High Speed Shutter vs. Ordinary Flash Sync

Many modern electronic 35mm cameras and flash units have a feature called "High Speed Sync", or "FP sync" or similar. This is a feature which allows the flash to synchronize with the focal plane shutter at all speeds up to the top shutter speed available on the camera.

This feature is useful for outdoor fill flash situations where you want to use a wide aperture, but have fast film loaded in the camera. But there is a somewhat widespread misunderstanding that high speed sync would be useful for stopping motion in action photography. Since it's called "high speed sync", it seems reasonable on the surface to presume that it would be good for photographing objects moving at high speed. But this is not the case. I did some experiments to demonstrate how much more effective ordinary old-fashioned flash sync is at stopping action.

To check motion-stopping abilities, I wanted a target that moved very fast, but very consistently. I have a small rotary Dremel tool that is capable of turning at 30,000 revolutions per minute. I painted some white lines on a cutting disk for this tool, to make the motion of the disk very apparent. Below is a photograph of the disk with the tool turned off.

(Throughout this document, click on any thumbnail for a bigger view.)


Next, I turned the tool on at its maximum speed. According to the specs, it should be 30,000 rpm, though I suspect the tool isn't very well calibrated. Note that, at 30,000 rpm, the tool makes a full revolution in 1/500 of a second, or 1/8 revolution in 1/4000 of a second, the maximum shutter speed available to me. I took a series of photos of the tool at decreasing shutter speeds, from 1/4000 of a sec to 1/500, shown below.

S 4000  
S 2000  
S 1000  
S 500  

The first thing you notice about the photos is the strong focal plane shutter distortion. The straight lines painted on the disk are no longer straight. The right side of the disk has more lines closer together than they ought to be, and the left side is blurred more. To understand this, realize that a focal plane shutter consists of two curtains. The curtains need approximately 1/250 of a second to get all the way across the film. At high shutter speeds, the first curtain starts across the film plane, and the second curtain rapidly follows it, a very short distance behind. The film is not all exposed at once, but only a narrow slit is exposed at any given time.

In this camera, the shutter moves from the top of the image to the bottom. Thus, you can tell that the disk was rotating counter-clockwise. In the 1/4000 picture, as the shutter moved from top to bottom, it passed four stripes on the right side, and they appear narrow. Only one or two stripes are on the left, and they appear wider. The right side was moving up past the shutter, while the left side was moving down along with the shutter.

The slower shutter speeds show similar patterns, but with much less contrast. That's because at 1/2000 of a second, the disk rotates 1/4 of a revolution. In other words, an entire stripe goes by each point during the time the film is exposed. At longer exposures, more than one stripe goes by, and they overlap, blurring together.

Ordinary Auto Flash (1/250 shutter speed)

Finally, I took a photo of the disk using an ordinary automatic flash (Sunpak 383 super), using the camera's sync speed of 1/250 of a second. With ordinary flash, the first curtain of the shutter opens fully, and then the flash electronically turns on and off, faster than any mechanical shutter could possibly open and close. Finally, the second shutter covers up the film. Automatic flashes at close range typically have durations in the neighborhood of 1/20,000 of a second, nearly an order of magnitude faster than camera shutters.

The tool was moving at the same speed for the ordinary flash sync photo as it was for the earlier high shutter speed photos. But note not only the absence of focal plane shutter distortion, but also the much reduced motion blur.

Incidentally, if you have a camera capable of high speed sync and want to try some of this for yourself, you can partially reproduce the experiment without even using film! My naked eye could see the action stopping ability of the auto flash at close range.

The problem with "high speed sync" for action photos is that it works by making the flash slow down. Since the camera's shutter can't open fully in less time than its max normal sync speed, the film can't be exposed all at once at high shutter speeds. Exposure must be controlled by the shutter's slit travelling across the film, exposing different parts of the film successively. Instead of giving one almost-instantaneous burst of light, the flash gives a sequence of many pulses of light, approximating a continuous light source during the time the shutter is open. So instead of freezing motion with the fast electronics of the flash, motion is stopped by the much slower and less effective mechanics of the moving shutter curtains. The maximum shutter speed is in the neighborhood of 1/4000 or 1/8000, but the fastest flash duration at ordinary sync is much faster. When using high speed sync, fast shutter speeds are only available at wide apertures and close ranges, precisely the conditions where ordinary auto flash produces its shortest durations.

When you change shutter speed from the ordinary sync speed to the "high speed sync" range, it's the only photographic situation I know of where speeding up the shutter causes the exposure time to slow down.

© 2001, Richard Cochran

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