6. I used bounce flash and my image is underexposed.
The camera calculates exposure based on what it sees through the lens. The camera assumes the flash light would fall directly on the subject. But the actual light that falls is lower in intensity because of scattering and absorption by the bouncing surface. So just increase flash power by making flash EV +1 or +2/3, whatever gets you a proper exposure.
That calculation works when you tilt the flash speed light head direct at the subject or scene. When you tilt the head at an angle e.g 45 degrees or 90 degrees the flash will automatically increase the flash intensity internally. Still at times the scene will not be properly exposed - not because of insufficient flash compensation but rather the nature of the location - whether it has low or high ceilings, color of the walls, or a dimly lit open space with the skies and heaven to bounce your flash back to earth.
Unteroffizer, That would still have the effects of fill-in flash plus the additional flash because the external flash operates off the pre-flash.
So you could just enable pre-flash and disable the rest to use it as remote control. That would make sure there is very very little effect of on-camera flash on your photo. :)
Using a diffuser on pop-up flash is already troublesome because you're losing intensity. Wouldn't work in daytime. And if you increase flash power, on-camera flash would make images flat. Double trouble. So just keep flash power highest and enable only the pre-flash. Solves the double trouble without a single trouble.
55-250mm standard lenses. Dual tube macro flash and external speedlit...