D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
OP Marianne Oelund Veteran Member • Posts: 7,030
Flash, PC sync and shutter timing

David314 wrote:

perhaps i missed it but did you ever figure out where the shutter is
in relation to the pc sync signal start and stop?

given that, i could relate the flash back to the shutter position

that's always been a question on my mind

The PC sync signal start edge is located at exactly the same point in the shutter cycle as the trigger pulse for the built-in flash, within about 1us. The PC sync ends 7.1ms later. However, the PC sync pulse is only issued when the built-in flash is off; you can only have one or the other.

I put together a setup this weekend, which provides inputs for the scope, corresponding to light shining on the sensor's bottom and top edges. This is an alternative way of measuring the shutter/flash timing, and is somewhat more direct. (The only difficulty is aligning the optical sensors to accurate edge positions, which took me a couple of hours of tweaking!)

This setup uses a halogen lamp shining into the D300's mirror box. The light reflected off the image sensor when the shutter is open is projected through a 200mm lens onto a pair of optotransistors. The 200mm lens is in symmetrical configuration (400mm from the D300's sensor, and 400mm from the optotransistors) so it projects an actual-size image of the D300's sensor. The optotransistors were shielded in foil, with a very small opening at the top (less than 1mm wide) for light, in order to restrict their sensitivity to a small range of shutter travel, right at the image sensor edges.
Here is the layout:

On the following scope traces, Chan. 1 is the lower-edge light sensor and Chan. 2 is the upper-edge sensor. Chan. 3 is the flash pulse, but I have added a second chan. 3 trace from a separate sweep to show the full-power pulse (at the bottom) in addition to the 1/128 power pulse. Time base is 1ms per division (10ms across the full screen width). You can make precise time measurements from these images, since the scale is exactly 100 pixels horizontal to 1ms.
The sequence is read as follows:

The rising edge of Chan. 1 is the front curtain uncovering the bottom edge of the image sensor. This edge starts off a little slow because the curtain has just started moving and is accelerating.

The rising edge of Chan. 2 is where the front curtain has fully opened (i.e. it's reached the top of the image sensor). This is a more abrupt edge since the curtain is moving quite fast at this point. You will note that the flash triggers very quickly after the top of the image sensor is fully exposed.

The falling edge of Chan. 1 is the rear curtain starting to cover the lower edge of the sensor, and the falling edge of Chan. 2 is the rear curtain fully closing. These falling edges have a somewhat slow decay since I used a rather large load resistor (22Kohms) for the optotransistors.

The a and b cursors mark the time period where the shutter is fully open. Flash pulses constrained to this range will illuminate the sensor evenly, but pulses which extend outside of this range will cause uneven illumination due to curtain shadowing.

This first trace shows the sequence at 1/250sec shutter speed. Cursor b shows where the rear curtain is starting to close; note that the full-power flash pulse extends beyond this point about 400us:

Here is the sequence for 1/320sec shutter speed. In this case, the camera has restricted the full-power flash pulse even more than at 1/250 shutter, but it still exceeds the rear curtain starting to close by about 400us:

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MOD TOF guy
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