Flash Sync Limit
A DSLR has two curtains which move across the sensor. They always move at the same speed regardless of shutter speed, taking about 1/300th second to cross the sensor. What controls exposure (indicated shutter speed) is the gap between the two moving curtains. At fast indicated shutter speeds such as 1/8000th the exposure is created by a very narrow slit between the curtains traveling across the smaller dimension of the sensor.

The flash can't fire until the first curtain is entirely across the sensor. That takes longer for full format sensors like the 5DmkII than 50D or 7D. That's why the sync limit for the 5DmkII is 1/200th and the others are 1/250th.

For correct exposure the ENTIRE flash burst must occur between the time the first curtain completely opens and the second starts to close ending the exposure in this sequence:

1) Press shutter button

2) First curtain starts to move (start of ambient exposure)

3) First curtain clears sensor entirely

4) Flash is fired

5) Second shutter starts to move (end of ambient exposure)

Two things may require slower shutter speeds with studio lights:

1) Signaling latency: When a flash is in the hotshoe it reacts immediately to the "fire flash" signal in step 3 above. A studio flash connected via cord will also fire immediately. But triggering the flash via radio, which in now pretty much the norm, adds a slight delay between the time the camera tells the flash to fire and the flash gets the message. That will cause the flash to fire after the second curtain has already started to close resulting in a black line across the frame.

2) Physical characteristics of studio flash vs. Speedlights: Because studio flashes have physically larger capacitors and flash tubes it takes a bit longer to come to full intensity than speedlights and the flash durations are much longer.

If you combine those two factors, a delay in triggering plus a long flash duration you might not get an even exposure across the frame at the specified 1/200th sync limit for the camera which as stated in the manual is FOR SPEEDLIGHTS. Canon can't control the variables created by studio lights so the warning in the manual is there so people how don't understand the physical limits of the gear and how it works don't have unrealistic expectations that 1/200th will work for all flashes.

Since it is the flash duration which stops motion in a flash shot the flash duration of the studio lights and control of the ambient light should be more of a concern than shutter speed.

Ambient light needs to be reduced as much as possible when using studio, not only to avoid ambient light produced blur, but because it is a different color temperature than the flash and will produce off color shadows.

If you have a situation where stop motion is needed studio lights are not the best tool for the job because of their longer flash durations which can't stop the action. That's where hypersync enters the equation.

With hypersync the timing of firing the studio flash is changed so it fires before the first curtain starts to open and a very fast shutter speed is used so the the slit the two curtains form get completely across the sensor during the time the flash tube is at peak brightness.

1) Press shutter button

2) Flash is fired before 1st shutter curtain opens. This is typically done by using a radio trigger which senses and reacts to the pre-flash signal from the hot shoe. A Canon EX flash in ETTL mode can also be used to trigger a studio light optically with its pre-flash.

3) First curtain starts to move (start of exposure)

4) Second shutter starts to move (end exposure) and slit between the sensor crosses entire sensor in 1/000th to 1/8000th sec. while flash tube is evenly illuminated.

Hypersync causes the camera to see the the single burst of flash as a continuous source of light and changes in shutter speed will affect exposure and to achieve fast shutter speeds like 1/4000th or 1/8000th powerful studio lights are needed.

High Speed FP sync (HSS) with speedlights works with the same sequence, but the difference vs. studio lights is that the speedlight pulses rapidly as the shutter slit moves across the frame vs. the flash emitting a single burst. There is no synchronization between camera and flash other than the camera telling the flash to start pulsing at 40,000Hz just before the first shutter curtain opens, versus telling it to fire a single burst after the first curtain clears the sensor.

HSS works with multiple EX flashes in both ETTL and M modes. Effective range of a single 580ex speedlight in HSS mode is about 7ft. at full power with no diffusion. Using two in a pattern of slave/key overlapping even centered master/fill will extend the effective range to about 10ft. Using diffusers will cut those ranges in half or more.

Holistic Concepts for Lighting
and Digital Photography

This tutorial is copyrighted by © Charles E. Gardner.
It may be reproduced for personal use, and referenced by link, but please to not copy and post it to your site.

You can contact me at: Chuck Gardner

For other tutorials see the Tutorial Table of Contents