Flash photography can come across as rather intimidating for photographers looking to expand their skillset especially if they only have experience using ambient light as their main light source.
Before starting to shoot with flash at all, we have to understand the technical side of photographing using strobes.
Flash is a quick burst of light and can be as fast as 1/40,0000 of a second which means your shutter speed, regardless of which one you use, will have little to no effect on your exposure, so, therefore, we control the flashes exposure by controlling the volume of light allowed into the lens, in other words, the aperture.
Most flashes these days have the ability to control the power output. The maximum power you have on your flash is represented by the flashes guide number. The higher the GN, the more powerful your flash is. The more powerful your flash, the bigger a subject you can illuminate and the greater distance you can shoot at from your subject.
So on your flash, you will have the ability to control your power output by means of fractions of your flashes' maximum power output (your GN). So for example, you can shoot at full power, or 1/1 power, you can reduce this if you like to 1/2 power, 1/4 power, 1/8 power, 1/16, 1/32, 1/64 as well as 1/128 power. Why would you want to reduce the power? To perhaps prevent your flash from illuminating the entire environment you're working in, or perhaps you wish to shoot at a wider aperture for creative reasons.
But what effects does your shutter speed actually have when using flash? Well, you may wish to alter the shutter speed to control the ambient light exposure when shooting in a mixed lighting environment, such as using flash at a wedding for fill-in flash to manage contrast, or for getting creative when shooting motion.
With that said, however, you may notice that your camera will automatically limit you to a maximum shutter speed of around 1/250th of a second in most cases when you have a flash connected to the camera via the hot-shoe mount. The reason for this is because after a certain shutter speed your physical shutter curtains operate differently and do not completely open during the exposure operation, when using flash in this instance, you will only have a segment of your flash's exposure in your final result. You can overcome this by "strobing" your flash in a "high sync speed" or "HSS" shooting mode in your flashes settings menu. This will allow you to freeze high-speed motion with using flash as well as shoot at a far wider open aperture with a faster shutter speed such as 1/1000th of a second. Keep in mind that if you are shooting off-camera flash, there is no physical connection between the flash and the camera body, so therefore your camera doesn't "know" you are shooting flash and will therefore not automatically limit your shutter speed to the "sync speed" or 1/250th of a second.
Lastly, and most importantly, we have to understand the characteristics of a flashlight. Flashes are a small and very bright white light source that instantly creates a harsh and high contrast lighting environment which is not ideal in most cases. It can kill the ambience and mood of an image very quickly if not used carefully. There are ways around this, however, that of managing the contrast caused by a flash. You can improve this by using advanced techniques such as bounce flash, or making use of flash accessories such as softboxes, diffusers, octaboxes and remote trigger systems that give you the advantage of shooting "off-camera flash" so you (as the photographer) have far more control over the direction of light in these scenarios.
Take a look at the video below by the YouTube channel: Mike Smith on Flash Photography For Beginners Part 1...