when to use flash?

[Overread]

hmm I will play the card and say that onboard flash can be used!
Its just that it needs the light from it sofening a little - a cheap trick is to use a bit of toilet roll (the soft smooth kind, not the sandpaper sort!). Take a short length of it and roll it up a few times and then place it infront of your flash (elastic bands can work - for me I poked the top and bottom round my canon 400Ds popup flash which held it there well enough most of the time).
That will diffuse the light from the flash and reduce its harshness - its certainly a very useful tool if you cannot afford a proper flash unit. Also there are some companies that do make diffusers for popup flashes - Lumiquest make one for several different camera models.

I have used this method to good effect when I did flower and some insect work - though I was also shooting in the bright parts of the day so natural light also helped ot break up harsh flashlight