Lighting 101: Flash Sync Speeds
Let's talk about flash sync speed and basically, right now I'm talking about sync speeds with DSLRs. In the old school days the way you would get a really fast flash sync is you would buy a medium-format camera like a Hasselblad or a Rolleiflex or some other cameras that could sync at all of their shutter speeds. You know, the old school 503CW Hasselblad could sync up to 1/500 of a second or 1/800 of a second depending on the lens 'cause the shutter was in the lens, not in the camera. So leaf shutters. And so that allowed you to get a faster flash sync than you could typically get in the old school days at 1/125 of a second or 1/60 of a second with film cameras. But these days and I have a Hasselblad with me, we could do it with that, but I figured most people aren't gonna know the Hasselblad. So we'll do it with with DLSRs which is a little more applicable. But in terms of how a shutter works here is 1/200 of a second, this graph, so you can see, watch, at one point the entire sensor ...
for just a split second is fully exposed to the light. And that's the reason your top sync speed on whatever camera you have is at that speed because that is the fastest shutter speed you can use where the sensor is fully exposed. If you go beyond that, say 1/320 of a second or 1/500 of a second, what happens is you'll see on the next slide, let's say 1/600 of a second. You see there is a slid moving over the sensor and you see this rear curtain. So the first one is the first curtain dropping, so most mechanical shutters in DSLR are a two-curtain shutter mechanism, where the first curtain drops, the second curtain drops to close the shutter and if we go to a 1/1,000 of a second here, you'll see there's just a slid moving down the shutter, so there's never any time at which the whole sensor is exposed, which is why if you try to shoot, I think there's another slide here, well, this is basically what I was just saying, this flash sync is that speed, so that you have the full sensor exposed. So for Canon cameras it's 1/200 of a second typically, for Nikon it's 1/250 of a second, some of them, I think, the DA10 can actually technically go to 1/320 of a second, but only if you're using Nikon Speedlights. Sonys, I think, are 1/200 of a second, somewhere in there as well. But if you are not using these fancy high speed flash techniques, this is what you see. At 1/200 of a second you have the full sensor exposed, at 1/400 of a second what's happening, you're basically imaging the backside of the second curtain as it closes. So that's like a shadow of the shutter blocking the light from hitting the sensor and you see it's a blur because you see the motion of the shutter coming in. So that's why you can kinda see through it a little bit. And obviously the 1/1,000 of a second you see nothing but black because the second curtain of the shutter is already completely closed or maybe there is just a little bit of gradation there, like light bleeding through before it finally closed. And at 1/8,000 of a second, puff, nothing, just pure black. So, we're all good with shutters? So, now let's talk about the definition of flash durations, we're talking about how long does that burst of light last coming out of the flash. And this is critical for what we're gonna be doing with the high sync stuff. So, this is a graph, this is how much light's coming out, the amplitude of the light, so how bright it is. First is time on the horizontal axis here and you can see that when a flash goes off, so a flash hit is basically, any power pack, it doesn't matter what brand it is, it has a bunch of capacitors in there that are storing up a bunch of electricity and when you push the button to make the flash happen it dumps a whole bunch of electricity into the xenon gas flashtube that's on the front of the flash head and it's exciting the xenon gas in there. I used to be a physicist, so I could tell you all the quantum mechanics of that, but you don't wanna know that, so, but basically what's happening is you get this huge burst of light right off the bat and then you get this long tail that descends down slowly to the bottom and when you see flash durations, maybe you're on B and H or some Samy's and you see the specifications for the flash duration for each flash, sometimes they say t.01 or t0. and so what those are, these are different specifications for how to measure the length of their flash burst, so t0.5 is kind of the standard of the industry and that means how long is that flash above 50% of its amplitude. So that means this is the time, we're calling that flash burst, whatever, 1/1,000 of a second at t0.5. But the reality is, there's this huge tail that lasts quite a bit longer for that flash, so t0.1 is actually a little bit more accurate because the t0.1 is how much of the flash curve is above 10% of the power. So it gives you a little bit more accurate idea of how long that burst of light lasts. So if it's 1/1,000 of a second at t.5, it might be 1/200 of a second or 1/300 of a second at t. and there's no calculation between the two because it just depends on the shape of the flash curve and on a lot of different things. So, I think brown color usually tells you the t.1, pretty much everybody else tells you the t.05 or 0.5. Regardless, it's a measurement that we can look at and kind of understand what's going on with the flash duration. So, here is what a 1/363 of a second is t.0. and there's actually this iconic light meter, the first one on the market. Just came out that can actually measure extremely accurately the flash duration of your flash. So that's how I was getting these numbers. And this is 1/5,350 of a second. So you can see there's a little motion blur in here. The reason there's motion blur in there 'cause I didn't turn off the lights in my studio to take this picture, but you get between these two, you see that fully blurred and stops the motion. So historically, if we're shooting action in the studio, what we'd wanna do is black out the studio and then we'd wanna use a strobe with a crazy fast flash duration, like 1/20,000 of a second, 1/30,000 of a second. And I think Profoto, the new Profoto B10 or whatever it is has 1/80,000 of a second, but that's at super low power. So it'd need to be pitch-black in here almost to actually make that useful for what we're gonna do. So that's the old school technique, you rely on a flash to stop the motion. The problem with that is that no matter how fast your flash duration is your shutter speed is still 1/250 of a second, so if there's any ambient light, you're gonna have a little motion blur, unless it's completely pitch-black in here. And then you would have the flash stopping it, but we can't have action, people flying around in pitch blackness, that would be a little dangerous, especially with parkour. So this technique we're using now allows us to really freeze the motion, so there is zero motion blur as you can see in this picture anywhere and still have the lights on.