D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
Part 2: Flash with High Shutter Speeds
1

D300 Shutter Timing

First, let's have a close look at the D300 shutter dynamics. The timing data provided by this sequence is important to understanding the time allowance for flash pulses. These photos show the D300 shutter curtains at various time points, during a 1/250 sec exposure (the exposure time is thus 4.0msec). The reference time t=0.00ms is when the D300 triggers its flash for the exposure (just after the front curtain is fully open).

Frame descriptions:
1. Front curtain at rest just before starting to move.
2-4. Front curtain starting to accelerate upwards.
5. Front curtain at lower edge of image sensor.
6-7. Front curtain traveling across sensor.
8. Front curtain at upper edge of image sensor.
9-10. Front curtain fully open and rear curtain starting to move.
11. Rear curtain almost at lower edge of image sensor.
12. Rear curtain midway across sensor.
13. Rear curtain at upper edge of sensor.
14. Rear curtain fully closed.
15. Rear curtain at peak travel (motion arrested).
16. Rear curtain after bouncing back to rest position.

For our purposes here, we want to know the exact transit time it takes for each curtain to travel just the full height of the image sensor. The amount of time available for a flash pulse is the exposure time (4.0ms here), minus the curtain transit time. Doing a little interpolation with frames 5, 8, 11 and 13 gives us a figure of 2.4ms for the transit time, so there is 4.0ms - 2.4ms = 1.6ms of time when the image sensor is fully exposed. The longest flash pulse which can be accommodated at 1/250 sec shutter speed, without causing any shutter curtain shadow, is thus 1.6ms.

When fast shutter speeds are selected, the D300 will not allow full-power flash pulses. Instead, the maximum power available is 1/1.3. As shown in Part 1, the pulse duration at this power level is about 920us, so at 1/250 sec shutter speed, there is plenty of margin.

The D300 also allows 1/320 sec shutter speeds by selecting 1/320 (Auto FP) from the menu. Note that the built-in flash is not FP mode capable (an external flash must be used for FP mode), so when this option is selected, the flash will still emit a single pulse. At this shutter speed, the exposure time is only 3.1ms, so the maximum pulse length which can be accommodated is 3.1ms - 2.4ms = 0.7ms, which is a little short.

In fact, it's too short to allow the use of 1/1.3 power without causing some shutter curtain shadowing on the image. This will show up on images as a narrow, slightly dark band across the very top edge. SteveL54 posted some examples on another thread. Although they pertain to use of the SB-800 flash, the effect on D300 built-in flash use is very similar. See http://forums.dpreview.com/forums/read.asp?forum=1021&message=30453868

If you will be using flash with your D300 at the highest 1/320 sec sync speed and high flash power settings, I recommend that you perform some experiments to determine if the curtain shadow problem will be significant to you.

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