Flash is a very wonderful and often times mystical thing. For something as simple as it is (a device that emits a blast of light), there are a lot of subtle nuances. There are three key components of using flash:
1) Synch speed
2) Aperture value
3) Shutter speed (one of the smaller components)
Synch speed is the fastest shutter speed at which your camera can use a flash. For some cameras it's 1/250sec, for others it's 1/60th. The most common shutter speed for flash is 1/60th as that's the slowest speed one can generally handhold a camera without noticible blur in photographs. Synch speed for your particular camera may vary, so for that you'd have to look it up in the manual. So, what does synch speed do? If your camera has a mechanical shutter that's a focal plane traverse shutter (leaf shutters on medium format (MF) cameras don't really have a synch speed, but they may have differenct "synch" settings, and on MF cameras it is usually called X synch). What happens is the shutter opens, then the rear curtain closes. If the rear curtain closes too fast, you'll have a nice,perfectly black area on your photo. Most modern cameras won't let you exceed the synch speed and older cameras have the synch speed marked in red on the shutter speed dial. Higher synch speeds (1/200th or higher) are usually only found on higher end cameras, namely dSLRs. Some of the nicer dSLRs have up to 1/500sec synch speeds. This is useul if you're trying to catch action with a strobe and need a fast shutter speed (and a flash).
Aperture controls how much light reaches the film or the digital sensor. For flash exposure, aperture is the primary exposure control. Close the aperture (larger f/ number) for less flash exposure, open it (smaller f/ number) for more flash exposure. Changing the shutter speed in the range of its maximum synch speed and about 1/50sec won't do anything to your exposure when using a flash. This is beacuse your flash will always fire within that shutter speed. See below for more.
Flash really doesn't care about shutter speed apart from the synch speed, however, you can shoot slower than the synch speed without problem. Why would you want to do that? If you wanted to balance flash light with ambient light, for example. You slow down the shutter speed until you get a good mix of flash with ambient, and by doing so you will have a more natural looking flash exposure. Another shutter aspect is something called "high speed synch". Not all cameras have this feature and what it does is it allows you to use shutter speeds faster than the synch speed. The flash fires a rapid series of bursts rather than just one for the length of the shutter opening. High speed synch provides a wider degree of flexibility with your flash.
What is flash exposure compensation (FEC)?
Flash compensation is simply the adjustment of the intensity of the flash burst. Flash compensation is used when your flash is putting out too little or too much light. Many sub-dSLR cameras do not offer this option. Say you are shooting a group of people in black suits. Your flash fires and it turns out that your photograph is too dark. You didn't get enough light coming from the flash. You can dial the FEC (usually located on a flash unit if external, and sometimes controllable from on the body of the camera, depending on the model) up from the "0" setting, usually up to 2 or 3 stops higher (or lower) than the "0" value.
While it may seem counter intuitive, if you decrease the flash compensation the effective range of the flash increases and if you increase flash compensation the effective range decreases. Why? Let's say your flash has a range of 100', if you increase the flash compensation, 100' would still be exposed correctly, but you requested the flash to overexpose, so the usable range is reduced to the longest distance one can get the correct "over" exposure. Conversely, if you reduce the flash compensation that same 100' distance be easily underexposed and thus the range for the correct "under" exposure is extended. Obviously, aperture size affects the flash range, regardless of the level of flash compensation being used.
Written by B52GUNR  . Parts added and changes made by DavisPhotog .
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by DavisPhotog
last modified: 2006-05-08 11:52:20 |
