Dockland wrote in post #4187442
Ive just ordered a Canon Speedlite 580EX II to my 40D and have read a little about High speed sync on flash. How does that work?
Can I take a picture @ 1/8000 and still get flash sync? I thought it was lockt to max 1/250? Can someone explain the benefits and pros and cons.
For you to understand "high speed sync", you first need to understand the concept of the focal plane shutter and the "max sync speed". At the end of this explanation I get back to the "high speed sync" thing.
Focal plane shutters (common on SLR cameras) consist of two "curtains", usually made of rubberized cloth (in old film cameras) or very thin metal. The first curtain (which I will call the "leading" curtain) normally covers the film or sensor, hiding it from the light coming through the lens. When you take a photo, the leading curtain moves across the film/sensor to expose it to the light. After the leading curtain has moved, another curtain (which I will call the "trailing" curtain) starts to move, again covering the film/sensor to hide the light from it.
At shutter speeds below the "Max Sync Speed", the leading curtain travels all the way across the film/sensor, fully opening the film/sensor to the light, before the trailing curtain starts to move. At higher shutter speeds, the trailing curtain starts to move before the leading curtain has completely travelled across the film/sensor. What happens to create the very fast "shutter speeds" is that an open slot between the two curtains travels across the film/sensor.
While old focal plane shutters (like in my Nikon F cameras from the 1960's) travelled horizontally, the shutters in most modern SLR's travel across the short distance of the film/sensor frame. The concept of "curtains" turns into one of "blades", but the travel concept is still the same. The leading blade moves first, uncovering the film/sensor, and the trailing blade follows, covering up the film/sensor.
The advantage of the blade style of focal plane shutter is that it can move across the whole film/sensor area faster than the old style curtains. Thus, the maximum sync speed is higher than in the old cameras (max 1/60 for my old Nikon F's, and 1/250 for the 20D).
The concept of a maximum sync speed, however, still applies. If you try to use a flash at higher shutter speeds (faster than the shutter speed at which the leading curtain/blade is fully open before the trailing curtain/blade starts to move), part of the film/sensor will be covered by one or the other of the curtains/blades when the flash (with a very short duration) goes off. Part of the film/sensor will not "see" the light from the flash, and that part of the image will be either black or very dark.
In the "high speed sync" mode, Speedlites emit a series of very short pulses of light for a long enough period to emulate a constant light source while the focal plane shutter moves across the film/sensor. The power output of the Speedlite is very low during the "high speed sync" operation because if it was putting out high power pulses it could not recycle fast enough.
The only use for "high speed sync" that makes any sense to me is for using the flash as a fill in an otherwise very brightly lit situation.