T3i high-speed sync with popup flash?

[daviddoria]

When shooting outside with a bright background, it would be nice to be able to use the popup flash as a fill light (for shadows on a face, etc.). However, the shutter speed is clamped to 1/200 when the popup flash is up, overexposing the scene. Is there a reason that the popup flash cannot do high-speed sync like external flashes do to allow a faster shutter speed and still get some fill?

Thanks,

David

[TCampbell · Accepted solution]

This has to do with how much power is available to the flash.  A more powerful flash can more easily handle high-speed sync.

Flash sync and high speed sync are related to how the curtain shutter works on any DSLR camera.  There are two doors.  One slides open and the other slides shut.  The reason there has to be two doors (instead of one) is because if there were only one, then the door would have to slide open, then reverse to slide shut. 

Suppose the following scenario were true:

Suppose it takes the shutter door a full 1 second to slide open.  If we set a 2 second exposure time, the following would have to happen if he had just one door.

- the shutter starts sliding open from left to rigth, allowing light to reach the pixels on the left edge of the sensor.  The right edge is still in darkness.

- as the door slides, more and more pixels are eposed to light.  

- one second later the door reaches the opposite end and all pixels are exposed to light, but the door must immediately reverse and begin closing because it will take another full second to slide shut.

- the door begins moving and immediately the pixels at the right-most edge are in darkness as they are covered by the shutter, but the pixels at the left edge are still exposed to light.

- the door finally reaches the opposite edge one second later and no pixels are exposed to light.  

Think about what just happened... the pixels at the "left" edge of the sensor got a 2 second exposure... but the pixels at the right edge of the sensor were barely exposed for a fraction of a second.  You'd have a horrible looking shot.

To fix this, the camera uses TWO doors... one starts sliding open... the other follows a moment later (based on shutter speed selected) and starts sliding shut.  Now ALL pixels are exposed for the same amount of time regardless of the shutter speed you use.

But the reality is these are mechanically sliding doors and it takes time to move them.  The flash sync speed is shortest possible exposure that can be shot where the first door has enough time to COMPLETELY slide open so that when the flash does fire, the entire sensor is exposed and will benefit from the flash.  THEN the second door can slide shut.   

If you aren't using flash and you set a very fast shutter speed (say 1/1000th) the second door actually starts closeing long before the first door has finished opening.  In effect the second door is "chasing" the first door creating a "slit" which sweeps across the sensor.

With this in mind... now think about what a flash has to do to provide for high-speed sync.

Suppose your camera has a max flash-sync speed of 1/200th, but you want to shoot at 1/400th.  

This means the first door has to start opening.  When it reaches the mid-point, the flash has to fire to illuminate the subject -- but half the sensor did not benefit from the flash.  The second door starts closing as the first door continues to open.  When the first door reaches the opposite edge, the second door will now be at the midpoint.  The flash needs to fire AGAIN - this time to expose the pixels on the 2nd half of the sensor.

This means the flash must fire TWICE in rapid succession and those pulses must be PRECISELY 1/800th second (half or your 1/400th second total exposure time) apart.    No flash would be able to recycle in just 1/800th sec if it provided a full power burst of light.  This means it must reserve at least half of it's power for the second burst.

If you increase the shutter speed to 1/800th... now the flash has to fire FOUR times and no single burst can be stronger than 25% of the flash's power -- and those bursts must be precisely timed to occur 1/1600th seconds apart from each other.

You can quickly see how this is going to require a flash with some power or you're not going to be able to do high-speed sync on a subject more than just a couple of feet away.

For daylight, the brightest exposure is going to be the sunny 16 rule... f/16 with the shutter speed set to the inverse of the ISO.  So for ISO 100, you can use 1/100th sec.  For f/8 you can use 1/200th (and now you're still below max flash-sync speed.)

[daviddoria]

Ok, so the idea is that the popup flash is just too weak to bother allowing high speed sync, as say 1/4 of the already low power will not even be noticeable in the scene? As you say though, if my subject is only a couple of feet away, shouldn't the camera at least let me try it and see if it helps?

David

[MikeSowsun]

No, High Speed Sync is not possible under any condition using the built-in flash. It is a physcal limitation. 

Rather than a single, strong burst, High Speed Sync tells the flash to send out an ultra-fast series of low-power, strobe pulses and the total light output is dramatically reduced. Because the strobe pulses are so close together, the light appears to be continuous. 

Even large external flashes like the 580EX II will have reduced output using High Speed Sync. The total output power of the flash is only about 1/4 of the power in normal sync mode.

The pop up flash simply does not have enough power to be effective.