How to: Track Ambient Automatically While Using Manual Flash
It works great for using manual flash against fast-dropping light after sunset, for instance. And you can control the flash and ambient very easily from your camera.
First off: As always with manual flash, you'll need to keep the flash-to-subject distance consistent. This either means shooting a relatively static set, or having an assistant track the subject with your flash.
Use Aperture Priority
In this mode, your aperture stays wherever you set it, and that means that your manual flash exposure will also remain constant. The shutter speed varies according to the ambient light. So it will track automatically.
This also gives you a neat control function over both flash and ambient exposures.
Let's say that your flash is putting an exposure value of f/5.6 on your subject, and the ambient (say, for the sky in the background) is f/5.6 at 1/60th of a second.
By setting the camera on AV f/5.6, it is going to choose a shutter speed at or very near 1/60th, which is just want you want. If you want to vary the sky value, simply do this with the exposure compensation dial. Your sky will track at your preferred lighting ratio -- even as the ambient light drops down after the sunset.
If you need to alter the exposure on your subject, simply adjust your aperture. Going to f/5.6 1/2 would drop the exposure on your (manually flashed) subject by half a stop. Going to f/4 1/2 would brighten them up. You get the idea.
And all the while, the relative brightness of the ambient background would track at whatever ratio you previously chose. Which is pretty sweet when your ambient is dropping by the second as dusk turns to dark. Because that is when the color of the light can get really interesting (and fast moving) and you do not want to be wasting time getting exposure readings off the sky.
And all the while you are keeping full control of your flash exposure, which is absolutely consistent.
UPDATE: I would think this goes without saying, but several commenters have correctly noted that you need to make sure your shutter speed does not creep up above your sync speed.