The Digital Photo Guy

Focal Plane Shutter and High Speed Sync

by on Jun.09, 2009, under Articles

There seems to be some confusion about the definition of a focal plane shutter so, hopefully, this will shed some light on the subject (pun definitely intended). A shutter is simply the camera mechanism that controls the length of time the medium (film or sensor) is exposed to light. A focal plane shutter is a design that places the shutter directly in front of the medium. This is the predominant shutter design in modern digital SLRs.

There are several variations of FP shutter designs but almost all incorporate 2 curtains to block the medium. The photo below shows a modern FP shutter in a Casio P&S circa 1999. As the shutter is released, the metal blades fall down, exposing the sensor. At the precise moment, a second set of blades falls down to block the sensor. At faster shutter speeds, the second set of blades (curtain) closely trails the first such that, in effect, a slit of light moves across the sensor.
Modern Metal Vertical Focal Plane Shutter

Modern Metal Vertical Focal Plane Shutter

This helps explain why digital SLRs have a maximum flash sync speed of about 1/250 second or less. At faster shutter speeds, the blades block off the sensor before the flash is able to fully expose the sensor. In other words, the first curtain opens, fully exposing the sensor. Just before the second curtain descends, the flash must fire while the entire sensor is exposed. Otherwise, a portion of the sensor is covered and produces a dark band across the image.

Older SLRs typically had horizontal curtains made of a rubberized fabric that moved from side to side but the concept was the same. The first curtain moved out of the way to expose the film and, after a predetermined time, the second curtain blocked the film.

Why is all this important today? Most modern flash units have a mode called High Speed Sync (FP Flash) that allows shutter speeds faster than the camera’s top sync speed of 1/125 to 1/250. It works by pulsing the flash to emit a series of flashes as the small slit travels across the sensor. Less expensive flashes like the Canon 420EX have a set pulse rate while more expensive units like the 580EX can be set to specific pulse rates.

When is this important? In a “typical” flash situation where the entire scene is dark, a slower shutter speed is fine. In fact, it is often desired because the shutter needs to stay open longer to capture the background ambient light. What if you want to take an outdoor portrait using a shallow depth of field. Setting your aperture to f/2.8 and ISO to 100, your shutter speed jumps up to 1/2000 second, way too fast for the flash. You could stop down the aperture to f/11 but that increases your DoF and you lose that nice creamy bokeh in the background. This is where FP Flash comes in handy. Set your flash to High Speed Sync (FP Flash) and blast away at 1/2000 second.
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