PTP
Picture Transfer Protocol
PTP (Picture Transfer Protocol) is the standard way cameras transfer images and accept commands over USB, without vendor-specific drivers.
PTP is the language a camera speaks to a computer or phone over USB. It lets the connected device list the photos on the camera, download full-resolution files, and in many cases trigger the shutter and read settings — all through one standardised protocol.
Because PTP is a standard, a tethering app can talk to Canon, Sony, Nikon and Fujifilm bodies over the same USB foundation, layering each vendor's extensions on top for camera-specific features.
How it works
PTP was standardised as ISO 15740 so that any computer could talk to any camera without a driver of its own. The connected device opens a session and asks for the camera's device information, which says who made it and which operations and events it supports. From then on it works with objects: each photo on the card is an object with a handle, a size and a format, and downloading a photo means asking for that object's data. Cameras also send events, such as “object added” when a new shot has been written.
Vendor extensions
The standard covers transfer well, but camera control varies. Canon, Nikon, Sony and Fujifilm each add their own operations and events on top of PTP, for remote-shooting modes and their own capture notifications. Tethering software reads the device information, sees which maker it's talking to and switches to that maker's extensions. That's why Sony bodies need their USB mode set to PC Remote before they'll tether, while most Canon and Nikon bodies need nothing set — see Sony camera tethering.
An example: a sports sideline
A sports photographer plugs a Nikon Z8 into a phone with a USB-C cable. Over PTP the phone learns about each new frame as the camera writes it, downloads the full-resolution file and checks it arrived intact. CloudTether's app logs show a median of about a third of a second per file from Z8 bodies over USB.
PTP in CloudTether
CloudTether talks PTP to every camera it supports over USB, with Canon, Nikon, Sony and Fujifilm extensions layered on top. It learns about new shots from the camera's own events and, as a fallback, by checking the card, so a frame isn't missed if an event is. Over Wi-Fi the same protocol runs across the network as PTP/IP. See camera tethering software for the whole flow.
Tips and common mistakes
- A camera usually holds one PTP session at a time: if the maker's app or a computer is connected, the phone can't be.
- If the camera turns up as a webcam or a USB drive instead of a camera, its USB mode is wrong — switch it to the PC, remote or PTP setting.
- Turn off the camera's Wi-Fi functions when using a cable; on several bodies they block USB communication.
See also
Put it into practice
CloudTether tethers your camera to your phone and delivers every shot to the cloud, live as you shoot — no laptop.