Hacking the GN calculations when using manual flash

Here’s how to hack it when you’re stumped as to what guide number to use with a manual flash. This is useful when you’re using an analog SLR that won’t sync the flash power automatically, or you’ve got a DSLR and want to fine-tune the amount of light the flash puts out. I can’t stand having to calculate this with formulas. We all may have seen this :

Aperture (f-stop) = GN X ISO/distance (in meters)

But do any of us know it by heart, or better yet, want to know it? And are we really going to take out a tape and measure the distance to the subject? I know I don’t feel like it. So how can we hack this? Well, we use what knowledge we have to ascertain the flash power we want, and then we adjust the GN (Guide Number) up or down. It works like this:

  • Higher GN means more power for the flash and consequently, more light
  • Higher f-stop means smaller aperture, and that translates to less light coming into the camera
  • Higher ISO means better sensitivity to the light that the aperture lets in
  • Higher distance means less light (remember, we’re using a flash, and the effective distance is limited)

So, what does this mean for us? Simple: we can adjust any of the four factors listed above to get the photo we want. Need more light? Boost the GN and/or the aperture. Can’t get more light, but want a better photo? Boost the ISO, but recognize the photo may be grainy. Can’t boost ISO? Decrease the distance between you and the subject.

Of course, keep in mind that when you boost aperture (choose a lower f-stop), you’ll decrease the depth of field. Think of the focus field as a loaf of bread. When you use a small aperture (large f-stop, 16 for example), you get the whole loaf in the shot. When you use a large aperture (small f-stop, 1.4 for example), you get only a slice in focus. You can effectively think of f-stops as slices of that loaf of bread. Larger f-stops means more slices. So if you’ve got objects in this photo of yours that reside at various points of focus (more slices), to keep them all in focus, you’ll need to keep the aperture fairly small (large f-stop). If you’re only interested in a particular object, by all means, increase the aperture (small f-stop), get more light that way, and use a lower GN. You’ll get more natural colors. Flash light can be harsh and wash out the nuances if overused, so the less you can use, the better off you are.

Don’t think I’ve forgotten to talk about shutter speed. Just realize that you won’t have too much flexibility there, in particular if you’re shooting handheld. Even with a tripod and manual flash, you can’t adjust the speed that much. Too slow, and any people in the photo will be blurry. Plus, the flash will be ineffective. It can’t stay lit for several seconds or more unless you use a bulb. Too fast, and you won’t get any light. Plus, if you’re syncing the flash with the camera, you’re limited by the top sync speed, which varies by camera and usually runs from 1/180 to 1/250 seconds. You’re better off playing with the other variables in the equation.

Remember, you don’t need to go to the trouble of using manual flash unless you have to. If you need to adjust flash intensity and your camera allows it, you can easily boost or decrease it through simple menu functions. Just look this up in your camera’s manual. You can usually use the +/- button, if your camera has one.

Hope this helps!


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