here's an article from pro-photographer..maybe it will help a bit..good luck
Another oft-requested feature addition that iPhone 4 brings is the inclusion of an LED-based "flash." This isn't a flash in the traditional sense of the termthere's no xenon flash tube and no waiting for its capacitors to charge. Instead, there is a relatively bright white LED that can light up in low-light situations. For the most dimly-lit interiors, this LED should be sufficient to light up subjects that are close-bya typical head-and-shoulders portrait, for instance.
There are some downsides to the LED flashes that I've experienced on other mobile phones, however. Typically, the LED is generally of the "cool white" variety, meaning the light output is slightly bluish. In most low-light situations, this contrasts sharply with the warm color temperature of indoor lighting, and can result in bluish faces on your subjects. The quality of the lighting leaves a lot to be desired, too. Typically, subjects are either still too dim, or are washed out by the LED. We'll have to wait until we get our hands on a shipping unit to really evaluate it, but our hope it that it can be turned off by the userif the quality of light sucks, we'd rather do without it.