most folks are content to go through life with their flash on camera, you know, like a tourist.
while you can achieve some great pictures with the camera and the flash doing the exposure math for you, one thing that you don't under normal circumstances is form and texture. a flash on camera could be likened to a studio fill light. you can get great exposure but the subject is a shape with no sense of roundness or drama. let's take the flash off the camera and connect it back using a sync. if i was doing event photography i would place it on a rotating bracket and connect to the camera with a TTL sync cord. the bracket would allow me to rotate the camera from horizontal to vertical keeping the flash positioned over the lens. i still have TTL but the distance between my lens and the flash have increased. the correct distance is 12-14 inches. this eliminates "red eye". red eye occurs when the flash is too close to the lens. light strikes the subject and reflect straight to create the image. the retina of the eye is illuminated by the flash and the blood vessels are recorded, elevating the flash cures this. also using bounce light will cure it but sometimes it it leaves a subject with no catch lights in the eyes. attaching a table spoon with a couple of ruble bands to the back of the tiled flash will put the sparkle back in to their eyes, but it's still just a fill light creating a shape. these refections are called "catch lights" and are best if at either the 11 o'clock or 1 o'clock position
okay on to manual flash. i'm going to try keep it simple so that you will try and grasp the concept and we will build on it later. for now in the interest of keep it simple stupid (KISS) me not you let's place the flash back on the camera and turn the flash on manual.
1. set the camera and flash exactly 8 feet from a subject.
2. make up a set of note cards, one each with each f/number that you camera has.
3. set the camera for iso 100
4. set the camera for 1/125 shutter speed
5. place each one at a time in from of the subject and set the aperture for what the card says, take the photo.

one of these photos will be perfect and the rest will either be dark or light. here's the fun part kids, take the f/number of the good shot and multiply it times the distance of the flash (in this case 8 feet) and you have the Guide Number of your flash. GN/D=A say what? the guide number divided by the distance between the flash and the subject equals the correct aperture for exposure. okay, this doesn't like like fun, well it can be. let's say that the guide number is 64. we can divide that number by the distance each time we move about or get smart. remember i set this test up using 8 feet? 8 is an aperture number also. now take a piece of paper and write down 8 over top of 8. let's create a scale now. in front of the bottom 8 write down 6, in front of 4, in front of it 3, and 2. these are now all now distances. remember everything in photography is double and half. the distances i gave you are round off f/numbers. if we moved up to 6 feet then have double the amount of light, therefore we have to reduce the aperture or stop down. write 11 over the 6. moving to 4 will double the exposure again so write 16 over it. basically take one aperture scale and write it down as distance. reverse another scale and align the correct exposure for 8 feet then and combination would correct. once this mastered then will work on with using this technique with a umbrella off camera position.

so what's the advantage of manual flash? it puts the same amount light every flash. it isn't influenced by the background, or the tonal range of the subject and it can't use with a variety of light modifiers. you can dial down manual flash to a lesser output. you can use a photo optical slave and use it in conjunction with another flash. for example, place a color gel over a flash and place it in the background. a red gel will look like neon on a black background and gradually fall off back into black, very cool. two things here, never fire the flash quickly and for a long period for the gel more melt. another and this is a little known secret, ready lights on flashes usually come up at about 75% power. this makes the flash to recycle faster than it really does. kinda like the estimated MPG sticker on a new car. try to match it... another thing about a background, the falloff should start to break at the subject shoulders any higher would suggest sainthood.
back to GN, it's only good for the ISO it calculated for. if you tested at ISO 100, then that's what it works for. if you want to work at ISO 200 (which is one stop faster than ISO 100 or twice as sensitive to light) multiply it by 2. if you want to work at ISO 400 (which is 2 stops faster than ISO 100 or 1 stop faster than ISO 200) double it again from the ISO 200 GN.
more to come on this topic, it's not easy but it can be rewarding.