Flashs can be used in different ways but before talking about it, let's compare flashes and permanent lightings. Flashes are emitting high light power during a fraction of seconds, typically 1/1000 seconds and even less depending on the device but also on the light power emitted: the more light power, the longer the flash duration will be. What would happen if lamps were used instead of flashes? Two issues will raise: (1) if the power of the lamp are the same, the heat produced will warm up the shooting place (2) the required electricity will be huge and a battery could not be embedded. But of course flashs cannot be used when doing movies where less powerful (LED) permanent lightings are used. However, Flashes might look tough to use because the effect of the flash light has to be imagined. As a workaround, most studio flashs come with a small low power permanent modeling light to appreciate the impact of the resulting light and shadows,...
Flashs might look less intuitive to use than permanent lighting, but it has a great advantage in photography: it makes it possible to control separately the lighting of the background and the lighting of the foreground if their distance is big enough. Because of the solid angle emitted by a flash, the light illumination coming from the flash is decreasing according to a 1/x2 law.
When using a flash, the camera sensor has to capture all the data at once because the flash duration in very short. It prohibits the use of an electronic shutter because scanning the sensors and collecting bytes of a picture take time: some sensor photosites will see the flash light while others not. Only mechanical shutter is possible and the synchro flash speed stands for the shortest duration (or fastest speed) that can be used by exposing all the photosites (or the film) at once. The maximum exposure time (the synchro flash) is usually in between 1/125 and 1/250 seconds. If you try to go faster, you might see black parts in the generated image corresponding to the non-exposed photosites. So, any time exposure can be used up to the synchro flash duration but it does not means you cannot freeze motion with smaller time exposure. Indeed, remember that the flash duration is around 1/1000 seconds and sometimes faster depending on the device. Moreover, if a flash and a camera can communicate with the same protocole, high speed synchro can be used to capture picture with shortest duration than the flash duration itself. How is it possible? By emitting several (but limited in power because it's not possible to recharge the condenser it between 2 flashes) flashes recorded by different areas of the sensor. All the bytes constituting the image are then gathered to generate a full image. Nevertheless there is a small delay between flashes... but generally not perceivable up to 1/8000 seconds.
Remember that because the flash duration is much shorter than the synchro flash speed, any time exposure can be used up to the exposure duration of the synchro flash. It implies that the light supplied by the flash is not dependent on the exposure duration! Whatever the exposure duration is, the illumination due to the flash will stay the same. It is only impacted by the aperture because it affects the quantity of light reaching the sensor. Therefore, the aperture can be set such as the flash light is as expected for the foreground and then the shutter speed of the camera can be set such as the background is as expected. If you want to shoot with a specific aperture, the only way is to adjust the power of the flash light.
studio context
The basic idea of shooting in a studio is to totally control the light i.e. to organize the flashs in order to get the proper illumination to each part of the subject. To reach this aim, the studio walls should absorb as much as possible the reflected light to avoid disturbances, but most of all, the ambiant light must neutralized by using powerful Flashes. It means that when shooting in studio, if the flash is off, the resulting image has to be totally black. Flashs operates as lamps where the lighting power of each flash is set in order to get the expected results: the background illuminance is also achieved by light flashes and the ambiant light does not impact it: the foreground/background separation is not useful in this context. A flashmeter helps to determine the right aperture by estimating the aperture for a given ISO sensitivity. The exposure time is usually set to the minimum duration of the synchro-flash to cancel the remaining ambiant light. The camera is set to manual: the minimum ISO sensitivity is set to maximize the image quality (by canceling amplification). The flash light power can be adjusted to set both the emitted light power as 1/1 (full flash power), 1/2 (half flash power), 1/4 (fourth flash power),... and proper aperture decided by the photographer. In a studio context, once the scene is properly exposed, the photographer can move around keeping the camera in manual mode (sensitivity, aperture and exposure are set), the exposure of the scene will not be affected.
fill-in context
Another way to use a flash is to add light in the foreground according to the foreground/background separation principle explained above: tuning the camera for getting a well lighten background and adjusting the flash power to properly expose the foreground. In this context, the flash power is usually adjusted thanks to a Through The Lens (TTL) measurement mechanism. When shooting, 2 light flashes are emitted: a small one for the camera to determine the right exposure of the subject in the foreground, and a final one, whose power is computed from the results of the first light flash, to properly expose the foreground. Flash compensation may vary the flash power but it is always related to the measurement resulting from the first flash. The advantage of the TTL mechanism is that it avoids the manual tuning of the flash power and the trade-off is that the illuminance of the scene depends on the first flash measurement i.e. on the position of the collimator in the viewfinder. TTL is an easy way to tune a flash power but it is less precise than manual setting.
using several flashes
Sometimes, in a studio context, several flashes should be remotely controlled by the camera: at least, the triggering, and frequently the power too. There are several ways. A flash can be installed on the camera and declared as master flash and before flashing, it sends pre-light flash for triggering other slave flashes triggering when receiving the light signal. This way of control can only be used in manual mode, it means in studio... where manual control is useful... Perfect... It's cheap however dependent on the diffusion of the light in the studio: the slave flashs have to receive the light signal. Alternatively, there is a more efficient way to control other flashes: using a Flash controller communicating with radio signal but all the devices have to talk the same radio protocole i.e. they should be from the same brand. With this way, TTL and HSS are possible.
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