Nikon D7000 manual Every Which WAY, Including UP, Avoiding Sync Speed Problems

Chapter 12 ■ Making Light Work for You 415

lightning-fast superheroes who always left a ghost trail behind them, that looks unnat- ural (see Figure 12.10, bottom).

So, Nikon provides rear (second) curtain sync to remedy the situation. In that mode, the shutter opens, as before. The shutter remains open for its designated duration, and the ghost image forms. If your subject moves from the left side of the frame to the right side, the ghost will move from left to right, too. Then, about 1.5 milliseconds before the second shutter curtain closes, the flash is triggered, producing a nice, sharp flash image ahead of the ghost image. Voilà! We have monsieur le Flash outrunning his own trail- ing image.

EVERY WHICH WAY, INCLUDING UP

Note that, although I describe the ghost effect in terms of subject matter that is moving left to right in a horizontally oriented composition, it can occur in any orientation, and with the subject moving in any direction. (Try photographing a falling rock, if you can, and you’ll see the same effect.) Nor are the ghost images affected by the fact that modern shutters travel vertically rather than horizontally. Secondary images are caused between the time the first curtain fully opens, and the second curtain begins to close. The direc- tion of travel of the shutter curtains, or the direction of your subject, does not matter.

Avoiding Sync Speed Problems

Using a shutter speed faster than 1/250th second can cause problems. Triggering the electronic flash only when the shutter is completely open makes a lot of sense if you think about what’s going on. To obtain shutter speeds faster than 1/250th second, the D7000 exposes only part of the sensor at one time, by starting the second curtain on its journey before the first curtain has completely opened, as shown in Figure 12.11. That effectively provides a briefer exposure as a slit of the shutter passes over the sur- face of the sensor. If the flash were to fire during the time when the first and second cur- tains partially obscured the sensor, only the slit that was actually open would be exposed.

You’d end up with only a narrow band, representing the portion of the sensor that was exposed when the picture is taken. For shutter speeds faster than 1/250th second, the second curtain begins moving before the first curtain reaches the bottom of the frame. As a result, a moving slit, the distance between the first and second curtains, exposes one portion of the sensor at a time as it moves from the top to the bottom. Figure 12.11 shows three views of our typical (but imaginary) focal plane shutter. At left is pictured the closed shutter; in the middle version you can see the first curtain has moved down about 1/4 of the distance to the top; and in the right-hand version, the second curtain has started to “chase” the first curtain across the frame towards the bottom.