SF 24D flash question

[agreenspan]

Hi everyone. I've got an important shoot tomorrow, and a(you won't believe it) stupid question about using an SF 24D and an M8.
I like the TTL function - just stick it on and go, but for most of the pics, I just want a little hint of fill flash - nothing more.
I tried lowering the EV to -3, and see no difference. Then, just for fun, while shooting wide open at f2, I told the flash that I was using f16, and viola! - less light output. Even the instruction book says that the effective flash distance will go down with smaller apertures.
I guess I'm stupid, but I assumed that the flash would fire more light with a smaller aperture than a large one?
Can someone smarter than me please help me understand this......
Should I use TTL, auto, or manual?
Thanks,
Stephen.

 

[RichardB]

I don't have an M8 but do have M7's and SF24D. What you are trying to do is use a camera system not designed for fill flash as a fill. The TTL Mode is out as it doesn't matter what you really do as the exposure is determined by the TTL sensor with whatever compensation you put in. In the AUTO Mode the correct flash is determined by a sensor on the SF24D which again determines the correct flash(really duration). So you are left with Manual which is a comparison of your correct ambient light exposure to one you calculate using basic flash principles.
Doing the above, you will quickly learn why 'fill flash' with an M is not fast and easy to do.
Rule Number 1 is, never do somehting untried on an "important shoot".-Dick