How to Use Guide Numbers in Manual Flash

drixxxdrixxx
edited February 2013 Posted in » Nikon D3100 Forum
I'm kind OF confused regarding Guide numbers. How do I use it to get proper exposure using manual flash?

Comments

  • rhinorhino
    edited February 2013
    Guide number divided by aperture value equals distance from flash to subject at ISO 100. If your flash is closer than the calculated distance you can decrease flash output. If you have the flash further from the subject than the calculated distance you can either change aperture if that works for you or you can increase ISO. Just remember that light falls off exponentially, so if you need your flash twice as far as your calculated distance you will need to increase your ISO sensitivity 4 times. For example, if you have a guide number of 80 feet and an aperture of f/8 the flash should be 10 feet from the subject at ISO 100. If for some reason you are limited to a distance of 20 feet then you will need four times the sensitivity of ISO 100, which is ISO 400.
  • drixxxdrixxx
    edited February 2013
    In your example, Im thinking that it's on-camera flash right? How about off-camera flash? Can I still do the same method?
  • rhinorhino
    The same principles hold true whether using on-camera flash or an off-camera speedlight. What's important to remember when using off-camera flash is that the distance in the calculations is always the distance from your subject to the flash, not the camera. Another important point is the fact that when attached to the camera hot shoe, many speedlights will narrow their light output to match the zoom of your lens. In this case, your flash guide number will be different at different zoom lenghts, so be sure to reference your flash manual to find the correct guide number in order to calculate required distance, aperture, and/or ISO.
Sign In or Register to comment.