March 14, 2008
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ARTICLE: The truth about high-speed flash shutter sync
Today I begin in earnest to blog, no, to write full-blown articles, about more serious technical matters, in an organized, informative fashion.
I’ve been reading a lot of the more technical technical articles by the late Galen Rowell recently, in the book “Galen Rowell’s Inner Game of Outdoor Photography”. It was loaned to me by the Skunkabilly, in exchange for my “Galen Rowell: A Retrospective” book that is less technical and more just a coffee table book of big, awe-inspiring outdoor photographs.
Galen was a wizard with flash in the outdoors, and I make my best attempts to apply the same lighting techniques even in my outdoor wedding photography. Especially at sunset, there are many traditional portrait photographers who simply settle for less than the whole scene, because they may be intimidated by the task of bringing everything into the same exposure range. Skunkabilly is a fine example of one who does NOT compromise, and his ninja-like flash skills add a strong dimension to usually un-compelling or impossible-to-capture scenes.
But I digress. I want to reveal one thing, a word of caution and an eye-opening fact, to those of you who may have an external flash unit (Nikon SB800, Canon 580 EX…) and are beginning to experiment with using fill-flash in bright daylight conditions now that you have access to the wonderful thing known as “high speed FP sync…”
What you may not know is how FP (focal plane) sync actually works, and the consequences that it has on your flash power.
Basically, to put it in the simplest possible layman’s terms:
Before you cross that “shutter speed sync” barrier, usually 1/180 sec or 1/250 sec for most cameras, or 1/500 sec. in the case of the legendary Nikon D70, your flash is just a PART of the exposure time. That means you get one WHOLE flash “pop” during the time that your shutter is open. After you pass that shutter speed barrier, however, your flash pop is actually LONGER than your shutter speed. So if your shutter speed is 1/1000 sec. you might only be “catching” half of your flash. And as any good photographer knows, HALF of any amount of light constitutes one whole stop, one EV. If you’ve got a 1/1000 sec. shutter speed dialed into a camera that normally only sync’s at 1/250, you’ve just built in a nice fat -2 EV exposure compensation!
Why did you have NO IDEA this was the case? Well, probably because you’ve just always left your flash in TTL mode, and the TTL was simply bumping up your flash power by 2 EV’s. If you never get too far away from your subject, and if you never crank your shutter speed TOO high, you may never discover this nasty little gremlin of FP sync…
To prove things to yourself, simply put your flash in fully manual power, and shoot pictures where your flash is the main light source. (Indoors / in low light) You may notice three things:
~ Your exposure gets about 1 stop darker for every stop you increase your shutter speed past the camera’s built-in sync speed.
~ If your flash is pointing straight forward, it should read out the distance your flash can reach, and this distance will decrease as well.
~ If your subject is really close and you really start to crank your shutter speed up towards 1/8000 sec, as you lower your flash power you’ll notice that you start to get some odd horizontal patterns in your light. Your shutter is moving so fast that it is starting to catch instants when there is NO flash at all!
So, what can you do? Well chances are this lovely little problem will NEVER affect you if you just leave your flash in TTL mode whenever you go into FP sync territory, and keep your subject close for safe measure. In TTL mode your flash can tell you what subject distances you have to work with, if your flash is pointed straight forward. For example with my camera set to 1/1000 sec, f/2.8 and ISO 400, my flash tells me that in TTL mode it can properly light anything from 2.8 to 31 feet, although of course not both at the same time lol… And this is with my flash zoomed all the way in to 105mm, mind you. SOME times, this will work. Other times, I’ll need to shoot at ISO 200 and f/4 maybe, which limits my shooting range to about 16 feet. If I’m shooting telephoto from far away to blur a background, it’s time to yank that flash OFF my camera and LEAVE it within 16 feet of my subject, commanding it wirelessly from my camera…
Take care, and I hope this article has been informative and easy to understand!
=Matt=PS: There ARE flash units out there that can sync up to 1/10,000th of a second, of course. (And faster!?) The kidn of stuff used to take pictures of bullets coming out of guns, etc. etc. (Talk to Muzikman if you wanna get into that…) But that equipment is expensive, hehehe…
Comments (4)
So many letters and numbers!
nice! im actually going to play around with this! i need to learn so much about flash photography!
is high speed FP sync the same as high speed sync?
These are very good points. One thing I like to point out is the flash duration in non highspeed synch is not actually the sync speed. Therefore exposing at 1/1000 with FP mode most likely is not 2EV less than a camera that X-syncs at 1/250.
The shutter is basically two curtains(although modern shutters are comprised of smaller pieces that makes up these curtains) that moves across the film(sensor) plane at a constant speed. The shutter speed is varied by changing the interval these 2 curtains starts moving. At low speeds the interval is long therefore the entire film plane is all exposed for the shutter speed it’s set at. As the shutter speed gets higher the interval is decreased up to a point where the 2 curtains just leaves a strip that moves across teh film plane(still at the same speed). At the shutter speed right before when the 2 curtains forms a strip smaller than the film plane, is as fast as the camera can fully expose the entire frame with a normal flash pop. This is the X-sync speed. If you fire a flash fires above that speed it will only expose roughly that strip because of the very short duration of a flash.
A flash like the SB-800 actually never changes the output intensity when it fires. It just varies it’s duration. Even at 1/1(full power) it has a duration much lower than the X-sync speed. 1/1050sec for the flash vs 1/250sec X-sync for let’s say a D3. To vary power it simply just changes the duration. So let’s say at it’s longest duration it’s still 4 times faster than the sync speed.
The way FP mode works as I understand is it has to pulse the flash at very short durations at high speed. To keep the flash output constant as the slit moves across the film plane(normal mode the flash output is actually like a sine wave). Therefore the power is lower than what you would get from X-sync flash.