I was looking to control a DSLR camera using the PTP protocol. A few libraries exist for the Arduino USB Host shield that I was looking to tinker with once my Photon gets delivered (https://github.com/felis/PTP_2.0).
After reading more posts from @mdma about this, I still don't see a definite answer on whether or not USB OTG will be possible on the Photon. Most recently, I saw this statement from another post (https://community.particle.io/t/usb-master-on-photon/12246/11):
Just a note to say I was wrong about OTG on the photon. I didn't realize there was some missing hardware, such as a 5v source, that would be needed to support OTG.
What hardware is missing to prevent the Photon from supporting OTG?