A hot shoe is a mounting point on the top of a camera to attach a flash unit.
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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.[1][2]
The trigger voltage (for a flash) between the center contact and the shoe have varied over the years, between manufacturers, and even in the same manufacturer. When the contacts with a shutter were mechanical contacts, the actual voltage did not matter too much as long as it did not cause arcing, but now with electronic triggering, it can cause problems.[3]
The ISO 10330 specification allows for a trigger voltage of 24 volts, some manufacturers, particularly Canon, ask for no more than 6 volts. Some older flashes may have a high voltage, sometimes in the hundreds of volts. [4].
It is possible to connect a high voltage triggering flash with a camera which can only tolerate 6 volts, through the use of an adaptor which isolates the two units. Also many radio triggers, e.g. PocketWizard, while giving a low voltage to the camera, can handle 200V from the flash port [5] thus isolating the camera from the flash's high voltage trigger.
Before the 1970s, many cameras had an "accessory shoe" or "cold shoe", intended to hold flashes that connected electronically via an outboard "PC cable" (not meaning a computer: the term goes back to the synchronization method of the "Prontor/Compur" shutters of the 1930s), or other accessories such as external light meters, special viewfinders, or rangefinders. These earlier accessory shoes were mostly the same U shape, and thus provided the template for the introduction of the hot shoe.
Canon, Nikon, Olympus, and Pentax use the standard ISO hot shoe with various proprietary electronic connections. Since 1988, Minolta has used a proprietary "iISO" connector, and Konica Minolta and Sony Alpha digital SLR cameras are based on Minolta designs and use the same connector.
There is still a use for "Cold shoes" i.e., a shoe mount without the "hot" connection. These are often used with off-camera flash, with the flash mounted on a light stand, where the sync signal comes from either a photo slave or a radio trigger.[6]
Manufacturers in the 21st century still make non-flash items which mount on the hot shoe of their cameras. For instance, for the Olympus XZ-1, one can buy items such as a stereo microphone or three different models of electronic viewfinder.[7]