D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
OP Marianne Oelund Veteran Member • Posts: 7,394
Obtaining photo evidence

David314 wrote:


the reason i ask is you can see the start and stop of the trigger
pulse using the sync signal very clearly

I'll have a look with the SB800 this evening, after dark (the setup requires a dark room, of course).

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

It's 160us. But if you look at the point where the phototransistor is fully "on" then the delay is virtually zero. I can't see how Nikon could mass-produce a camera like this and tighten the timing tolerances any further. There must be some allowance for shutter transit time variations, and the camera needs to work over a fairly wide temperature range. We are talking about a tiny fraction of the transit time, and shutter movement of less than 1mm when the shutter speed is many meters per second.

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)

so - if i could see a picture of the shutter, the flash, and maybe an
LED being driven by the sync signal all in the same frame - that
would show quite graphically what is going on and would remove any
issue with sensing the edge and tweaking the photo transistor window

Precise confirmation would be nice, but there are some difficulties. Remember that the photographing camera is an SLR also, and has moving shutter curtains which take 2.5-3ms to traverse its sensor. One couldn't achieve the photo you want with shutter speeds of 1/250 or 1/320 since the sensor is exposed for a relatively long time, so max shutter speed would be needed. In that case, only a narrow slot of the sensor is uncovered at any single instant; in order to pick up the flash light, one would need to construct a light bar which runs down the full height of the image. Then there is a small problem of the shutter "race" since both camera's shutters are moving at the same time, and at slightly different rates (the D3 is a bit slower). It would be difficult, if even possible, to reconstruct the timing from the result.

Probably the best photo evidence would simply be to have the photographing camera on Bulb in a dark room, and use the D300's own flash light, via a mirror, to take the photo. This gives only a single time point (showing the shutter curtain position when the flash fires) and wouldn't provide an actual measurement of the delay, but would just be confirmation that the sensor isn't shaded at that moment. Repeating this at flash full power might be able to show how far the rear curtain moves over the sensor by the end of the flash pulse, but it would be rather faint in the photo and probably no more valuable as data, than the image recorded by the D300 (which we already have). Personally, I'd rather spend my time going on to look at other camera models.

All things considered, I think the measurements already done are more trustworthy and meaningful, especially the shutter photo sequence with timings. I suspect you haven't yet fully utilized the information available from those, as they really do contain the answer you're seeking (Hint: Do a little interpolation).

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