Sync or swim

A reminder to all flash photographers: you need your shutter speed to be below the camera’s flash synch speed.

What does this mean? Let me explain.

The flash fires for the briefest period, of course. Like 1/2000th of a second. That is why we call it a flash.

So when it fires, if the light is to reach the entire film or sensor, the shutter needs to be totally open at that point.

That much is obvious. But what is not obvious is that there is an engineering limitation in your shutter. Beyond a certain shutter speed, the camera’s synch speed, the shutter never totally opens. Instead, a small (increasingly narrow) slit travels across the shutter to give each pixel a brief exposure time.That’s cool – the shutter does not have to be super-fast and expensive and you get a fast shutter speed.

But this gets in the way when you are using flash. When you fire during those short exposure times (on most modern cameras, faster than about 1/200th second), the light does not reach the entire sensor. Look at this example I shot to illustrate this, at speeds from 1/200th to 1/1000th sec:


You can see that as I exceed the sync speed, the light only reaches part of the shutter.

You should also note that especially when using external flashes with Pocketwizards or similar, flash takes time to set up. My 1Ds MKIII has a synch speed f 1/25oth second but as you see, at that speed it is already beginning to cut off. Best stay a bit below your synch speed (I typically set my shutter, when I am using studio flash, to 1/125th second).

(There is a way to overcome that: fast flash, which some high end flash units offer. This continuously, all the time that the shutter travels, pulses the flash at a very rapid rate, so that the slit, as it travels across the sensor, has light coming in throughout its travel time. It works great – do use it when taking flash images outside – but it uses a lot of energy, and hence decreases the range of your flash.)

(Advanced tip: I know of at least one photographer who uses this effect to introduce an electronic version of a neutral density filter or a barn door: he sets his camera to 1/320th second while using flash, and turns the camera upside down. That makes the top part of the image dark, at least as far as the flash part of the light is concerned!)

0 thoughts on “Sync or swim

  1. Am I right in thinking that (on camera) flash technology has not advanced much recently? Yes, we have tweaked the camera-flash interface with ETTL and its competitors to adjust the intensity and width of the flash according to the subject and lens.

    But the basic flash tube has not changed much in many years. Maybe it has increased in efficiently – more flashes per battery charge – but it is still a very short duration light. You talk of pulsing the flash – but why can’t we make one that stays on for longer at full intensity?

    Or is the problem that humans would not be able to endure that intensity of light for more than a short period of time?

    Is it the same for the studio flash units you use? I know zero about studio lighting.

    Incidentally – why can’t the flash unit report its orientation to the camera so it can be recorded in the EXIF? I did some tests bouncing a 580EX flash different ways but had to resort to writing down the direction the flash was pointed for each shot. How old fashioned!

  2. Flash is done by exciting a gas to a high energy level, and by then allowing it to release that (and a lot of photons in the process) all at once. That’s why it is a quick flash. So we’re pretty much stuck with that The rapid cycling is a way around that.

    We could make them stay on for longer but they’d heat up tremendously – not ideal. That is, I think, the main constraint – well, that and energy needed.

    As for reporting orientation – yes, agreed, it would be useful. Though for real analysis you would need to know about he flash orientation and what was where it was shining (wall, ceiling, and THEIR orientation).

    I am just teaching four wedding pros flash techniques tonight… small world, as it were.


    • So what we need in addition to the flash reporting its orientation is a small camera (like that on a mobile phone) mounted on the flash head and thus pointing where it is pointing.

      Then the flash would take a picture of what it is pointing at when it fires and download that little picture to the camera as a sidecar file. It can have the extension .ian since I thought of it.

      I think Canon could charge a couple of hundred extra dollars on the 590EX flash for that.

  3. Hi Michael,

    So in my camera’s menu (Nikon D300), what should I leave the flash sync speed set to? There are all sorts of shutter speeds to chose from: 1/30s, 1/60s, 1/125s, . . . 1/250s, 1/250s AUTO FP, 1/320s AUTO FP. Surely we don’t need to set this to match the shutter speed for each shot? If not, then which setting is the most appropriate. The default is 1/250 (non AUTO FP). Does this setting work for all shutter speeds slower than 1/250? If my shutter exceeds 1/250 do I need to change to 1/350 AUTO FP?

    The manual’s explanation of how the camera is affected by these settings is extremely poor. Any help would be greatly appreciated.

    Many thanks

  4. Hi Dave!

    Yes – the manuals are extremely confusing. Useless, pretty much. It’s not you – it’s the manuals.

    I’d leave it set to the default, although you can also choose to go to lower speeds. What does this mean? This means, when using flash, what shutter speed will your camera use. FP flash on Nikon is what Canon calls “fast flash” – the flash pulses at a high rate instead of firing one flash, so that the entire shutter is exposed. But at a price: you use more (i.e. lose) flash power.

    Test this. Set it to different values and see what the camera [a] does, and [b] allows, in:

    – Aperture mode (A)
    – Program mode (P)

    This will help you understand!


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