When "Speedlight Present" Is Absent

So far I've neutered a total of four Nikon Speedlights (one SB-24, two SB-26s,  and one SB-28). As a precaution, I've blue-taped, or installed blue velcro, on each of these units, primarily as a reminder that the Speedlight Present pin has been disabled. This is not an issue with the D70s they were modified to be used with, but this modification could cause some problems if any of these modified speedlights wander into a camera bag with my D300 or my D7000, two heavy-use cameras that can't perform the sync-at-all-speeds magic.

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.

When shutter speeds briefer than this maximum synchronization speed are chosen, it is impossible for all of the pixels to be lit at once. At these speeds, a slit (gap) separating the two curtains travels across the sensor. All of the photosites get their serving of light, but not all at once.  The slit could be made to vary in width, depending on how much light is required to properly illuminate the image. A wider slit translates into more time for each pixel to absorb the light necessary for proper exposure. A narrower slit, less.

I made a series of photos to illustrate the slit, and what could happen if I deploy the wrong speedlight. Because optical lens systems actually invert the image, I modified the samples as a nod to technical accuracy. If you inadvertently put a neutered speedlight on a camera incapable of high speed synchronization, like a Nikon D90, you'll get results similar to these:

1/200 of a second                                            1/250 of a second                                          1/320 of a second

At 1/200 of a second, the first curtain has advanced and disappeared into the lower reaches of the cameras innards. The second curtain waits briefly before advancing, giving each pixel its full 1/200 of second. The entire surface isn't illuminated all at once: the  pixels at the top edge are lit before those at the bottom.

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

In these last three images, you can see that the second curtain is in motion a bit sooner for each decrease in exposure time. Remember that that the flash is set to fire when first curtain is completely hidden by the lower edge of the frame, and that the rear curtain follows, leaving the proper gap for each exposure setting.

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.