I started the series of posts about Nikon Advanced Wireless Lighting (see Nikon CLS Advanced Wireless Lighting, Part I) but quickly realized that there are some interesting aspects of the non-wireless intelligent through-the-lens (i-TTL) system as well.
The i-TTL system typically works like this: immediately before firing the main flash, the flash fires a series of pre-flashes. The reflected light from the scene is detected by the camera. Combined with other information from the camera’s 3D Color Matrix metering system, the camera determines a proper flash output and fires the flash.
Nikon’s i-TTL has two modes: Balanced Fill-Flash mode and Standard mode. The Balanced Fill-Flash mode automatically balances the output of the Nikon Speedlight and the scene’s ambient light while the standard mode ignores the background and tries to bring the scene to a standard level. The standard mode is automatically activated when spot metering is selected.
How many pre-flashes?
The answer is one or two. As shown in the following graph, the last pulse is the final flash output. The first one or two is (are) the pre-flash.
The tests were conducted with a Nikon D200 with its built-in flash unit and a SB-800 Speedlight mounted on hotshoe. With the built-in flash I normally see two pre-flashes. The 2nd one appears to be weaker than the first one. If I put the lens cap on so the camera won’t detect the reflected light from pre-flash, there is only one pre-flash. In that case, the final flash appears to be at its maximum power. With a SB-800, it is a different story. I normally see only one pre-flash. If I put the lens cap on, I see two pre-flashes, the 2nd one is stronger than the first one. I cannot quite figure out the reason for the behavior with the built-in flash. The SB-800 behavior makes sense: if the first pre-flash is too weak, camera asks for a second one with higher output power.
How fast is the pre-flash to final flash process?
It is pretty fast. Normally the whole sequence takes about 0.1 second or less. Still I hear complaints about pre-flash causing closed eyes in photos. I guess people can really close eyelid faster than 0.1 second.
With flash value lock button pressed, the camera performs the pre-flash sequences but does not fire the final flash actually take the photo. The calculated flash output amount is used for the flash output when the shutter is pressed. I observed the same pre-flash behavior as above.
Red-eye reduction flash sync mode
On the D200 I have, the red-eye reduction flash sync mode simply turns on the focus assist lamp for a short period of time one second before the flash to cause pupils in subject’s eyes to contract. With a SB-800, the flash emits three short pulses for this purpose. Notice this delay between the last red-eye reduction pulse and the pre-flash is only about 0.4 second. The last two pulses in the graph above are the pre-flash and the final flash.
When the depth-of-field preview button is pressed, the camera by default cause flash (built-in or shoe-mount) to emit a modeling flash, which is 70 short pulses within ~one second.
Commander mode TTL
If I set the built-in flash to Commander Mode and within Commander Mode sub menu set Built-in to TTL mode with both Group A and B set to Off, the flash sequence behaves just like when I set the built-in flash to TTL mode.
Aperture Auto (AA) mode
Some people offered AA mode as a solution for closed eye problem saying AA mode has no pre-flash. This is untrue. It still has pre-flash but the delay between pre-flash and final flash is much shorter. The following graph shows a comparison between AA mode and TTL mode. I guess no human eye can close faster than 50ms.
Auto FP High-Speed Sync
If you are not familiar with high-speed sync concept, please read this. In this mode, the flash sends out very fast pulses. The total duration of the pulses are longer than the shutter speed. In my test, when shutter speed is 1/4000s, the pulses lasted ~1/200s. The pulses are too fast for the maximum 96KHz sampling rate I used to resolve.
Keywords: CLS, Creative Lighting System, Flash photography, i-TTL, Nikon