Flash photography is interesting. In particular, flash sync speed. (In some cameras, there is a "flash sync speed" for S-mode, and "flash shutter speed" for A and P modes. For this discussion, I will just use the general term of flash sync speed to mean both)
The theory behind it all is that the flash goes off for a very short time relative to the shutter speed. See picture above. The exposure from flash is the same regardless of shutter speed.
Night time indoor flash photography:
The image is frozen by the flash regardless of shutter setting (which is typically much longer than flash exposure time). Sync speed is not an issue then. To get maximum range, use A mode and set aperture to maximum. P mode can sometimes set itself to less than maximum aperture. Here is where a bright f2.8 lens starts to pay off, as every f-stop counts in night time indoor flash photography.
Daylight fill-in flash photography:
The key point to remember is that with flash, sync speed is the fastest shutter speed you can use, regardless of what your camera is capable of without flash on. Setting the sync speed to the highest possible gives you two advantages that cheaper SLR's (including my Nikon D90, having maximum sync speed of only 1/60s) don't have: a) stop motion, b) better range; hence, smaller flash still produces good result.
When sync speed is set high, the camera can shoot at larger aperture, which means the flash does not work as hard, which means it will recycle faster. Faster sync speed helps you to get enough flash power to balance with direct sunlight.
1. Contribution from flash is the same regardless of shutter speed. Only distance, aperture, and ISO affects it.
2. Different ISO and apertures don't change the ratio between flash and ambient light.
3. Ratio between flash and ambient light is affected by distance, shutter speed, and flash power.
4. A faster sync speed gives more range. Set at larger aperture, less power from flash is required.
The above information is gleaned from this website:
All said and done, flash photography requires one to really know the limitations of his own camera and his flash. What works on one camera may not work as well on another. Try changing sync speed, shutter speed, aperture and ISO under different lighting condition to see what happens! Finally, dont forget to try out TTL vs TTL-BL where ambient light is weaker than subject, and vice-versa.