A hot shoe is a mounting point on the top of a camera to attach a flash unit and other compatible accessories. It takes the form of an angled metal bracket surrounding a metal contact point which shorts an electrical connection between camera and accessory for standard, brand-independent flash synchronization.
The hot shoe is a development of the standardised "accessory shoe", with no flash contacts, formerly fitted to cameras to hold accessories such as a rangefinder, or flash connected by a cable.
The dimensions of the hot shoe are defined by the International Organization for Standardization in ISO 518:2006. Details such as trigger voltage are not standardised; electrical incompatibilities are still possible between brands.
The hot shoe is shaped somewhat like an inverted, squared-off "U" of metal. The matching adapter on the bottom of the flash unit slides in from the back of the camera and is sometimes secured by a clamping screw on the flash. In the center of the "U" is a metal contact point. This is used for standard, brand-independent flash synchronization. Normally the metal of the shoe and the metal of the contact are electrically isolated from each other. To fire the flash, these two pieces are shorted together. The flash unit sets up a circuit between shoe and contact—when it is completed by the camera, the flash fires.
In addition to the central contact point, many cameras have additional metal contacts within the "U" of the hot shoe. These are proprietary connectors that allow for more communication between the camera and a "dedicated flash". A dedicated flash can communicate information about its power rating to the camera, set camera settings automatically, transmit color temperature data about the emitted light, and can be commanded to light a focus-assist light or fire a lower-powered pre-flash for focus-assist, metering assist or red-eye effect reduction.
The physical dimensions of the "standard hot shoe" are defined by the International Organization for Standardization ISO 518:2006.
Before the 1970s, many cameras had an "accessory shoe" intended to hold accessories including flashes that connected electrically via a cable, external light meters, special viewfinders, or rangefinders. These earlier accessory shoes were of standard shape and had no electrical contacts; contacts were added to produce the hot shoe.
In 2014, camera accessory manufacturer Cactus combined these electronic extensions into a multi-brand hot shoe on their wireless flash transceiver V6. With multi-brand ISO hot shoe, cameras and flashes from different manufacturers work together.
In 1988 Minolta switched to use a 4-pin proprietary slide-on auto-lock "iISO" connector. A compatible 7-pin variant, which allows battery-less accessories to be powered by the camera's battery were also made, but not widely used. Konica Minolta and Sony Alpha digital SLR cameras are based on Minolta designs and used the same connector, officially named Auto-lock Accessory Shoe, as well up to 2012. Since the electrical protocol remained mostly compatible, TTL and non-TTL adapters exist to adapt ISO-based flashes to iISO hotshoes and vice versa.
Sony also used a variety of other proprietary hotshoes for other digital cameras, including the ISO-based 6-pin Cyber-shot hotshoe, the 16-pin Active Interface Shoe (AIS) and the ISO-based 16-pin Intelligent Accessory Shoe (IAS). Some of their NEX cameras used a proprietary Smart Accessory Terminal (versions 1 and 2). In September 2012, Sony announced a new ISO-based 21+3 pin Multi Interface Shoe for use with their future digital cameras of the Alpha, NEX, Handycam, NXCAM and Cyber-shot series. This quick-lock hotshoe is mechanically and electrically compatible with a standard 2-pin ISO-518 hotshoe, but electrically compatible with the previous Auto-lock Accessory Shoe with extensions, so that passive adapters ADP-AMA and ADP-MAA allow to use digital-ready iISO flashes on new cameras and some new Multi Interface Shoe equipment on older cameras, while providing compatibility with standard ISO-based equipment as well.
Canon uses a non-ISO-based 13+1 pin hot shoe, named Mini Advanced Shoe on some of its camcorders.