The Leaf Shutter Advantage over High Speed Sync for Strobes
The use of Leaf Shutters (LS) have been a mainstay of large and medium format cameras since the advent of photography. Moving into the modern era of photography, their limitation has been in how fast they can operate, with Focal Plane Shutters (FPS) easily exceeding their fastest captures with 1/8000th+ speeds available.
Leaf shutters, however, have a number of benefits over focal plane shutters, including near vibration-free capture, but the most predominant is being able to sync flash at speeds far faster than the focal plane shutter will allow.
Why is this important? The combination of leaf shutters and strobe gives you the power to diminish or even overpower the light of our (relatively massive) star sitting 93 million miles away.
In Phase One 645 bodies, the Leaf Shutter operates automatically between 1 second and 1/1600th, with the FPS taking over above and below those thresholds, with the fastest possible exposure of the FPS at 1/4000th. And with the release of Feature Update #8 firmware from Phase One, the overall responsiveness and speed of the XF camera while using Leaf Shutters has been greatly optimized and refined.
Leaf shutters provide the utility of limiting the intrusion of unwanted light into a scene, whether from overhead lights in a makeshift studio, modeling lights from your strobe heads, or stray window light.
Focal Plane Shutter vs Leaf Shutter Mechanics
Without the leaf shutter involved, the fastest shutter speed you can use and still sync a strobed flash into is limited by the last time value the focal plane shutter is entirely open all at one time. Faster than that, the focal plane curtains chase each other thru the frame, with the digital sensor (or film) never having the opportunity to see the entire frame open at once.
FPS Shutter Gifs courtesy of the Pocket Wizard Wiki:
Modern 35mm DSLR’s typically have a sync speed between 1/125th and 1/250th with most medium format cameras fixed at 1/125th due to their larger physical size. If you’ve ever recorded a partial frame accidentally, you’ll know what I’m talking about.
1/160th, 1/200th, 1/250th, 1/320th, 1/500th, 1/640th FPS strobe exposures on Phase One 645DF with LS disabled:
In many situations, this results in even the inability to shoot at all apertures even in a studio environment because the warmth of the modeling lights perverts the color temperature of the daylight balanced strobes supplying primary light for your scene.
When the leaf shutter fires, however, as the blades close it has more of the appearance of a leaf or a starfish, producing no harsh lines of limitation.
This motion allows for fast sync speeds with very little light falloff, allowing you to shave ambient exposure out of your scene, whether it’s the ambient/modeling lights in your studio, unwanted objects on location, or simply to shape light in a way outdoors mid-day that isn’t wholly dependent on the sun. (As you increase shutter speed to its maximum, the flash duration of whatever strobe system you’re using becomes important in order to get most of the flash to bloom within the available open shutter… see my prior blog on the subject: High Speed Flash Sync Using The Phase One XF And Leaf Shutter Lenses)
Modern leaf shutters in the newest systems still only operate up to 1/800th or 1/1000th mechanically, but strategies are employed electronically to halve those times to 1/1600th in Phase One cameras and up to 1/2000th in Hasselblad medium format. (Phase One Industrial cameras can actually operate up to 1/2500th)
Cameras that don’t make use of Leaf Shutters can employ a strategy called High Speed Sync (HSS) to sync strobe at shutter speeds faster than the focal plane shutter limitation. This methodology entails your HSS enabled strobe to fire a rapid burst of flashes in order that the burst of light extends for the entire period of the mechanical exposure, creating a somewhat constant light out of your strobe for the duration of the exposure. I would love to have shot a video of the difference between a head firing once or as a burst, but with only 240fps capture available to me, the effect change would have been illustrated only minimally.
HSS is indeed a novel way of solving the flash sync speed limitation in FPS systems, but until I tested it against Leaf Shutters, I really didn’t know its power or limitations.
The following image sequence demonstrates strobe syncing through escalating shutter speeds between the Phase One XF and the Canon 5Ds when paired to a Profoto B10 Plus (rated at 500w/s) through a 4’ Octabank at 8’.
- FPS sync of the XF is 1/125th and the Canon 1/200th.
- The XF was using its internal Profoto transmitter, the Canon used the Profoto Air-TTL C transmitter.
- Both Cameras shot at base ISO: IQ4-150 ISO 50, Canon 5Ds ISO 100
- Both cameras were set to rear curtain sync, but at 1/250th the Canon was moved to HSS. At 1/2000th, the XF moves to FPS and no strobe is fired.
- In all cases with HSS, the Profoto B10 plus was firing at full power, whereas the unit was set to Power Level 8 (125 w/s) for all of the XF exposures until the last two frames of the animation where I turned it up to 9 (250 w/s), and then 10 (500 w/s).
The HSS images demonstrate that while strobe syncing is available, the ratio of light being recorded falls at the same rate as the underexposure of the entire scene. While the strobe light being captured by the Leaf Shutter does fall off as speeds increase, by powering up the strobe in the last two frames of the animation, I was still able to exceed my original grey card exposure considerably. There is also a relative inefficiency of having to burst thousands of micro-flashes as battery power is consumed faster for the same number of frames shot HSS, even though the overall output is significantly less light introduced per frame.
Still frames of above animated sequence: (Note: I equalized exposure on the grey card to start with, the IQ4-150 had at least a 1/2-stop more light gathering power at what should have been the same resulting exposure. )
Still frames of the above animated sequence:
This test demonstrates an undeniable advantage to Leaf Shutter to have better control of your shooting environment than a grip truck full of able bodies, stands and blackout material can provide. I had always thought that HSS likely compared pretty well to the Leaf Shutter lenses I’ve been shooting commercially for the last 10+ years, but I was actually surprised how little of a comparison could be made between the systems.