During FaceTime calls, I’ve observed that when the other participant rotates their device, the image on my end sometimes reorients or shifts in a way that reveals more of their surroundings than expected.
Question: Are you sure that their preview inset isn't showing them the same video they're transmitting? Depending on device rotating it may change what is being sent, but I'm pretty sure they'll still see what they're sending.
The exact rotation behavior does depend on the sending device. If it is a recent and higher-end device with a better camera, it may auto-crop to match the orientation of the receiving device. In those cases, rotating the sending device has no effect on crop or aspect ratio, but rotating the receiving device will change the aspect ratio being received, and the crop may change somewhat. That's separate from the auto-framing available on Macs and iPads.
On an older or lower-end device, the video being sent matches the aspect ratio of the sensor. In those cases, rotating the sending device does rotate the transmitted video as well.
I'm not sure that I see this as much of a privacy issue; at least as far as I know the preview image will show what is being sent to the other side, but fundamentally you're pointing a camera on a handheld device at your face so the risk of a changing crop--particularly when you've physically rotated your own device--is no greater than accidentally bumping the device.
Is there a way to disable this automatic orientation adjustment or limit the visible frame strictly to what the sender sees on their own screen?
You can turn off the auto-framing on an iPad, but again, are you sure that the preview image isn't showing what's being sent? As far as I've ever experienced, it will change to match.
Additionally, I’ve experienced inconsistent behavior when using Screen Lock during FaceTime calls...
This is a really interesting one; I'm assuming what's happening is that the sending device's camera isn't high-enough resolution to do auto-aspect-ratio, and so it's instead trying to send what it can by cropping the low-res sensor to match the target device aspect ratio. If the result is a cropped image of lower resolution that may be the intended behavior, or it may be trying but not entirely succeeding due to a bug.
Is there a setting that allows FaceTime to display the video feed in its native aspect ratio without scaling or cropping? I would prefer to see the full frame as captured by the camera, even if that results in black borders (letterboxing or pillarboxing), instead of having the image resized or altered.
On the receiving device, as far as I've ever experienced this is exactly what happens if (and only if) the sending device doesn't have a high-enough resolution camera to do automatic aspect ratio. The video sent will match the orientation of the sending device, and the receiving device will letterbox or pillarbox to display that video based on its own screen orientation. If the sending device does have a better camera, it will auto-rotate the video to match the orientation of the receiving device.
I believe there is a slight amount of cropping depending on whether the receiving device is an iPad or iPhone: the iPhone will chop a little of the sides off if it's in the same vertical orientation as the sending device, so the screen is still filled, even though the video being received is 4:3, although in my experience the sending device will actually display this in the preview--you'll see a skinnier preview window if you're talking to someone on an iPhone in portrait mode than an iPad in portrait mode.
As for the sending end, not that I know of--I think it will try to match the orientation and screen ratio of the receiving device as best the front-facing camera allows (assuming that you don't have the auto-framing turned on). In my experience this does occasionally glitch--most frequently, the video gets stuck rotated wrong. But it's supposed to work that way, and for the most part does.
As to your final question, unless I've misinterpreted your description of something, it sounds like everything you've experienced is the designed and intended behavior. FaceTime isn't a precise system full of options, it's designed to adjust automatically as needed to let both people have the best possible chat given the hardware of the devices on each end, which can and does include varying amounts of automatic cropping and scaling.
Mostly this works, occasionally it doesn't, but if you have an issue with exactly where the borders of the frame lie, you should probably just use a different video calling app.
