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Shutter speeds for flash

The role of the shutter in flash photography


Imagine this scenario. You are holding a camera in a completely-darkened room. You open its shutter and keep it open. Since there is no light, the film, or the image sensor if you are using a digital camera, will not be exposed no matter how long the shutter remains open. Suddenly, there is a brief burst of light - a flash - then the room goes completely black again. Since the shutter was open, the film/sensor was obviously exposed to the light, and a picture was made.

How important was shutter speed in making that exposure? It played no part at all, since the amount of light striking the film/sensor was controlled by the duration of the flash and by whatever lens aperture was selected.

The picture may have been overexposed, under-exposed or just right. The important point is that the shutter had no control over the amount of light striking the film/sensor.

Now, let’s imagine the same scenario - you with your camera in a completely-darkened room - only this time, you have set your shutter speed to 1/500 sec, a fast shutter speed, and your electronic flash is connected to your camera. You trip the shutter release. The shutter opens and the flash goes off. What do you think happens to the film/sensor in this case?

To help you answer that, let us go back and look in slow motion at what actually took place in the camera.

  • When you pressed the shutter release button, the shutter began to open, and was only partially open when the flash went off.
  • The duration of the flash was so incredibly brief that the shutter was still opening when the flash shut down.
  • So, only part of the frame was exposed.
  • Seems hard to believe, doesn’t it? But, the duration of electronic flash is really fast, around 1/1,000 sec. and faster.
  • So, you can see that, unless the shutter is already wide open when the flash goes off, only part of the film will be exposed. This means that the operation of the camera and the flash must be synchronized so that the shutter completely opens first, then the flash burst can occur to expose the entire frame. After that, the shutter can close.


    FLASH SYNCHRONIZATION

    The foregoing explanation was greatly simplified in order for you to understand the function of the shutter when film or a camera's image sensor is exposed by electronic flash. In practice, the shutter does not need to be kept fully open before and after the flash is activated; it just needs to be synchronized with the flash so it is open long enough for the light from the flash to make a complete exposure of the frame.

    Most flash pictures are not taken in completely-dark conditions. There is usually some degree of ambient light. It may be a single houselamp bulb or it may be bright sunlight. If the shutter is held open before and after a flash exposure, ambient light (particularly strong ambient light) could cause over-exposure. So, a shutter speed must be selected that (1) permits both the light from the flash to record properly on the film/sensor and that (2) controls the amount of ambient light reaching the film/sensor. This is where flash synchronization comes into play.

    We have already seen that a fast shutter speed of 1/500 sec with a camera won’t do the job. And we know that if we keep the shutter open for too long, too much ambient light will enter the camera and ruin the exposure. How about a one-second shutter speed? That will work for the flash, but one second may still be too long for bright ambient light, and it is certainly too long for hand-holding the camera. Okay, how about half-a-second? Sure, but you could have the same problem - probably too slow a shutter speed for hand-holding depending on the ambient light. You can see that the question becomes: "How fast a shutter speed can you use for flash?"

    The answer depends on the model of camera and the flash it uses, but it is safe to say that most cameras will synchronize electronic flash at 1/60 sec or slower shutter speeds. Some will synchronize at faster speeds, from 1/80 sec. to as fast as 1/250 sec. Generally, though, any faster than 1/250 sec., and the flash will expose only part of the film/sensor.


    “THAT’S NOT TRUE. MY 35 mm CAMERA SYNCHRONIZES AT VERY FAST SPEEDS”
    Ah, you caught us! Well, sort of. Some sophisticated, high-end 35 mm camera systems are seemingly able to break the rule. They have special flash units that communicate with the camera and can be controlled to emit light at an extremely rapid rate so the camera’s shutter can be operated at extremely fast speeds - as high as 1/4000 sec. - for flash pictures in very bright light. This is called “high-speed sync flash” and is not commonly-found with most popular camera systems. It works because there is more than one flash emitted while the shutter is open. The bursts are extremely-fast, one right after the other, like a super high-speed strobe light.


    The finger is pointing at the camera's shutter speed dial. All shutter speeds from 1/2 sec and up are indicated by white numbers except 1/250 sec., which is red - which tells you that this particular camera's fastest shutter speed for flash is 1/250 sec.
    The finger is pointing at the camera's shutter speed dial. All shutter speeds from 1/2 sec and up are indicated by white numbers except 1/250 sec., which is red - which tells you that this particular camera's fastest shutter speed for flash is 1/250 sec.

    HOW DO I KNOW MY CAMERA'S HIGHEST SHUTTER SPEED FOR FLASH?

    The best answer is to check your camera's manual under "Flash," "Exposure Controls," "Setting the Shutter Speed" or some similar heading.

    Don't have a manual any longer? Then, look at your camera's shutter speed dial for an "X" or a colored line between two shutter speed numbers, or some unusual marking other than the normal shutter speed numbers. One number may be in a different color than the others. (For example, the Nikon F5 camera has red numerals - shown in the picture at left - indicating that 1/250 sec. is the camera's fastest shutter speed for flash synchronization.) Odds are, the line, mark or unusual number signifies the highest shutter speed setting for flash synchronization. "X" is a definitive mark to which you set your shutter speed when you are going to use electronic flash. It is often called the "X setting" or the "X synchronization setting."

    If you can't find any such clues that provide your 35mm camera's highest shutter speed for flash synchronization, your camera dealer will be able to help. You may also be able to find a copy of your camera's manual on-line. In the meantime, set your shutter speed to 1/60 sec. or slower, and it will always do the trick.




    Further information...

    Flash synchronization explained