Odd shutter speed behaviour with iPhone 7 telephoto camera

This is an intentional behavior. You're familiar with the one over focal length rule? In general, exposure time needs to be at least one over the focal length in order avoid blur from handshake. The telephoto lens is a ~56mm lens, so 1/50 of a second follows the rule. Try setting the exposure time longer and then take some handheld photos. Hard to get a sharp result unless your hands are very, very steady.

The focal length of the wide angle lens is ~28mm, but as you note, auto exposure will go as low as 1/15s. This is because the wide angle camera has OIS, which can optically stabilize the lens to counter handshake.

Yes, I realise this and know the reasons why. I've built my own AI system to replace this though, and it will detect if say the phone is in a tripod and allow longer exposure times.

On the wide lens this works as expected: exposures are typically limited to 1/15s, if I hold still I get 1/5 to 1/10, and if I put the phone down I get 1/3s.

On the telephoto lens though, it stays at 1/50s unless I use the manual controls. I'm setting the AVCaptureDevice's activeVideoMaxFrameDuration as normal but it has no effect.

So there's a difference in behaviour between the two cameras here.

(Unrelated, but if there's any way that 1/3 second max exposure could be increased it'd make a lot of my customers happy and sell a fair few iPhones 🙂)