This is the specific rule which applies to 121 cockpits:
https://www.ecfr.gov/current/title-1...ection-121.542
Executive summary:
You cannot use any electronic device during block time which has the *ability* to connect via a wireless system, unless it's approved per your OPSPEC. Examples of approved devices: EFB's, and typically personal cell phones while on the ground with the brake set, to contact the company.
So you can use old school film cameras, and older digital cameras which don't have any wireless. Obviously not during sterile cockpit though.
People have gotten in trouble over this, usually after they post stuff on SM. I have a buddy who had his career derailed by such an event years ago.
That's the cliff's notes, and should keep you out of trouble. But you can go down a lot of rabbit holes with this...
there may be some unintended consequence with the wording of the law. You cannot use a device in airplane mode, doesn't matter if it's connected or not.
While the genesis of this was two NWA pilots who supposedly got distracted by a laptop and overflew their destination, the other concern behind the rule is that PEDs might interfere with aircraft systems.