Originally posted by Eberbachl Can you force a higher shutter speed than 1/180s
Look up 'focal plane shutter' and 'leaf shutter' for more information.
Leaf shutters are often built into the lens itself. Because of this, they are rare on smaller cameras with interchangeable lenses. It adds substantially to the cost to build a shutter mechanism into every lens. To get vignetting in an image caused by the shutter, the shutter would have to be reasonably circular like the aperture. This may be true of higher-end leaf shutters, but many leaf shutters are just one or two blades that flip across the optical path inside the lens.
Focal plane shutters are built into the camera body itself and are located at the back of the optical path near the sensor/film. A focal plane shutter has to be as big as the sensor/film so there is a much greater mass to move.
A focal plane shutter is typically composed of two curtains. When you press the shutter release, one curtain jumps across the opening to expose the sensor/film to light. At the specified interval, a second curtain jumps across the opening to stop the exposure. On current Pentax bodies, it takes 1/180th of a second for a curtain to completely cross the opening. The curtain movement speed is the same for every shot you take, regardless of your shutter speed.
Here is what that means- For all shutter speeds up to 1/180th of a second, the first curtain completely opens before the second curtain starts moving. That is why flash sync is 1/180th. It is the fastest speed in which the shutter is completely open. The flash duration is MUCH shorter than 1/180, so the camera can leisurely trigger the flash anywhere within that 1/180 (or longer) time span. So how do we get shutter speeds faster than 1/180th? Simple. The second curtain doesn't wait for the first curtain to fully open before starting its dash across the opening. The result is a moving slit across the face of the sensor/film. The higher the shutter speed the narrower the slit.
The electronic flash is still much faster than the highest shutter speed for the camera. If it goes off at any shutter speed higher than 1/180, Your sensor/film will capture a slit with black borders. Your shutter speed determines the size of the slit. I guess that's vignetting of sorts, but most folks won't see much artistic value in this.
So how does high shutter speed (HSS) flash work? The flash strobes to create multiple exposures on the sensor/film, one flash for each width of the travelling slit.
This is also why HSS flash is mostly useless for stop-action photography. You aren't taking a single fast exposure of a moving object. You are taking many fast exposures. The result is a ghost-like repetition of the subject for every flash. Further, to strobe like this, the flash can only put out a fraction of its normal power with each strobe so working distances are much shorter. There are two situations where HSS flash is of value - first, when you WANT this multiple exposure streaking for artistic reasons; second, when the subject ISN'T moving but the background is very bright requiring a higher shutter speed and you need fill flash on the subject.
This limitation of HSS flash is likely why Pentax didn't bother trying to build this capability into the dinky built-in flash on its current dSLR models.