D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
OP Marianne Oelund Veteran Member • Posts: 7,109
Close, but not quite there

David314 wrote:

as far as i can tell with the speed lights turn the flash off and on
quick enough to avoid the dark band

Not quite. At 1/250 shutter, since the curtain transit time is 2.4ms (and because the camera does not initiate the flash pulse before the shutter is fully open), there is at the very most 4.0ms - 2.4ms = 1.6ms for a flash pulse without shadowing. As shown by both your trace and my measurements, the flash continues producing light until 1.8ms after it's triggered. Thus the rear curtain has started closing over the image sensor for at least 0.2ms by the time the flash turns off, and this results in a narrow band at the top edge of the image that is slightly darker.

You can demonstrate this for yourself. Take two photos of a uniform surface, one with the SB800 at full power, and another at half power (both at 1/250 shutter). Then display the images side-by-side on your computer, and you will see the darkening at the top in the full-power image.

for instance SB800 at full power but 1/250 shutter

Note that although the flash has been set for full power, it is not actually producing full power, as it has been limited in duration by the camera. A full power discharge continues to decay exponentially for many milliseconds and exhibits no cutoff edge.

If you want to see the true peak characteristic of the flash output, you need to place your detector a bit farther away from the flash, as it's clipped the signal.

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