Avoiding Sync Speed Problems

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Using a shutter speed faster than 1/200th second can cause problems. Triggering the electronic flash only when the shutter is completely open makes a lot of sense if you think about what's going on. To obtain shutter speeds faster than 1/200th second, the T2i exposes only part of the sensor at one time, by starting the second curtain on its journey before the first curtain has completely opened, as shown in Figure 7.9. That effectively provides a briefer exposure as the open slit between the blades of the shutter passes over the surface of the sensor. If the flash were to fire during the time when the first and second curtains partially obscured the sensor, only the slit that was actually open would be exposed.

Figure 7.9 A closed shutter (left); partially open shutter as the first curtain begins to move downwards (middle); only part of the sensor is exposed as the slit moves (right).

You'd end up with only a narrow band, representing the portion of the sensor that was exposed when the picture is taken. For shutter speeds faster than 1/200th second, the second curtain begins moving before the first curtain reaches the top of the frame. As a result, a moving slit, the distance between the first and second curtains, exposes one portion of the sensor at a time as it moves from the bottom to the top. Figure 7.9 shows three views of our typical (but imaginary) focal plane shutter. At left is pictured the closed shutter; in the middle version you can see the first curtain has moved down about 1/4 of the distance from the top; and in the right-hand version, the second curtain has started to "chase" the first curtain across the frame towards the bottom.

If the flash is triggered while this slit is moving, only the exposed portion of the sensor will receive any illumination. You end up with a photo like the one shown in Figure 7.10. Note that a band across the bottom of the image is black. That's a shadow of the second shutter curtain, which had started to move when the flash was triggered. Sharp-eyed readers will wonder why the black band is at the bottom of the frame rather than at the top, where the second curtain begins its journey. The answer is simple: your lens flips the image upside down and forms it on the sensor in a reversed position. You never notice that, because the camera is smart enough to show you the pixels that make up

If a shutter speed faster than 1/200th second is used, you can end up photographing only a portion of the image.

If a shutter speed faster than 1/200th second is used, you can end up photographing only a portion of the image.

your photo in their proper orientation during picture review. But this image flip is why, if your sensor gets dirty and you detect a spot of dust in the upper half of a test photo, if cleaning manually, you need to look for the speck in the bottom half of the sensor.

I generally end up with sync speed problems only when shooting in the studio, using studio flash units rather than my T2i's built-in flash or a Canon dedicated Speedlite. That's because if you're using either type of "smart" flash, the camera knows that a strobe is attached, and remedies any unintentional goof in shutter speed settings. If you happen to set the T2i's shutter to a faster speed in Tv or M mode, the camera will automatically adjust the shutter speed down to 1/200th second. In Av, P, or any of the Basic Zone modes, where the T2i selects the shutter speed, it will never choose a shutter speed higher than 1/200th second when using flash. In P mode, shutter speed is automatically set between 1/60th to 1/200th second when using flash.

But when using a non-dedicated flash, such as a studio unit plugged into the T2i's hot shoe connector, the camera has no way of knowing that a flash is connected, so shutter speeds faster than 1/200th second can be set inadvertently. Note that the T2i can use a feature called high-speed sync that allows shutter speeds faster than 1/200th second with certain external dedicated Canon flash units. When using high-speed sync, the flash fires a continuous serious of bursts at reduced power for the entire duration of the exposure, so that the illumination is able to expose the sensor as the slit moves. High-speed sync is set using the controls that adjust the compatible external flash.

To use high-speed sync, just follow these steps.

1. First, make sure that C.Fn I-03 (Flash sync speed in Av mode) is set to 0 (Auto, the default). That allows the flash to synchronize at any speed, from 30 seconds to 1/200th second (and beyond). If you choose either 1. 1/200-1/60 sec. auto or 2. 1/200 sec. (fixed), then high-speed sync is disabled.

2. Mount or connect a compatible Canon Speedlite, such as the 580 EX II, to the T2i, and turn it on.

3. Choose a shutter speed higher than 1/200th second.

4. Press the Flash/H/>>> button (on the 580 EX II, it's the button between the Mode and Zoom buttons) until the flash icon with an H subscript is displayed on the top line of the flash unit's LCD. The same icon will be shown at left in the viewfinder of the T2i. If you choose a shutter speed of 1/200th second or slower, the icon will not display in the viewfinder.

5. Keep in mind that the higher the shutter speed, the less the effective range of the flash. With speeds of more than 1/1000th second, your flash range may be only a few feet.

6. When finished using high-speed sync, press the button on the flash unit one more time to disable the feature.

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Responses

  • Karri Sumiala
    How to sync a canon 550D with external studio flash?
    8 years ago
  • lauren
    Does t2i has high sync speed?
    8 years ago
  • futsum
    Can you use faster flash sync speeds rebel t2i?
    8 years ago
  • brad
    What is curtain 1 and curtain 2 in canon rebel t2i?
    8 years ago
  • Gundolpho
    Can I use high speed sync with a Canon 550D?
    8 years ago
  • Osman Medhane
    What is shutter sync canon t2i?
    5 years ago

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