About Flash: Part Two

Now what you know about the power& distance limitations of a flash, lets see some stuff you can do with a flash.
There are two ways to use a flash:
1. As the primary source of light
2. As a secondary source (fill flash)

As a secondary source, you would expose the image for ambient light and let the camera adjust the flash strength via TTL(through the lens) metering, while giving a bias if needed(+1 flash EV will double the power, -1 will half the power that the camera has otherwise calculated using TTL metering). Av(/A) and Tv(/S) modes expose for ambient light and use flash as a 'filler'. Also called slow sync when long shutter speeds are used (slower than about 1/30s)

As a primary source, u underexpose the background by a lot and use the flash's power to completely light your subject(or whatever part of it you want to light up). In a reasonably dark place, P(rogram) mode does this... it will not expose for ambient light, only for the flash via TTL.  Will usually use a shutter speed of 1/60-1/200s. When used in this manner, the flash freezes the subjects movement and leaves no noticeable blur, because the flash strobe has a very short time duration (~1/10,000s) and the camera is NOT exposing for ambient light.

When to use the flash?
Indoors: When there is insufficient light or to balance a subjects exposure with backlighting if a window is open in the background
Outdoors: for portraits/ group snaps, to lessen harsh shadows and to create catchlights (the twinkle in the eye that you usually see)

In this example, I used flash to balance the indoor and window exposure. It was about 3pm and had I not used flash, there would be two possible outcomes depending on how you expose the snap:
i) The subjects would be properly exposed, but the window in the background would be completely blown out(highlights clipped), ruining the snap
ii) The window would be exposed correctly but the subjects would be silhouettes!
The only way out was to correctly expose ambient light of the window with the lightmeter while letting the camera use TTL-metered flash to expose the subjects.

Flashes used directly can produce harsh shadows and ruin texture and it is advisable to diffuse them i.e. make the source seem bigger. There are many ways to do this, the most effective is bouncing it off the ceiling/wall. This changes the light source from being hard to soft.

I may have mentioned in the past about the X-sync speed of a flash, which is ~1/200s on most cams, and it wont allow you to shoot faster than this with the flash on. However you can use an external flash which supports high-speed sync to use flash at higher shutter speeds. This is especially useful for shooting portraits outdoors when you want to use a large aperture to blur the background as well as use the flash to prevent harsh shadows.