This is a slippery slope and with the ability for phones to zoom/photograph pretty much anything at any angle. I find it ridiculous a flight crew thinks they can police/monitor image capturing.
In one experience I eyewitness on UAL in premium class, a FA demanded a man (olive complexion) share/display his phone to verify he wasn't filming anything prior to take-off. It was embarrassing and really saddening as the summary of the situation is the poor chap simply had poor vision and had to keep his phone at eye level to view its content.
Now, would the FA make a similar request of a 10 yr kid who is on their first flight. A different circumstance, but flight crew either should enforce a no phone policy or no restriction phone policy but selective enforcement will result in this type of scenario (complaint, lawsuit, etc.) because it will always come across to the individual as if they are targeted or profiled. Another option, they would have to make the emergency announcement include the restriction on using a camera on the plane to verify no one is disobeying the rules.