What is Guide Number?
The further away the subject is from the flash unit, the more energy is needed to give sufficient illumination for proper exposure. If the power of the flash stays constant, it is necessary to open up the camera's aperture as the distance between the flash unit and the subject is increased. The distance and the aperture value for proper exposure are connected by this formula:
subject distance * f-stop value = constant = Guide Number
Guide numbers are valid only for a given speed of film, and typically are expressed for ISO 100.
In order to double the guide number, four times more energy needs to be stored in the capacitor. This puts practical limits to guide numbers, since the size, weight, and cost of the capacitor increase as its capacity is increased.
Doubling of film speed will increase the guide number by a factor of 1.4 . Going from ISO 100 to ISO 400 film, the guide number is doubled. Going from ISO 100 to ISO 25, the guide number is halved.
What is TTL flash?
TTL flash is a type of flash that is metered using through-the-lens metering.
TTL means "through the lens". Flash metering is through the lens, using the camera's metering system, which looks through the lens you're using.
In TTL flash, the flash works with the camera's internal metering system to determine flash output for proper exposure of the scene.
The advantage of using a through-the-lens flash metering system is that the photographer is not required to perform the calculation of flash output, which eliminates a margin of error. This saves time while shooting, ensuring that less action is missed. In basic photography, TTL metering ensures a reasonably correct exposure while using flash.
What is Wireless TTL flash?
Wireless TTL is a proprietary, closed-loop system with the capacity to transmit firing and flash exposure information to single or multiple compatible TTL flash units set off-the-camera.
These off-the-camera TTL flash units are usually called Remote / Slave(s).
Exposure for the TTL Remote / Slave(s) is metered in-camera, through the lens (TTL). Flash Exposure Compensation for the TTL Remote / Slave(s) is controlled from the camera's built-in flash or a master flash connected to the camera through the hot shoe or a TTL cord. The TTL Remote / Slave(s) will fire in synchronized with the triggering source, without having the user to worry about the pre-flashes.
What is Automatic flash?
On the front of an automatic flash unit, a sensor reads the light reflected from the subject that is produced by the flash. When this sensor is satisfied with the amount of light received for correct exposure, it automatically shuts off the flash.
The closer the subject is to the flash, the quicker the sensor shuts off the light.
An automatic flash unit usually provides several choices of aperture (f/stop) to the user for controlling the depth of field.
By knowing the guide number of the flash you can determine the effective flash-to-subject range for correct exposure of the subject.
What is Slave flash?
A slave flash usually has a sensor built-in that allows it to be triggered from another flash. It can be placed anywhere within the sensor's triggering range or near the flash you will use to trigger it.
It fires in synchronisation with the main flash that's connected to the camera.
Some more sophisticated slave flashes provide options to the user, such that the user can choose to get the slave flash in synchronized with the main flash or skipping several flashes before firing.
You could use the slave flash to light nooks and crannies in interior shots where the on camera flash can't reach such as cupboards, alcoves, underneath arches etc. In portraiture it can be used to provide a hair or rim light for a model shoot or a side light for enhanced lighting. You can point it up or down to graduate a background too.
It can also be used as a power boost to the built-in flash when you want to shoot at a longer range. You could even use two or three to create more complex lighting effect.
What is Auto Focus?
Cameras have big trouble focusing in low light. Usually they have built-in lights (white or infrared) to add light to the scene for focusing.
Some flash units have similar focusing aid lights built in. These are typically more powerful than the ones available on the camera bodies.
What is Radio Trigger?
A radio trigger set basically consists of two parts. The transmitter module goes in the camera hot shoe, and a receiver module attaches to the flash, usually via PC sync connector, or some models use a shoe foot connector.
The transmitter sends signals via radio frequencies to the receiver to fire the attached flash once the shutter release button has been pressed.
Radio has advantage of great range. It can go through/around most obstacles, and is not affected by bright sunlight.
Radio Triggers comes in two types. One type does not transmit TTL flash exposure information. The devices of this type are usually not Wireless TTL, nor are they compatible with or required Wireless TTL. The other type is TTL-compatible. These devices usually have microcontroller inside that allow them to transmit TTL signals.
What is High-Speed Synchronization?
For some applications (such as daytime fill-in flash) it is important that the subject would be illuminated for a longer time than the typical duration of a flash of 1/1000 seconds.
With many flash units, there is nothing you can do.
Some newer flash units, however, offer a high-speed synchronization mode, which works by emitting a rapid series of short flash pulses. As the slit between the shutter curtains moves across the frame, these short pulses illuminate the scene in a rapid sequence, eventually resulting in exposing the complete frame.
The flash unit distributes its light energy to the whole scene in all cases. With normal (low-speed) synchronization, all of the light which is collected from the scene by the lens, is available to expose the film. In high-speed mode, part of the frame is blocked by the shutter at any given moment, and therefore part of the flash energy is wasted. As a consequence, flash units have lower guide numbers in high-speed mode than in normal mode.
What is Second Curtain Synchronization?
When one uses long exposures to blur the motion of a moving subject, it is sometimes nice to add a flash picture of the subject into the same scene, to create a better illusion of movement.
With the standard construction of shutters, the flash is usually fired as soon as the shutter gets completely opened. This results in ugly pictures where the movement trails are ahead of the flash image. For nicer looking results, it is therefore good to synchronize the flash just before the shutter is about to close. Then the movement blur is behind the subject, providing a better visual clue to the motion.
While the technology to achieve this is quite simple, it requires support from the camera body, and in cases of some manufacturers, is implemented via proprietary communication between the flash unit and the camera body, being therefore available on only selected flash units.
What is Trigger Voltage?
The trigger voltage is the amount of voltage between the strobe's two primary hotshoe contacts (center pin and rail). This voltage will be discharged by the strobe through the camera's hot shoe when the strobe fires.
Some more primitive flash units apply a very high voltage (200-300 volts). These voltages may damage the sensitive electronics in modern digital cameras. The acceptable trigger voltage for all modern digital cameras is typically around 6 volts.