Shutter/Flash Synchronization

Uploaded by snapify on 17.02.2010

There are several shutter types:
electronic shutters in consumer cameras,
leaf shutters in medium and large format lenses,
and the focal-plane shutter, found in every single-lens reflex camera.
Let's take a closer look at how the focal-plane shutter works,
in particular how it synchronizes with small electronic flashes.
We'll use a simplified model to illustrate the concepts.
The sensor sits in the back of the camera.
If the SLR mirror is up, the subject's light will be focused on the sensor.
The sensor is protected from this light by 2 curtains in front of it.
They're called first and second curtains,
or front and rear curtains,
not because of their physical location
but because of the order in which they participate in making an exposure.
The full sequence goes like this.
The second curtain rises into the ready position.
The exposure begins as the first curtain moves down,
allowing the light to hit the sensor.
After a long time,
the exposure ends with the 2nd curtain moving down in front of the sensor,
in the same direction and speed as the 1st curtain movement.
Finally the first curtain moves back to its original position.
This coordination makes sure all parts of the sensor
are exposed for the same amount of time.
During the exposure,
the curtains always move in the same direction and at the same speed
regardless of what the exposure is.
The exposure length is determined by the time lag
in between the motion of the first and second curtains.
For shorter exposures,
the 2nd curtain can start actually covering the sensor
before the first curtain has completed its movement.
For really short exposures, this slit can be quite narrow,
sometimes as little as a millimeter as it moves down the sensor.
Let's switch our attention to electronic flashes for a moment.
They have some characteristics that are important
when you synchronize them with the focal plane shutter.
What we'll see applies specifically to small
shoe-mount electronic flashes,
not the big studio flashes which are entirely different.
When the flash is triggered it reaches peak intensity very quickly
and then trails off at an ever slower rate.
This tail end of the flash lasts far longer than the quick rise to full intensity.
We can get a better sense of this behavior
by graphing the flash's intensity over time.
Once again we see the quick rise to peak intensity
followed by an ever-diminishing rate of decline
in what's called an exponential decay.
So how long does a flash last?
Engineers have come up with a practical answer.
Given the peak flash intensity,
note the half-intensity level and declare that the flash duration is
the length of time the intensity is above 50% of its maximum.
This is called the t. 5 time.
The problem with this is that
it ignores a big chunk of the tail of the flash intensity
that can still contribute noticeably to the exposure.
So engineers came up with another way to quantify the flashes duration:
the period of time the flash is at least 1/10 of its maximum intensity.
This is called the t. 1 time.
If your owner's manual lists a flash duration
without specifying whether it's the t. 5 or the t. 1 time,
they're using the t. 5 time.
As a rule of thumb,
you can estimate the t. 1 time for flash duration
as about 3 times the t. 5 time.
A typical t. 5 time will be somewhere
in the neighborhood of 1/2 to 1 millisecond
corresponding to a t. 1 equivalent of about 2 to 3 milliseconds.
Flashes often feature fractional power settings
so you can cut down the illumination over some range of stops.
Here's how it works.
The flash simply cuts off power to the flash tube
once the required fractional light volume has been reached.
For power settings below 1/8th, the flash will be clipped
before it even reaches its normal peak intensity.
So, the lower the power setting, the shorter the flash duration.
Without the issue of a long tail,
quantifying the flash duration is simple:
it's the time from beginning to cut-off, as you would expect.
Let's get back to the focal plane shutter
and see how it synchronizes with a flash.
Once the 1st curtain is fully open,
the flash is triggered adding light to the sensor
in addition to the ambient light
which continues to accumulate during the exposure.
Finally the 2nd curtain closes. How long does all this take?
A typical shutter curtain takes 2-4 milliseconds
to move into position.
We saw the t. 1 flash duration was about 2 to 3 milliseconds at full power.
Total this up and get something on the order
of 4 to 8 milliseconds or a 250th to a 125th of a second.
If this sounds familiar, it's a typical flash sync speed-
your camera's published fastest shutter speed
that can be synchronized with a full-power flash.
Now you know where that came from.
If you're using your camera on manual control,
you can force it to trigger a flash at higher shutter speeds
but there's a consequence.
Part of the image will not have any of the
flash's illumination contributing to the exposure
because the 2nd curtain has already started
covering the sensor when the 1st curtain
is fully opened to trigger the flash.
There are certain cases where you don't care about this dark band because
that part of the image is inconsequential or unlit by flash anyway so
you're free to use a higher shutter speed if needed.
You can test your shutter's limits
by exposing a series of flash-lit images
at faster and faster shutter speeds.
Starting a little above the published sync speed,
a dark band can be seen,
the shadow cast by the shutter's 2nd curtain
already partially covering the sensor
when the flash is triggered.
There are situations where we can use this knowledge to our advantage.
For example,
exposed at more than 3 times faster than nominal sync speed,
the 2nd curtain is blocking the flash on the lower half of this image.
It’s not a problem in this case
plus the faster shutter speed knocks the ambient exposure down almost 2 stops.
You may wonder why
our shutter model shows the curtains moving down from the top
whereas our images show the curtain’s shadows on the bottoms of the images.
Just remember the image focused onto your sensor by the lens
is reversed and upside down.
There are a few ways you can get around the sync-speed limitations
if you have the right equipment.
For example, this older Nikon D70 will switch
to electronic shutter mode at high shutter speeds.
Here's a simplified view of how that works.
The 1st curtain opens as usual,
then the sensor is activated and the flash is triggered.
Once the exposure duration is complete,
the sensor is deactivated and the shutters close.
With this electronic shutter the exposure is not controlled
by the shutter curtains,
but by how long the sensor is gathering light.
This is not a common capability because it requires
a lot of processing in the sensor electronics
that might otherwise be devoted to gathering a higher-quality image.
Another option,
if you have the right camera and flash combination,
is high-speed sync.
In this case,
the focal plane shutter behaves as usual
but the flash sends out a series of low-power high-frequency bursts of light
during the shutter slit's movement across the sensor.
The compromise with this approach is that
the amount of flash's light hitting each part of the sensor
is much lower due to the brief low-power pulses of light.
Finally, let's talk about "second curtain sync" or "rear curtain sync".
This is just an alternate way of triggering your flash.
Instead of firing the flash just as the first curtain completely opens,
the flash is fired just before the 2nd curtain begins to close.
Whether you use 2nd curtain sync or 1st curtain sync
makes no difference whatsoever unless your subject meets 2 conditions.
If ambient light contributes to the exposure
and there is noticeable movement during that exposure,
then whether you use 2nd curtain sync or 1st curtain sync will make a difference.
The difference is in how motion blur is rendered.
Second-curtain sync will create a more natural looking image
by making the motion blur appear to follow the direction of movement.
In this 2-second exposure,
a ball bounces from left to right
and the flash component of the exposure
freezes the ball at the beginning of the exposure
because of the use of 1st or front-curtain sync.
A more natural-looking image results
if we switch to 2nd-curtain sync
where the flash freezes the ball at the end of the 2-second exposure.
That wraps up our quick tour of focal plane shutters and flash synchronization.
Keep in mind that we simplified things a bit to get the basic concepts down.
Also, very little or none of what we've said applies to other types of shutters or flashes.
As always, the best way to get familiar with your own gear’s capabilities
is to get out, shoot, and experiment.