Android screen, but there are several apps that can do it.
The problem is that, in every search I tried, they are buried in with a much larger number of apps that stream to TVs, and are difficult to find.
user3179186 suggested using the HTTP Stream app, as described in Cast Android Screen to Android - YouTube, but as a link-only answer it was deleted.
I tried it, and it's more than adequate for my purposes.
Basically what it does is act as a mini-web-server running in the background.
It makes a single web page available on the local wi-fi network, a page that simply shows whatever is currently on the host device's screen.
Anyone on the local network can use a web browser to see it, so it is not only useful for acting as a remote camera, it can allow several people to see whatever you are doing on their own devices (rather than having all the heads together trying to see the one screen).
So it's perfect for "look at these photos" or "look, this is how you do it" situations.
The only two possible drawbacks I've found so far are that it doesn't provide audio, and the video has a slight time lag.
[Note to user3179186, feel free to repost a more suitable version of your answer.
Make it better than this, and I'll delete mine and accept yours.]