DavidR wrote:im currently waiting on my new DSLR body as im upgrading from my trusty d70s and was wondering why with the new(er) dslrs flash sync is restricted to say 1/250 while i can get 1/500 from my sq-a and sync at any shutter speed with the d70s. is this a shutter issue, ie do the better shutters restrict flash sync? any thoughts people!
This isn;t quite a simple question, as you're mentioning a number of different cameras, and they each employ dfferent technologies.
Let's start with the SQ-A - and I'll include the ETR-S, 'Blads, Rolleis, and most MF and LF cameras in this category. These cameras generally employ a leaf shutter, which is (usually) embedded within the lens. These sorts of lenses open from the center towards the outside, and then close towards the center again, often remaining closed until the camera is recocked.
Because they open from the center, the emulsion/sensor is fully exposed to the whole of the subject matter while the shutter has been opened, and thus the flash is able to be synched with the camera at all speeds that the shutter is capable of achieving. But the physical aspects of what the shutter needs to do in order to close, then open, then close again cause a restriction on the maximum speeds the shutter can achieve, and thus your 1/500 is not only the maximum sync speed, it's also the maximum shutter speed.
Typical SLRs, digital or otherwise, use a focal plane shutter. This uses a pair of curtains that travel across the focal plane. The first curtain opens the shutter, and the second curtain closes it. At some (very fast) speeds the distance (time) between these two events is very short, and this provides for a moving slit traveling across the focal plane.
As the shutter speed slows, this distance (time) increases, to a point where the slit no longer exists, and the shutter is fully open for some period of time, before the second curtain closes the shutter.
For correct flash sync, the maximum time will be the minimum time where the shutter is fully open. Anything faster will cause a dark band to appear on the image, being a shadow from the curtains that are not yet fully open, or which have already started to close.
Many older cameras (Canon A1/AV1/AE1 etc, Olympus OM series) employed a horizontal shutter movement. This means that the shutter traveled across the focal plane, which is a greater distance than a vertical movement required. Hence those camera had a sync speed of 1/60, while others had 1/125, and some employed titanium to achieve 1/250.
The D200 fits into this category of camera.
Other cameras take advantage of other technologies to achieve faster sync speeds: The D70 has an electronic shutter built into the sensor. This requires the camera to fully open the mechanical shutter, and then the electronic shutter actually makes the exposure. Being electronic, this doesn't rely on physical movement, and therefore faster speeds are able to be achieved. Because just one flash pulse is made, full power flash is able to be used with this method.
An alternative method uses the computing power in the camera to calculate the locations of the shutter curtains as they traverse the focal plane, and it fires multiple flashes as the shutter's slit travels across that focal plane, only firing the second (and subsequent) pulses when it reaches a point not yet exposed, and so on.
This obviously requires that the flash intensity be degraded to compensate for the pulsing it will be doing in order to make an exposure.
HTH.
Posted: Mon Nov 12, 2007 12:36 pm
