Flash Technique – The 1st and 2nd curtains explained

A DSLR camera gives the option to fire the flash on the 1st or 2nd/rear curtain, which can produces different photographs in the same conditions.  This article explains what it means, and how to use it.

Explanation of the 1st and 2nd curtain

The curtains we are talking about are the curtains of the focal plane shutter, present on most DLSR camera.

Focal plane shutter - image from Wikipedia
Focal plane shutter – Photo courtesy of Wikipedia

The shutter mechanism consists of 2 curtains in front of the sensor. Prior to take a picture the 2nd curtain is open and 1st curtain is closed so that no light can reach the sensor. To take a picture, the first curtain opens which exposes the sensor to the light. At the end of the exposure time, the 2nd curtain closes and blocks the light again. Assuming that the duration of the exposure is significantly longer than the duration of the flash strobe (typically 1ms), it is possible to fire the flash either at the beginning (1st curtain) or at the end (2nd curtain) of the exposure.

Difference between an exposure with flash on 1st and 2nd curtain

Difference between an exposure with flash on 1st and 2nd curtain

Influence of the 1st and 2nd curtain on a photograph

Now let’s see what differences it makes on a picture when setting the flash on the 1st or 2nd curtain. The series of pictures below shows a tennis ball falling down, captured with different settings of flash and ambient light.

  • The trails are caused by the ambient light, independently of the flash settings (Pic 1, 2, 3)
  • The flash freezes the movement independently of being on 1st or 2nd curtains (Pic 2,3,4,5)
  • The flash on 1st curtain captures first the freezing of the flash, and secondly the trails from ambient light (Pic 2)
  • The flash on 2nd curtain captures first the trails of ambient light and secondly the freeze from the flash (Pic 3)
  • If you want a sharp picture with no trails from ambient light, underexpose the ambient light or use a fast shutter speed, independently of using 1st or 2nd curtain (Pic 4,5)
Influence of flash on 1st and 2nd curtain, with or without ambient light

Influence of setting the flash on 1st and 2nd curtain when photographing a falling ball, with and without ambient light

 Recommendations

When you want to capture trails from ambient light in a way that looks natural, set the flash on the 2nd curtain. You can see on picture 2 that the ball seems to go up with the flash on 1st curtain which is not its natural motion. On the picture 3 the ball seems to fall down thanks to the flash on 2nd curtain.

If you do not want to see trails from ambient light you have to adjust your ambient exposure accordingly and in that case it is better to set the flash on the 1st curtain. The flash on 1st curtain fires when you press the shutter, but the flash on 2nd curtain fire after a delay and may cause you to miss the shot.

Another reason to set the flash on the 1st curtain Is when using a TTL flash. When using TTL flash, a burst of pre-flash fires just before the picture is taken, it is used by the camera to calculate the power of the flash required for the picture. The TTL pre-flash is not noticeable when the flash is on 1st curtain, because both happen within a very short interval. But when the flash is on the 2nd curtain, the TTL pre-flash and the flash occur at a noticeably different moment in time, and 2 distinct flashes are visible. If you are unlucky the TTL pre-flash will cause your model to blink their eyes when the flash on 2nd curtain occurs.

Finally, if the shutter speed is as fast, or faster than the High-Speed-Sync speed , it makes no difference to set the flash on 1st or 2nd curtain.

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