FP-Sync vs X-Sync

[Kirks518]

If I'm correct, FP-Sync triggers the flash slightly early to allow the flash bulb to reach it's full output. This isn't necessary with X-Sync, as a strobe fires at full (or selected fraction thereof) immediately.

A flash that has HSS (High speed Sync) capabilities (for today's modern digitals) sends multiple pulses of the strobe for a long period of time when triggered, allowing flash to be lit during the shutter slit travel. I believe in a digi cam, you also have to change a setting to allow for higher shutter speeds when a flash is connected (in my 50D I know I do), whereas with film cameras, there isn't a computer-based setting to allow for this.

With that in mind, can you use a flash that has HSS at higher than sync speed if you plug it in to the FP-Sync? I think the point in which the flash gets triggered on FP or X would be one determining factor. My thought is that with the HSS being triggered early by FP, the frame would be lit from the moment the first curtain starts it's travel, all the way through completion. Or, can HSS be selected on the flash and used with X-Sync? Anyone know if any of what I'm thinking is correct and would work?

 

[wiltw]

Simple answer is NO the HSS flash will not work with a film SLR...the camera has to send a Canon ETTL command to the flash to actually put it into HSS mode, whereas setting 'HSS' on the flash merely ENABLES the use of that mode in the flash.

 

[mdarnton]

The modern flash isn't firing earlier to enable higher shutter speeds--it's going into an entirely different mode, a fluttering or longer sustained burn than the 1/1000 or so normal electronic flash blast. It's not about timing the blast; it's about making it longer so that the narrower slit of the higher shutter speed receives illumination for the entire time it is passing in front of the sensor.

The normal sync speed for any camera is the highest speed at which there is still a point when the whole sensor/film is exposed, when the light fires. One curtain of the shutter opens, the flash fires, then the second curtain follows to close the shutter. Higher than that, the shutter becomes a smaller slit traveling across the sensor/film at the same rate as at slower speeds, and the light is limited by the width of the slit. If you fire a normal flash at one of those higher speeds, all you get is the spot on the sensor that the open smaller slit is over at that instant--usually at the leading edge of the shutter travel, smaller and smaller the higher the speed. I don't think modern digital cameras will permit you to make this mistake, but old film cameras will, however, so when your camera is empty, you can open the back and fire off a few flash exposures at higher speeds and see for yourself through the back what's going on, watching for the flash-illuminated slits. While you're at it, you can observe how FP won't do the job.

 

[Chan Tran]

The first film camera with HSS was the Olympus OM-4T. Since then there were many 35mm film cameras have this capabilities. HSS is not a digital only feature.

 

[GRHazelton]

Please don't confuse M synch with FP. M and FP both delay the shutter opening to allow the flash to reach full output, but FP relies on a bulb which has a - comparatively - long burn time to allow the focal plane shutter's slit at any shutter speed to traverse the film gate. This travel time is why X synch on focal plane shutters is a comparatively low shutter speed, that speed is the fastest one at which the slit is the entire width or for a vertical travel shutter height of the film gate. FP bulbs - if you can find them! - will synch with a focal plane shutter at any speed.

Leaf shutters, on the other hand, are "fully" open almost instantaneously, since the opening is at the lens' nodal point, as is the diaphragm. Leaf shutters can synch at all speeds for X or M. IIRC the typical M synch delay is about 20 milliseconds.

 

[Kirks518]

Not confusing sync's. The M645 only has X-sync and FP-sync. I guess Mamiya felt that the M and FP were close enough to only have FP.

 

[GRHazelton]

Not confusing sync's. The M645 only has X-sync and FP-sync. I guess Mamiya felt that the M and FP were close enough to only have FP.

Point taken. No insult intended. Some of our younger members probably have no first hand experience with flashbulbs.