Basic rules about flash, the output is measured in f stops and for portable flash the time is far shorter than your shutter speed. So for example, if the sun is bright and you want to make the overall ambient light area dark, using the “sunny 16 rule” 1 over ISO @ f 16 would be 1/160th at f16 with ISO 160 which would mean that to make the ambient light area 1 stop darker than the flash lit scene, you would need the flash exposure to be at f 22 which would underexpose the ambient light area by 1 stop. Using the SF 58 flash, you can use HSS to sync the S2 up to 1/4000th of a second but with reduced flash output. So applying the above sunny day scene and the SF58, the exposure of 1/160 @ f16 would equal 1/2500 @ f4 so if you wanted a 2 stop darker ambient light then set the speed at 1/2500 and aperture @ f8. The flash will output to f8 all of the area that it is pointed to and the ambient light will be 2 stops darker. Or in some cases used to balance out the indoor scene with the outdoor scene.
Using flash as a fill flash and look unobtrusive will require you to dial down the flash +/- to -2/3 or more, I prefer to dial it in at -1 to -2 stops. This will fill in the eye sockets and add a little catch light to the eyes. At the same time I also add a little warming filter over the flash with a colored theatrical gel from Lee filters a 1/4 CTO (Color Trans Orange) is the equivalent of 1/4 the strength of a daylight to tungsten conversion filter. This gives the illusion that the light is coming from a natural source such as a lamp instead of the harsh blue from the strobe. The amount of fill depends on the effect that you are going after, the term “fill” should be just that, to fill in the shadows and even out the harsh lighting and not overpower the ambient light. So just the opposite of the fast shutter, small f stop of the first example, now you use the long shutter speed with large aperture to balance out the flash to ambient light. Thus the – compensation for flash. For example, an interior scene lit by a mix of daylight and incandescent lights with the subject next to a large window. The shadow cast is blocked up but you do not want to overpower the scene destroying the light from the window. The meter reading says 1/125 @ f 5.6 ISO 160, you put on the flash and set it for – 1 which in effect will put the flash exposure @f4, this will lighten up the shadows with out blasting out the ambient light. Again, remember to use a little warming filter over the flash as this will give the illusion that the light is coming from a lamp in the room.
The shutter speed is always for the ambient light while the f stop is for flash. In the photo of my son and daughter in law, I used a Hasselblad 501CM and CFV back @ ISO 100. Nikon SB800 flash in a softbox and gold reflector. I set the exposure for 1/500 @ f8 and the flash output at 1/2 power (No TTL connection). The ambient light exposure would be 1/500th @ f6.3 so about 1/2 stop darker background to balance out the light and bring focus to the couple.
In the shot at the beach, I used the flash to light the entire shot and brought the background about 1 stop darker by using direct flash from the Nikon SB800 at full power and exposing the shot at 1/500 @ f11. The setting sun which will normally get the sky white is now with texture and under control.