Because there is no direct communication between the camera and the speedlight, the ISO and aperture settings of the speedlight must be set to match those of the camera. In use, one should definitely NOT chose the camera's shutter priority exposure setting because N-TTL relies on a constant F-Stop in order to work.
Here’s how it works. Assume that both the speedlight and the camera are set to ISO 200 and an aperture of F 5.6, and that the two are attached. Let's make a theoretical test to see how N-TTL exposure automation works.
- Imagine that the flash/camera combination is placed five feet from a blank white wall. Take a photo. Based on a ISO and aperture settings, the flash will provide enough light to properly expose the wall with a burst of light, which in our hypothetically example might last 1/2000th of a second.
- Next, imagine moving the flash to a distance of ten feet from the wall. The inverse square law tells us that when we double the distance, we must increase the amount of light by a factor of four. When a photo is taken, the N-TTL sensor will measure the light bouncing back from the wall and increase the flash duration to 1/500 of a second, quadrupling the light output.
- The opposite is also true. If the distance is reduced to 2.5 feet, the flash duration would be cut to one-quarter, or 1/8,000 of a second.
One important footnote in the N-TTL story is the inclusion of Thyristor Circuitry. In early units, any excess flash power that wasn't needed to expose the film was "dumped" in a quench tube and lost forever. The power was essentially wasted, meaning that every shot, no matter how close, was at full discharge. The introduction of the thyristor allowed any excess power to be re-cycled within the unit, allowing for a much faster recycle time and longer battery life.
N-TTL Automation In Action: I tested the consistency of N-TTL exposure by mounting a 60mm 2.8 Macro Nikkor on a D600 body, ISO set to 200, aperture set to F 7.1*, shutter speed 1/160 of a second. I mounted an SB-26 in the hotshoe, set it to F 5.6 at ISO 200, and set the camera to manual focus since I would be photographing a blank white wall and there wouldn't be anything to focus on. Four exposures were made at distances of three, six, nine, and twelve feet. The camera and flash settings were not changed between shots. The composite (Photo #3) was made from the histogram displays from the respective images with three feet at the extreme left, and twelve at the extreme right.
You can see from the "white" histogram (the one at the top), that the exposure is very consistent from shot to shot. Each time the flash moves farther from the wall, the sensor allows more light to escape. When enough light for a proper exposure has been dispensed, the excess power is shunted away from the flash tub and returned for use during the next flash.
This series of photos demonstrates (to my satisfaction, at least), the N-TTL flash automation is capable of providing consistent exposures over a variety of distances. This neither proves nor disproves the accuracy of the exposure, only that given a constant set of exposure conditions, the discharge accurately dispenses the correct amount of light over the four tested distances. By adjusting the size of the lens aperture, the results can be fine tuned to taste. But at least we know that Non Through The Lens exposure automation is capable of accurately compensating for changes in flash to subject distances.
*Several old timers recommended selecting a lens aperture 2/3 of a stop smaller than the one suggested by the flash. This would, in theory, underexpose your flash images. I'm willing to take this on faith.