# 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?

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.

The relevant parts for an SB-24 on automatic are:

An automatic flash is a great aid in doing fill flash, and it greatly simplifies things. First, set exposure for the background scene, using any shutter speed at or slower than the maximum synch speed. Next, choose an automatic mode on your flash that (a) matches your distance and (b) is one of the combinations that matches the background scene.

Example:

1. Flash has automatic modes of f/2-f/16
2. Background exposure is 1/125 and f/16 (other choices were 1/60 and f/22; 1/250 and f/11; 1/500 and f/8; 1/1000 and f/5.6)
3. Set flash to f/16 mode (provided your subject is close enough)

[...]

Half Power Fill: pick an aperture/shutter speed that is one f-stop smaller than the flash mode you are using. So if the background exposure is 1/125 and f/16, pick the flash auto mode for f/11.

There are sections for doing it all-manual (you'll need to know the guide number) and other even more esoteric variations.

My simple rule for flash use is the combination of f8,GN,ISO at 10ft(3m) and I'll work any power or distance adjustment based on that initial setting.

Having said that, for fill use, obviously I have to find a good combination of shutter speed+f8+ISO for ambient without my shutter going above the flash sync speed. Normally, half the maximum sync speed.

If my flash syncs at 1/500 then my ideal combination would be 1/250+f8+ISO at 10ft. Based on this, especially with that combination of 1/250 and f8, then my GN and its power output is dependent on the ISO.

For example, if the ISO is 400 and assuming the ambient light would give me a shutter speed of 1/250 or near it, then the GN would be GN13 @ 1/1

or GN36 @ 1/2 ISO 100 or GN13 @ 1/4 ISO 1600 or GN18 @ 1/4 ISO 800

and so on

How much light the fill needs is of course dependent on how deep the shadow is. Once, I get my GN+f8+10ft+ISO+shutter combination, then it's just a matter of fine tuning by moving closer or farther by a bit.

Hope this helps.

Manual flash dates from the B&W film days. (Actually from the tintype days.) And B&W film is pretty forgiving of exposure errors. If there were a simple rule of thumb, it would already be known. Basically, there are two methods. You set the camera for the ambient exposure and then adjust it to accommodate the flash power, or you set the camera to the aperture you need for the flash and adjust the exposure to accommodate the ambient light intensity. And sometimes you might start out with one method and find that it doesn't work with the situation in front of you so you switch to the other method.

(Many manual flashes come with full, 1/2, 1/4, etc. power settings which makes the first method much easier to work with.)

You are typically going to want the fill light to be 1/2 to 1 stop below the ambient exposure. But there are so many variables that you are going to have to develop a photographer's eye before it ever becomes easy. That was the beauty of all manual film photography. It was a lifelong pursuit of excellence. Automation takes the fun out of it. Make the journey the objective and you will love it all the more when you achieve success.