Flash photography is basically adding light to the subject when there is not enough there already for you to make a correct exposure for what your intensions are for your image. It could be to just get a balanced exposure or it could be to freeze action.
All flash units have a guide number; because light intensity falls off with distance, the distance from camera and flashlight to the subject is directly linked to working out the guide number.
Guide number (GN) = distance in meters multiplied by aperture.
Looking at Canon speedlight manuals I see that they give you a guide number to work the distance out yourself but in my Nikon speed light manual they state the distance in meters and feet at 100 ISO, this make it easier I suggest.
The most basic way to use a flashlight is to generally illuminate the whole scene like my example below taken in a nightclub. fill in flash
When I took this shot it was very dark and to the side of a DJ while he was mixing as in the next image you can see just how dark it was. The image was still shot with a high ISO of 1600 and this is probably higher than I needed as I had used the flash at such a short range of about four feet but, the aperture was set to F/5 so the flash is always going to help you to get sharp well exposed images in such a dark corner of a nightclub.
Fill-in flash is another way to use a speedlight. Fill-in is generally where you are lightening up unsightly shadows that you do not want in an image. A classic example of fill-in is when you are shooting a portrait on the beach and the sun is setting behind the subject providing a strong backlight, without the fill-in the portrait would be a silhouette. But, if you use the flash on the right power setting the face will be nicely lit. and you will still get the sunset in the background.
Here is a good example by the photographer Anthony Aceldy, he used fill in flash for this shot otherwise the face would be a lot darker.
Another way you can use your speedlight is to bounce the flash off a wall, ceiling or reflector. This gives a softer more diffused light quality and the light is more evenly spread but you must remember that this method reduces the power of the flash so you may have to turn the power up on the speedlight depending on what strength you want. When using bounce flash, most speed lights have a turnable head so that the flash is pointing at an angle to the side, up or even behind on some models so that this becomes easier. It is probably worth making sure your speedlight can do this when purchasing.
Slow sync flash is something you can use on a wide range of good cameras and a speedlight. basically you are firing a burst of flashlight with a slower shutter speed. It is used to capture action and is extra helpful when using a panning technique with your subject, or it is used to capture subjects in low light as in a nightclub for instance.
You have two modes on the speedlight called 1st curtain and 2nd curtain modes (Canon) and on the Nikon it is called SLOW mode and REAR mode this refers to 1st and 2nd curtain modes respectively.
In slow mode, when you have the camera set up with a slow sutterspeed say, 1 or 2 seconds and the ISO is high enough to expose the scene to how you want it you then set the aperture. If you choose a tighter aperture (higher F number) this will have a stronger effect in brighter light than subdued. Anyway enough of the camera settings. When you shoot in these settings with SLOW mode or 1st curtain, the flash “goes off” when the first curtain opens. The flash fires and freezes the image of for the example below I used a bottle of whiskey. As I shot the image I moved the camera slightly to the left and you can see the burred effect around the bottle but the bottle is pretty sharp as it has been frozen in the center of the image.
For the next image I have used the setting of REAR mode or 2nd curtain for Canon users. This time the focused bottle is slightly to the right in the image. Even though I moved the camera to the left as before and the bottle was in the middle of the frame as before, this time the more prominent sighting of the bottle is to the right because as I moved the frame of the shot to the left the flash fired slightly later.
Both of the above images were shot at ISO 320 and F/5 with shutter speeds of 2 and 1 seconds respectively.
This technique is very good for shooting images of people in nightclubs and I am keen to try this in the future. It is also good for shooting atmospheric images of moving things like sports people indoors (if flash is allowed) or a common image I have seen is a frozen image of a BMX rider while jumping with a blurred background. I will probably be using REAR mode as it will be the subject moving and not me moving the camera and I would want my subject to be in front of the blurred motion.
High speed sync is another function to be used on speedlights. When setting your flash to high speed sync you should have FP showing on your settings, this stands for focal plane.
Usually DSLR cameras have a flash sync speed of up to 1/250th of a second and any faster the shutter curtains inside the camera would be in the frame but; if you set your speedlight to high speed sync, the speedlight produces a continual low pulse of flashes which goes for longer than a normal flash time and allows the camera to be synced to faster shutterspeeds.
I have just tried my camera at 1/8000th of a second at ISO 100 F/16 and this was in a pretty dimly lit room at night and the image came out very usable, I was pretty amazed actually because if you are reading this you should have a good idea of how little light is shone onto the cameras sensor at 1/8000th and F/16!
Why would you want to use this? well, if you was outside in daylight and you wanted to freeze any action but still have fill in flash for whatever reason and you wanted a wide aperture to narrow the depth of field to create bokeh or to make the subject stand out you would have to have a much higher shutter speed to get an even exposure so that is why you’d need to set the speedlight to high speed sync.
Another type of flash that most speedlights can perform is called stroboscopic flash this is where it fires a rapid burst of lower power flashes to capture multiple images of a moving subject in one frame. It is worthy of shooting against. a dark background to stop overexposure of the background.
Here is an example of a stroboscopic image of a golfer and his swing, photographer unknown.