The Speedlight Present feature is a safety device that prevents a camera from firing at any shutter setting that exceeds the manufacturer's top synchronization speed. When functioning normally, Speedlight Present prevents the user from accidentally selecting a speed faster than 1/500 of a second for the D70; 1/250 for the C90, D300 and D7000; or 1/200 for the D80. These speeds are not arbitrary; they are the minimum speed where the entire sensor is exposed to the light coming through the lens for a single instant.
The shutter mechanism used in the digital single lens reflex has its roots in the original Leica camera, which dates back to 1925. It consisted of two rubberized cloth curtains that marched past the film using a mechanical clockwork mechanism. Early versions moved from right to left, probably as a concession to the positioning of the gears and springs required to make the system work. At some point, the shutters were redesigned to accommodate a metal curtain that moved from top to bottom, something like a roll-top desk. Here's how the two-curtain shutter works when the camera is set to the maximum flash synchronization speed. When the shutter release is pressed:
- The first curtain falls, exposing the top of the sensor first, until the entire sensor is exposed.
- At this exact moment, the flash is fired, allowing all of the photosites (photo-optical sensor that yields one pixel of data) of the sensor to be lit simultaneously.
- The second curtain falls, first covering the top of the sensor, until the sensor is completely covered.
|1/200 of a second 1/250 of a second 1/320 of a second|
At 1/250 of a second, the top synchronization speed for the Nikon D90 I used for these photos. The second curtain will advance the moment the first has disappeared past the lower edge.
At 1/320, you can see a darkened area at the top edge of the frame. The second curtain started its advance before the first curtain fully disappeared, creating a gap between the two that was not as wide as the frame. Since the flash fired the moment the first curtain had completely disappeared, we catch the leading edge of the second curtain as it advances into the image area.
|1/400 of a second 1/500 of a second 1/640 of a second|
I've been really pleased with the modified speedlights, as it gives me the option to carry one as my third, backup unit. I can now stuff a D70 in my camera bag (or carry a spare on my neck) and know that I can use the magic all-speed-synchronization if I need it, and not have to worry about finding a neutered SD-17 flash extension cable to keep with the flash,or the extra hand needed to use it.