The Nuts and Bolts of off-camer flash – part 4, miscellaneous topics

Note: this is the final part of a 4 part series -
PART 1 – basics

PART 2 – manual flash

PART 3 – TTL wireless

Let’s talk about sync speed.
Sync speed is probably the most misunderstood topic for folks starting out with off-camera lighting, and for good reason.  There is a ton of seemingly contradictory information out there, misinformation, disinformation and downright wrong information… but really it’s not all that complicated, so let’s break it down.   First off to understand “sync speed” we must first understand how an SLR shutter works.  SLRs (whether digital or film) all (generally) use a type of shutter known as a “focal plane” shutter -

Obviously the job of a shutter is to expose your film or sensor for a prescribed period of time (usually less than a second).  The way a focal plane shutter work is by using 2 “curtains” that travel across the film plane.  The first (or “leading”) curtain starts by moving outward, exposing the film plane, and when it is done the “trailing” curtain follows it closing off the exposure.  Now what happens when the shutter speed gets faster than a certain point is that the 2nd curtain starts its travel before the first curtain has cleared the film plane.  In other words, the film/sensor is *never completely exposed* as a whole.   As the exposure get faster and faster, there is an increasingly narrow “slit” created by the two curtains that kind of “paints” itself across the film.  While this poses no problem for a shot using a continuous light source, for flash this is quite problematic.  Since normally the actual duration of the flash burst is much faster than the actual exposure, if one of the curtains is in front of the film plane when the flash “pops” it will partially block the light, yeilding the dreaded “black bars” across part of the image.

(Ben Mathis of the Lighting-Practice blog has a great animation of this here)

So in essence when you talk about “sync speed” this is what you are referring to – the maximum shutter speed at which the film is completely exposed at a point of the exposure.

Now if you think abou thtis process, it is a actually a hard physical limit.  There is physically *no way* to change how the shutter curtains operate, or the maximum speed at which they are fully open.  In other words, when folks throw around the term “cheating” or “hacking” the sync speed, it actually has *nothing to do* with the physical x-sync per-se, which is a common point of confusion.  What they are talking about is actually different ways of manipulating the flash burst to “work around” they shutter limitation.

*of course having said that*, let me first point out the exception to the rule

Invariably, during any sync speed discussion, someone will inevitably chime in with “you must be wrong since I normally at up to 1/2000 or so all the time with flash”.    They are, in fact, correct but it is a special case – they are generally using a Nikon D70/D40 when they report this.   There is a unique feature on some Nikon DSLRs (and a few other cameras), which is that they actually don’t use a purely mechanical focal plane shutter – they use what is called a “hybrid” Electronic/mechanical shutter.   This works similarly to a regular focal plane shutter, in that there are the normal first and second curtain, however after a certain point (usually 1/60th) any exposure faster than that does *not* precipitate an increase in the physical shutter speed – the curtains still open/close at 1/60th, however the camera merely “grabs” a smaller and smaller “slice” of the exposure recorded by the CCD.  In other words, even at 1/2000, the shutter is still opening for 1/60th, and then a tiny slice of that exposure is recorded.  The practical ramifications of this is that in essence the shutter is *always*  fully open for the eposure (for all practical purposes) – you never have to worry about the curtains blocking the flash.
Now for those of us who don’t have one of these cameras, we need a different method if we want to use flash with shutter speeds higher than the sync.

The first, and most common is known as HSS (High Speed Sync), sometimes also referred to as “focal plane sync” (FP sync).  Remember that at speeds higher than the max sync speed, the shutter curtains are essentially creating a “slit” that travels across the film plane.  In HSS, the flash will “strobe”, firing a series of superfast bursts (instead of one single burst) that are timed to match up with the shutter movement, ensuring that the whole frame is exposed uniformly as the “shutter slit” passes across each part.  HSS works great, although there are 2 major downsides.

1) it is proprietary – because the flash needs to “talk” to the camera in order to ensure that it’s pulses are timed properly with the shutter movement, HSS is limited to the manufacturers TTL capable flash units, so manual flashes and studio strobes are out .  This also means that triggering solutions like cables/pocketwizards/any manual triggering will *not* work (remember they only provide a “dumb” fire signal, no communication which is needed for HSS).  This means you are limited to using the manufacturers wireless ttl triggering if you want HSS off camera (and Radiopoppers potentially) with the associated drawbacks)

2) because the flash has to fire a bunch of little “pulses” across the entire exposure from a single charge of the capacitor (as opposed to one big burst), the flash output is *drastically* decreased.  HSS really eats the output of your flash (hence Dave Hobby & Joe McNally’s desert shoot using *7* SB800s

That being said, if you are using system flashes with something like radiopoppers (or in an indoor/studio situation) HSS is probably the easiest way to sync past the max sync speed of your camera.

Another (and much less optimal) method actually involves turning the flash into a conitnuous light source!!

Consider:  even though the flash may seem to be an instantaneous burst, it still takes time for the burst to occur.  (this depends on the flash and the power, but often in the range of 1/4000-1/7000 second) that is the time that the flash is physically “on” putting out light.  Now consider if your shutter speed is faster than that, the sensor “sees” the flash output for its entire exposure!  in essence the flash has become a continuous light source as far as the camera is conernced.  Now this isn’t a particularly flexible solution (requires very specific parameters from both camera and flash in order to work), however in the right circumstances in a pinch it will do…  (one thing to remember if you are doing this as that as a “continuous light” the flash contribution to the exposure is now affected by shutter speed just as ambient is!)

I personally shoot with a Canon 5d which has a max sync of 1/200. for faster, I generally use HSS with radiopoppers, or in a pinch the Canon G9 has a hybrid electro-mechanical shutter as described above (similar to the D70/D40) which allows syncing up to 1/2000 or 1/2500 pretty reliably.   Between those 2 methods, I cover all my bases pretty well.

Anyway, that about wraps it up for “the nuts and bolts of off-camera flash”.  I hope this series has been informative, and I may add/refine/clarify it as time goes on

-Ed Z