Are you chasing your shadow or is your shadow chasing you?

Second curtain use of camera flash – Part 1

By Katriel


Knowing the difference between first and second curtain use of a flash in photography increases the options a photographer has when shooting action. Let’s take as an example a runner jogging at night.  If the goal is to show the path of the jogger as s/he passed across the camera’s field of view you could just simply use a slow shutter speed, however, you would lose a lot of detail in the subject. By firing your flash on its default setting (assuming you’ve never made changes to the flash) with a slow shutter speed you could freeze the subject and still capture the ghostly trail. But what if you want the trail to appear on the opposite side? This is where second curtain sync comes in.

So what is second-curtain sync or first -urtain sync?

Digital SLR cameras use plane focal shutters to help control the length of time light strikes a sensor. For the most part the shutter consists of 2 sets of metal or cloth blades. These sets of blades create a blind between the sensor and the light coming through the lens’ aperture except for a slit of space between the blinds when they travel across the sensor.  Each blind is called a curtain.



When you press the shutter release button one of the curtains moves across the sensor exposing the sensor to light.  There will come a point in time when the entire sensor is exposed to light before the 2nd curtain begins its travel if the photographer is  using a shutter speed less than or equal to 1/60 of a second. (Many higher end cameras today can use what’s called –high sync speeds of 1/125 of second or faster .) 

A flash’s sync speed (or synchronization speed) is the fastest shutter speed a photographer can set which will leave the entire sensor completely open to light for a brief period. A flash needs to fire during the time the entire sensor is exposed to light; otherwise, you will see an image of the second curtain as it travels across the sensor.  That part of your photograph will appear black.  If you still use a film camera it will look like you did not advance the negative completely and as result only recorded part of the scene.

The time lapse of a flash is much faster than the sync speed.  Depending on the hardware a flash’s burst can be anywhere from 1/500 of second to over 1/4000 of second which is fast enough to freeze most moving subjects.  In the world of photography then that leaves plenty of time for ambient light to also be recorded onto the sensor.  When you use 1st curtain flash, ambient light is recorded after the flash burst.  When you use 2nd curtain flash, the burst of light the flash fires occurs right before the 2nd curtain begins to move; obscuring light from the sensor. The ambient light is recorded prior to the burst of light.  With slow shutter speeds moving subjects will be recorded blurry except at the point of the flash burst.  This can lead to lead to some very interesting effects when shooting action such as having your shadow chase you or you chase your shadow (assuming you’re the subject of the picture).

So what else do you need to know before taking shots using 2nd curtain synchronization? Plenty. Such as why using high-sync speeds may cause problems if trying to capture a motion trail. Also the shooting mode to which you’ve set your camera may prevent you from using 2nd curtain sync.   Another consideration is the tone of your subject in comparison to the background and how much of the background or ambient light you actually want to record. You may want to record less ambient light than the shutter speed you set will produce. Knowing the inverse square law is helpful when you want to minimize ambient light.  Part 2 will cover these issues.