EVOLUTION:
FIRE IN THE HOLE -
In the beginning of flash the output was determined “by hand”. Lord help us if the assistant just happened to have "big hands". The amount of gunpowder “that fit in your hand”, when you loaded the powder holder. I would imagine the first auto flash was a series of measuring cups to determine exposure. We know a few probably went nuclear when photographing in a silver mine with methane gas. After the light, the helper standing there, still smoldering said to the boss "hey boss, that was F-296 for sure.
OLDER FIXED FLASHES -
The camera triggers the flash and allows the flash to send out its maximum discharge that it is set for. Older flashes only had one setting referred to as a full dump or nuclear. In this mode the camera and flash work as two independent pieces of equipment. Since the camera does not know how much power the flash will put out, this required the shooter to adjust the aperture, for exposure.
FLASH METERS -
A heavy flash user with multiple setups and no flash meter is a trekker without a compass. In studio work with fairly powerful studio lights, flash meters measure the flash output, this determines the camera aperture. You simply measure the flash at the subject distance and compute the aperture to be used. I use and swear by the Sekonic 358, many pros do.
THEN CAME SENSORS -
In the second generation of flashes, a new innovation called a sensor is used for measuring flash output. This sensor is activated when the flash is triggered. The light strikes the subject and bounces back. At that point the sensor cuts the flash discharge and the exposure is complete. "Thyristor and squelch circuits" became the new buzzwords. Most of these simple sensor models operated in auto mode on the flash.
Some flashes have sub modes determined by aperture and distance benchmarks and used colored identifiers on the flash to illustrate the sub-mode you were in. One writer called them "preset levels". On a Vivitar thats the purple yellow and blue modes.
OFF CAMERA SENSOR -
Some of these earlier flashes had a removable sensor by the use of an extension cable so one can move the flash to an off camera position. The sensor works regardless of it’s relation to the head of the flash. This can produce more flattering light with objects and portraits.
THE BIRTH OF TTL -
As the cameras became more sophisticated, it became apparent the camera could determine these settings and convey the signals to the flash when needed. The camera can tell the flash what aperture and ISO sensitivity to be used or considered. Most of the better units have complex signal devices if some parameters are exceeded. The sensor placed in the bowels of the camera now take the bounced signal off the “film plane” in the film camera and referred the same way in the digital camera. Thus the measurement is taken directly off the “contact area” or sensor area of the system. When you press the shutter release, the following in sequence happens:
Since communication is the key element in this equation provision has to be made for this signal information. Simply put more information is shared with the camera and flash. Thus manufacturers have proprietary systems or different ways of sending these signals. Generally this means no compatibility between brands and lots of little contacts in the hot shoes not matching the other guys’ contact pattern. Different sensor signals and algorithms make them incompatible.
TIMING AND MOUNTAIN CLIMBING -
Here is a simple explanation I found on the web. A flash discharges but does not reach its peak power immediately. Instead, it starts at the bottom of the mountain and then reaches the lowest halfway house (about a third of the way) and continues to the top. Maybe I should of called it a third-way house. It peaks! Now it starts the descent till it passes the same halfway house a little slower than the climbing phase. Thus almost 2/3 of the trip up and 2/3 of the trip down offer the best shot of the flash. This is called flash duration or usable power.
Flash bracketing on some models of cameras and flash combinations are similar to simple exposure bracketing, except that that flash is the illumination source. Few know about this feature. For rapid-fire Flash Bracketing the flash must be able to recycle very quickly—almost instantaneously. This might mean manual settings and low power output meaning close distances. This is what some pros do. More on this later.
SYNCHRONIZATION SPEED –
Because flash is so fast and needs a slower set of parameters to fit within. In other words the shutter opens, a flash of 50,000/sec takes place and the shutter closes. To achieve a proper flash exposure the flash must discharge within the time frame set by the shutter movements. BUT these are shutter curtains. They travel and travel takes time. When a manufacturer tells you his “flash synch speed”. Thats is the maximum shutter speed that allows the full frame to expose when the flash is active. exceed this speed and the picture will be black on either the top , bottom, or sides.