point of clarification...
if doing this in aperture priority, wouldn't you have to make sure to stop the lens down to get a longer shutter speed,, if you weere using the nifty fifty and set aperture at 1.7, your shutter speed might be too high...???
I don't think brand of camera matters, nor whether you shoot aperture priority ,shutter priority, or manual
with a compatible flash unit, the camera will meter for ambient light and the flash is adjusted accordingly
the key is making sure the shutter speed is slow, generally longer than 1/60th
Let me explain using an example. I have my camera set up in my office right now. It is at ISO 200.
In program mode with no flash, it meters for 1/5s and f/4. When I turn on the flash, it meters for 1/60 and f/4. The camera anticipates using the light of the flash to light the scene, so it increase 30x.
Now I switch to AV mode. With no flash and the aperture set to f/4, it still picks a shutter speed of 1/5s. When I turn on the flash and keep the shutter speed at f/4, it picks a shutter speed of 1/10s. It anticipates getting some light from the flash, but it only doubles the shutter speed instead of increasing it thirtyfold. That means that anything not lit by the flash will still be visible in the picture.
In manual mode, I can pick any shutter speed and aperture and the camera will try to make up the difference using the flash.
Now, as to your first question, I can set whatever aperture I want. The camera will set the shutter speed to be a little faster than normal to leave some headroom for the flash. It is possible to get a shutter speed higher than the max sync speed on the flash. In that case, you'll want to either stop down or switch the flash into high speed sync mode.
I think that the brand does matter, but I'm not sure. I only know how Canon handles it. In Program mode, it meters under the assumption that the flash will be the primary source of light. In AV mode, it meters under the assumption that the flash will only be supplemental and that the ambient light will be primary. In Manual mode, it adjusts the flash to bring the subject up to "proper" exposure but otherwise does what you told it to do.
If you are indoors in a lowlight setting, try manual mode. Set the ISO at the highest tolerable level for your camera. Set the shutter speed at the lowest you are comfortable hand holding. Open the aperture as wide as possible. If this is overexposing, then you don't need the flash and you can switch to your favorite auto-exposure mode. If it is under exposing, turn on the flash. The flash will compensate for the underexposure. The manual work assured that you are getting the most ambient light possible in the situation and only using the flash as little as necessary.
Just for you, MICKEY88, here is a shot I took with a Minolta film SLR (I think it was a 7000i) camera dragging the shutter. It was taken about 20 years ago on New Years. I can't remember with certainty how I got the shot, but I'm guessing it was something like this. I switched to bulb mode (like manual, except that the shutter stays open as long as you are holding the release button) and my widest aperture. Then I turned on the flash. I counted on the E-TTL flash metering to correctly adjust the flash to illuminate my friend. I pressed the remote release trigger as my friend was starting to light the bottle rockets. The shutter opened, the flash fired, the bottle rockets lit, my friend ran, the bottle rockets launched, I released the remote trigger, and the flash closed. The original looked better. This isn't a great scan.
