"SB unit into softbox, camera left @ f/4" I read. The first time I saw something like that I went and looked at the back of my SB units to see how I could magically punch in f/4. And alas, I was mistaken, and misunderstood as are many newcomers to strobes.
Lets back up and think about this carefully...
We know that aperture is the only thing (ignore ISO) that realistically affects the flash lit portion of the exposure, given that flash durations are normally faster than 1/1000th sec.
We also know about the inverse square law L = 1/(r^2), where L = luminoscity and r = distance, so the amount of light on a subject is controlled by distance.
Given both of these statements, even if a strobe allowed you to punch in f/4 into the strobe, it would mean nothing.
What the original statement is really saying is:
the light reaching the subject will require an aperture of f/4 on the camera for a correct exposure, given the strobe's distance, power and light modifiers used.
And to get to this destination, theres the 2 choices:
- setup, shoot and chimp the histogram until it looks fine OR
- setup, take a incident flash reading and decide whether the flash output is strong/weak enough and punch in the aperture to the camera
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