While shooting my toy car with flash on, using Macro mode with 350D, the shutter speed
was F/4, ISO 400 and 1/60s. When I manually set F/4, ISO 400 in AV, the shutter speed
was 1/15s. Does my 350D adjust the power of built in flash?
Please read you camera manual for flash duration, if you set speed at 1s, or 1/60s (camera lens at same f-stop), the flash output will still be the same, what other factor the exposure when you shoot with flash on different shutter speed is the ambient light. if you need to adjust the flash output, you can use the flash compensation setting.