>>3810788There are three ways to do flash:
1. TTL: everything is automatic like autoexposure. You pick your shutter speed (same or slower than max sync speed of your camera), pick the aperture, and the flash does the rest. TTL is proprietary to each camera and flash model and not interchangeable between brands. Also it's possible with later (late '70s, '80s), electronic SLRs that have the circuitry for it.
2. Thyristor: there is a sensor on the flash that measures exposure as the flash is firing, and cuts the flash when adequate exposure is reached. You choose an aperture, then set the aperture and ISO on the flash and you're good to go. You can use any thyristor flash on any camera, since the circuitry and calculation is done on the flash and is independent of camera. Those flashes predate TTL and are cheap and plentiful.
3. Manual: you do the calculation yourself. You find the guide number on the flash - note whether it's in feet or metres. Then you focus on your subject. You check the lens to see the focusing distance, i.e. how far away the subject is from you. Then divide the guide number by that distance and you get the aperture you need to set on the camera - remember to use the same units for the distance and guide number, be it metres or feet. What you get is the correct aperture setting for ISO100. Adjust aperture accordingly for different ISOs.
Lastly keep in mind that shutter speed should be set at the max sync speed of your camera or lower. Usually that is 1/60 or 1/125.
Also remember that shutter speed affects ambient exposure and not flash exposure. You control flash exposure with aperture (or power setting on the flash).
