Gee... Flash Exposure is complicated

[Samerr9]

I've read books, they all tell me to use trial and error for manual. There must be some way to use an educated guess...

Use a lightmeter!

 

[Tiberius47]

As an example, if my ambient exposure is already what I want, but my subject is 3 stop underexposed, what flash setting should I use?

You can open the lens aperture 3 stops...

Not without increasing the ambient by three stops as well!

 

[fjrabon]

As an example, if my ambient exposure is already what I want, but my subject is 3 stop underexposed, what flash setting should I use?

You can open the lens aperture 3 stops...

Not without increasing the ambient by three stops as well!

Yeah, that post puzzled me. I know KmH knew that, but what he said was worded in such a way that you could easily take it to mean that aperture would only change the flashed part of the exposure. Which is oddly a certain misconception that a lot of newbies also have. I've seen people on here say many times that Shutter speed only controls ambient, aperture controls flashed part.

Just as a general guide for the OP:

Shutter speed has no effect on flashed parts of the exposure, only affects ambient. At very low values it can affect motion and camera shake blur. While this normally works against you, you can use it to your creative advantage as well.
Aperture affects both ambient and flashed parts. It makes your whole exposure brighter or darker. It also affects depth of field
ISO affects both ambient and flashed as well. At big values it affects noise, but this usually shouldn't be an issue if you're using flash.
Flash power affects only flashed parts. Power only affects levels (well besides recycle times)
Flash distance affects only flashed parts. Flash distance also affects the softness of the light. Closer = softer.

Getting ambient and flash to work together is both one of the hardest, and most rewarding things you can do in photography. When you've got it going right, you feel like a conductor of a symphony. When it's going wrong you feel like you're trying to herd cats.

 

[Village Idiot]

As an example, if my ambient exposure is already what I want, but my subject is 3 stop underexposed, what flash setting should I use?

You can open the lens aperture 3 stops...

Not without increasing the ambient by three stops as well!

Turn the flash up 3 stops if it's not already having an effect on your ambient or open the aperture 3 stops and up your shutter speed up three stops since apeture will make both subject and ambient 3 stops higher and shutter speed will make only ambient 3 stops darker. If you hit your x-sync you either have to use an ND filter to darken the entire exposure and adjust aperture and shutter speed accordingly, use the high speed sync feature if you have a flash capable of doing so, "trick" the exposure by going over your x-sync and either cropping the black band or framing it so the band won't show because of what is being exposed by the flash, or use a devices like a Pocket Wizard that uses it's proprietary hyper sync tech to adjust timing so you can use faster shutter speeds than the camera's x-sync.

Basically. Start reading places like the Strobist 101, Light Science and Magic by Phil Hunter, and any other material you can get your hands on about flash. Coming onto TPF and asking "How does flash work" will probably do more harm than good as you're getting basic concepts and advanced concepts both thrown at you right now, which may make it hard to understand and you're getting the same information delivered differently by different people.