D300 built-in flash, Part 1: Blinding speed

Started Dec 25, 2008 | Discussions thread
Marianne Oelund OP Veteran Member • Posts: 6,632
Part 4: Commander Mode Timings

Basic Pulse and Data Encoding

Commands and data sent by the D300's built-in flash in Commander Mode use a low-power pulse of duration 12-15us, which is 1/320 of full power, or 1.3 stops lower in energy than a Manual mode 1/128 power pulse. The only exceptions to this are the camera's own monitor flashes if TTL mode is used (which are 1/16 and 1/64 power), and the flash used during the actual exposure. In the latter case, the camera will revert to the 1/320 power flash pulse if the built-in flash is set to Off, in order to synchronize the remote flash units.

This brings up a question of CLS reliability if the built-in flash is set to TTL mode and fires at very low power during the exposure. As shown in earlier postings, this pulse can be as short as 4us and have a power of only 1/3300 of full power. In my own experiments, CLS operation is reliable at distances up to at least 20 feet, even when the remote flash sensors are not facing the D300, but I would still recommend setting the built-in flash to Off instead of TTL in cases where flash power will be very low.

Since the data sent by the D300 flash for Commander Mode uses pulses of uniform amplitude, the information is encoded by pulse interval alone. Intervals range from 150us to 600us, in steps of 75us.

Maximum-Length Command Sequence

Let's jump in with both feet, and start with a look at the most complex command sequence, which corresponds to all groups (including the D300 itself) set to TTL mode. Note that EV and power settings do not affect the length of the command sequence.

Here is a trace of the sequence, with marks and labels denoting the segments as discussed below. As before, you can make timing measurements from these traces, since 100 pixels horizontally corresponds to 20ms:

Here is the content and range of timings for each of the command segments:
1. Shutter Release Delay

This is quite variable, and can be anywhere from 2ms to 17ms. There is no correlation between the length of this delay, and the details of the command sequence itself. The variation is random, and you will see a number of different values in the scope traces here.
2. Channel Number and Flash Mode Data

This cluster of pulses contains the channel number, and the user's selected modes for the external flash groups. It consists of about 7 to 12 individual pulses, and takes 2.2ms to send. Following the cluster, there is a processing delay of 5-6ms.
3. D300 Flash Monitor Pulses

If TTL mode is selected for the built-in flash, the monitor pulses will appear here. The first is at 1/16 power, and a second pulse at 1/64 power may also be used if the subject returns a high level of light from the first pulse. The total time for this segment is 18-19ms if two pulses are used, or 8-9ms for a single pulse. This segment will be skipped if the built-in flash is set to Manual, or Off.
4. Group A Monitor Pulses

This segment begins with a short (5-6 pulse) cluster which identifies the group being addressed (group A). This is followed by a pair of pulses which trigger the remote flash(es) to produce a 20us monitor pulse. If in TTL mode and this monitor pulse is insufficient, a second one (which will be 120us) is requested 10ms later. Total time for this segment is
25-26ms for TTL mode if two monitor pulses are used,
15-16ms for TTL mode if one monitor pulse is used, or
21-22ms for AA mode (always single monitor pulse).
This segment is skipped if Group A mode is Manual or Off.

5. Group B Monitor Pulses
This segment is as described for Group A above.

6. Power Level Data

If a TTL exposure mode is used for Group A or Group B, this segment begins with a 6ms processing delay. There is no processing delay if Manual mode only is used for external groups. Following this is a cluster of pulses which communicate the calculated power levels to the remote flashes. The cluster duration is 2.2ms for a single external flash group, or 3.3ms for two groups, thus the total time for this segment is 2.2ms, 3.3ms, 8.2ms or 9.3ms.
7. Shutter Cycle

This is the mechanical shutter cycle. For 14-bit recording, time from the start of this segment to exposure is 90ms; for 12-bit RAW or JPEG, it is 47ms.

Summary

Using the above data, you can now calculate the maximum and minimum shutter delays for any combination of Commander Mode settings - just add together the max or min timings for each of the 7 segments. For example, maximum shutter delay for the all-TTL Commander Mode sequence shown above is (17 + 8 + 19 + 26 + 26 + 9 + 90)ms = 195ms for 14-bit RAW, or 152ms for 12-bit RAW or JPEG.

Example Variations

Following are four examples of shorter Commander sequences. Each of these are the same as the maximum-time all-TTL sequence described above, except for one setting parameter, which is listed on the tracing. The segment which is affected by the changed setting is between the a and b cursors. All of these traces were taken with 14-bit recording.

Speeding Things Up

With some subjects, the long Commander Mode delays and pre-flash sequences will be problematic. One solution is to use Manual mode only for all groups, which eliminates most of the communication delay, as shown here:

FV Lock with Commander Mode

Use of this feature provides best speed, and retains the mode flexibility of CLS. When FV lock is entered, the D300 executes the full Commander sequence, but ends with a special power-level data cluster (there is no 6ms processing delay associated with this). When the shutter release is subsequently pressed, the D300 only needs to send the actual power-level data cluster (2.2ms or 3.3ms), then immediately starts the mechanical shutter cycle.

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