Light question

[Mikey10159]

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The flash on the iPhone 4 is a two stage, the second stage it briefly flashes considerably brighter. If I'm not mistaken flashlight apps seem to use the less bright portion of the two stage flash. Why? And even if it is the brighter of the two why don't apps offer the option to switch between bright and dull if the phone has the ability?

Thanks

 

[testcard]

I believe the first stage provides light to allow the camera to focus in very dark environments. Once the focus is set, the main flash fires to expose the subject.

 

[tygrubb]

I think this may be standard TTL flash behavior. TTL stands for 'through-the-lens', and refers to a method that the camera uses to calculate exposure: the camera sends out a pre-flash of a known amount and registers the amount of light reflected back from the scene, then uses this info to calculate and fire the main flash pulse at the proper amount to obtain (what it thinks is) the correct exposure.

As the poster above me also correctly pointed out, it could also be using the pre-flash to light the scene enough to obtain a focus lock.

Both of these methods (in proprietary variants) are standard on all modern cameras (compact/point-and-shoot and DSLRs) that have the ability to control an internal or external flash unit.

I don't think there are discreet brightness settings for the flash (at least on typical flash units, don't know about the iPhone 4 flash specifically), it's just either on of off; the amount of light emitted is controlled by the length of the light pulse. Flashlight apps just turn the bulb on continuously. The 'dull' flashes you see are in actuality the same luminosity (same amount of photos emitted at any given time), but just have a lower apparent brightness (less total photons are emitted).