How do I manually calculate fill flash?

My main flash is a Nikon SB-24, used on a Nikon D200. Hence, iTTL doesn't work. I've had no issues using A-mode (the thyristor in the flash determines flash output) or M-mode (manual on the flash) when shooting indoors, but I'm wondering how to use it for fill outdoors?

Are there some simple rules I can use to determine the settings for fill?

Answer

If you understand exposure and the relationship between shutter speed, ISO, and f-stop. You can follow this. If not, you'll need to learn that. You'll also need to understand how much a "stop" is.

SB-24 has an advertised Guide Number of 118 (ft) @ ISO 100. Calculate settings for proper exposure and use that as a base. You can find TONS of info on the net on using guide numbers, but here's a quick down and dirty:

Note: This is not as difficult as it seems here. It's just complicated to explain.

1. 
   

   Figure your exposure settings you want for the key.

   
   


2. 
   

   Think about how much fill you want. Fill is generally either full exposure or up to one stop under the key.

   
   


3. 
   

   Calculate flash setting using the settings you've chosen for your exposure and the guide number. Distance = GN/f-stop

   
   
   


4. 
   

   Adjust to get the effect you want.

   
   

Say you're shooting at f/8 @ISO 200 (remember, shutter speed isn't as factor except that you need to stay below your flash sync speed) and you want your fill a bit lower than the key.

The calculation tells us to place the flash 14.75 feet away if we were at ISO 100. We're at ISO 200 so we can drop the flash down to half power or move it further away. This is for proper exposure. If we want our fill lower than the key, we can drop the power down a touch, or move it back a bit more. If you want the flash closer, drop the power even more.

When moving the flash around you need to consider the Inverse Square Law concerning light falloff.