For many participants in our flash courses, flashing on the second curtain is always a big issue. I have never really understood this. In my opinion, there are very few reasons why you should flash on the second curtain at all. As a rule, you are better off using the first curtain.
Two light sources
When flashing, except in complete darkness, we are dealing with two light sources: The ambient light and the flash light. So the exposure of the image is composed of two components. Depending on the combination of the parts (a lot of flash, a little ambient light, or a little flash and a lot of ambient light), the finished photo can look very different. And since the exposure is composed of two parts, the exposure on the first or second curtain can also make a difference. Can. But doesn’t have to.
Static subjects
If the subject is not moving, it makes no difference whether the flash is fired immediately after opening the shutter or shortly before closing it. The result is absolutely identical.
Moving subjects and short exposure times
With short exposure times, say shorter than 1/30 second, it also makes no real difference whether the flash is fired immediately after the shutter is opened or immediately before the shutter is closed. As a result, the photos will not differ, since most objects do not move fast enough to have traveled any appreciable distance within the image field.
Moving subjects and long exposure times
Ah, finally we get to the point. With long exposure times in combination with the flash, it really makes a difference whether you flash on the first or second curtain. This is because long exposure times capture the motion trails of moving objects. Without flash, the object is completely out of focus. If you flash additionally, then the very short flash provides a sharp image of the object in combination with the motion trails created by the long exposure.
In the image above, this can be seen very nicely in the person in the background, who moved a little during the exposure time of 1.6 seconds and thus left a motion trail on the photo.
And why does it now make a difference whether you flash on the first or second curtain? In some cases, the motion trails are supposed to follow the moving object, not precede it. The following two photos will illustrate this:
In this case, the flash was fired on the first curtain. The flash was therefore fired immediately after the cyclist moved into the picture. During the remaining exposure time of 2 seconds, only the light from the headlight and the red taillight are visible. However, the light trails seem to precede the bicycle. This does not meet our expectations.
In the following picture, we flashed on the second curtain.
Since the flash fired just before the cyclist left the picture, the light traces are behind the bicycle, as desired.
The small problem here is that the time of pressing the shutter release, the exposure time and the speed of the cyclist must be precisely coordinated so that the cyclist is exactly on the right edge of the image when the flash is fired. This can only be achieved under very carefully controlled conditions. And this is what most experiments look like:
Quite often you can see only the light traces and nothing from the bike… Various attempts were therefore necessary to get a usable picture. For example, if you want to try the flash on the second curtain at a sporting event, you need a lot of luck to get the desired result.
Therefore, whenever the direction of movement is not really important, you should use the first curtain. The big advantage is that you have control over when the flash fires and can therefore determine what is in focus. The movement trails are then more or less left to chance, but the chances of success are much greater.
In the shot above, the two dancers were long out of the frame at the end of the exposure. If the flash had been fired on the second curtain, it would have only illuminated the empty space in front of the camera. However, since the flash was fired on the first curtain, a nice composition could be achieved.
As always, please feel free to comment. And enjoy using your flash!
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