One of the most important concepts in flash photography is sync speed. This is the fastest shutter speed that your camera can use with normal flash. For most DSLRs, the sync speed is 1/250″. Your camera’s sync speed may be faster or slower — so check your user manual if you don’t know.
Why can’t I use flash at any shutter speed I want? This is a fair question. It has to do with the design of your camera’s shutter. If you use a DSLR or an old-school 35mm film camera, your shutter mechanism has two curtains in front of the sensor/film plane. During the exposure, the 1st/front curtain opens and then the 2nd/rear curtain closes. The difference in timing between the movement of the two curtains is your shutter speed.
At many shutter speeds, the 1st-curtain will completely clear the sensor before the 2nd-curtain begins to close. If your flash fires at any of these speeds, the entire sensor will see the flash — which is what you normally need to make a flash photo. Your sync speed is the fastest shutter speed at which the 1st-curtain completely clears the sensor before the 2nd-curtain begins to move.
At faster shutter speeds, the 2nd-curtain begins to close before the 1st-curtain is completely open. The shutter literally becomes a slit between the curtains that moves across the sensor. So, there is no point at which the flash can illuminate the entire sensor.
High-Speed Sync is a special flash mode that changes the way your Speedlite fires. Rather than fire one big pulse of light, HSS turns the Speedlite into a machine-gun of light that fires an insanely-fast series of pulses. Essentially the Speedlite becomes a continuous light source for a very brief period of time. The downside is that HSS consumes a ton of power. So, your Speedlite in HSS is 2.5 stops dimmer than it is in normal mode. There are loads of reasons to use HSS. Click here for the many articles I’ve written on HSS.
More Tid-Bits About Flash and Sync
Your Camera Might Not Know That You Are Using Flash
If your camera senses that a Speedlite is attached and turned on, then it will impose the limitation of sync speed. However, there are several situations when the camera will not know that you are using a flash. In these cases, the camera will fire at any shutter speed you specify. As shown above, when the shutter speed exceeds the sync speed, you’ll get a black band on a portion of your photograph. So, you need to be mindful of the sync speed in the following situations:
- using a Nikon Speedlight with a Canon camera or vice-versa
- using a radio trigger to activate your off-camera flash
- using a PC-sync cord
- using an infra-red trigger to fire flashes via optical slave sensors
Why Outdoor Flash “Overexposes” Your Photos
Using your flash outdoors in bright sunlight is a smart idea because your camera cannot record the full ranges of brights and darks that you can see. Outdoors, in bright sunlight, the shadows will often be too dark. Turning your flash on in bright sunlight will throw “fill flash” into the shadows so that the camera can record those important details.
If you’re using a wide-aperture to create shallow depth-of-field that hides distracting background details, then under bright sun, you’re also using a fast shutter speed. So, when you turn your Speedlite on for fill flash, the camera slows the shutter back down to the sync speed — even if the camera is in shutter-priority (Tv) or manual mode. So, if you just turned your Speedlite on and your photo looks way over-exposed, you need to activate high-speed sync so that the camera can return to using fast shutter speeds.
Some Cameras Have Very Fast Sync Speeds
For most of us, the sync speed is imposed by the mechanics of the common two-curtain shutter used in SLRs. If your camera does not have a two-curtain shutter, then you likely have a much faster sync speed or no sync speed at all. For instance, the shutter in a view camera lens is made of blades that open from the center out to the edges (much like the design of the aperture mechanism). So with view cameras and some medium-format cameras, sync speeds can be very fast. Another example is the digital cameras that have electronic shutters. With these, the sensor literally turns on and off very quickly. So the sync speed can be 1/500″ or faster.
X-Sync Is The Same As Sync Speed
Old-schoolers will refer to sync-speed as the X-Sync because cameras used to have an X on their shutter dials to indicate the maximum speed with flash. So, when you read “X-Sync” just think “Sync Speed.”
For more details on sync speed and flash photography in general, check out my Speedliter’s Handbook.
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