D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
OP Marianne Oelund Veteran Member • Posts: 7,753
Closest Possible Look

David314 wrote:

your cursor A position is showing the top of the sensor uncovering
200usec before the flash is firing - that seems like a lot of time

so again to me - as practiced skeptic I would look for confirmation
of this

and that confirmation would be seeing the shutter open and the flash
not going off - and on the other end, the flash going off and the
shutter partially closed (although as you previously pointed out
there is other evidence of this)

I've put together a 5-frame composite of new shutter photos from this evening. These are like those shown in Part 2, but with more detail, and each one has its accompanying oscillograph so you can see the timing measurement yourself. The D300 shutter speed for these was 1/250sec.

The scope traces show the PC sync output (which as determined before is simultaneous with the built-in flash trigger, or 10us before the SB800 trigger) on chan. 4 (green) and the flash pulse which illuminated the photo on chan. 3 (magenta). Time base for the main scope window is 400us/div, except 100us/div on the third frame where the timing is extremely close.

The first two frames show the front curtain approaching its fully-open position, at 438us and 242us before the PC sync pulse. The third frame is at 9.7us before the PC sync pulse (how's that for manual coordination?). By interpolating with the 2nd and 3rd frames, we can calculate that the front curtain is at the top row of sensor pixels 195us before the PC sync pulse.

Frame 4 shows the rear curtain starting to move, and frame 5 shows the rear curtain just at the lowest row of sensor pixels, 1250us after the PC sync signal's starting edge. Again, this is at 1/250 shutter; for the 1/320 shutter value we subtract 875us from that time, so the rear curtain will start to cover the image sensor at only 375us after the PC sync start. That's only enough time for a 1/2.5 power pulse from the built-in flash, or 1/4 power -2/3ev from the SB800.

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