Adjusting Flash Power (how) ?

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thelamont

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Hi All, I'm new here, and new to DSLR's as well ..... great site btw.....

Been reading up a bit on flash lighting, I understand the need to reduce the power on your fill in flash, but how is this done ?

Do flashes come with a variable power switch or do you just dial in a different 'f' number ?

Hoping this don't sound too dopey .... :thinking:
 
Do flashes come with a variable power switch or do you just dial in a different 'f' number ?
Click to expand...

Most flashes do have a way of controlling the power, even if it has to be done via the controls on camera for built in and some hotshoe flashes. Or you can just alter the aperture as you say.

There are times that you might need to keep the aperture where it is for controlling the area in focus and then the only option is to reduce the flash brightness. Which can be turning it down, or if it's off camera, moving it a bit further away from the subject. :)
 
Flashes (decent ones anyway) have their power listed in fractions. 1/1 is full power, 1/2 half power, 1/4 is quarter power etc.

How this equates to shooting with flash is that dropping the power by half costs you one stop of light each time. To compensate for this one stop of light you need to open your aperture up by a stop. Remember that most SLR's adjust in either half or 1/3 stops so make sure you dial in the appropriate adjustment.

Any changes you make to the aperture will also affect ambient (available light that is already there like the sun or a lamp) so you will need to adjust your shutter speed to compensate for that. Changing shutter speed has no effect on the flash as the flash duration is shorter than the fastest speed your camera will sync with the flash (usually 1/250 sec).

Hope this makes, check out strobist.com or Zack Arias blog for some more indepth information.
 
Remember when moving the flash back twice the distance you will reduce the light 4 times. Move it back 3 times and you will only get 1/9 of the power. This is known as INVERSE SQUARE LAW.
 
Thanks for the replies.

Got an old Canon speedlight 177 flash here from my good old 35mm days.
No power control switch, but is aperture adjustable.
Was just wanting to use it as a fill in light when i get a setup going here.

Oh ..... and i do know about the good old inverse square law..... (y)
 
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