Shutter Speed

Shutter speed is one of the two main components of exposure. The total amount of light that reaches the sensor is the product of intensity and time. For the same exposure you can have lots of light for a short time, or less light for a longer time. Intensity is controlled by the aperture and time by the shutter.

Inside a camera are two blades or curtains that open and close to expose the sensor to light. How long they are open is called the exposure time or “shutter speed”.

The less light there is the longer the shutter must stay open. At night you may need exposures of minutes. But, normally we are dealing with small fractions of a second.

A typical shutter speed might be 1/60th of a second. Cameras display this just using the bottom part of the fraction. So 1/60th of a second would be shown as 60. A quarter of a second is displayed as 4. For exposures of a second or longer the camera use a little mark to indicate whole seconds, not fractions.

Common shutter speeds are 60, 125, 250, 500, and a 1,000. Note the doubling at each step. This relates to stops as we discussed in the episode on exposure.

These shutter speeds are one whole stop apart. In the days of film that was all we had, but now there are also third-stop settings between them. Such as 160 and 200 between 125 and 250. These allow finer control of exposure.

You will remember that Victorians had to keep very still when having their picture taken. This was because the shutter stayed open for a very long time and any movement would blur the image. This effect is called motion blur.

Any movement or shaking of the camera will also be visible in the image. You can reduce camera shake by putting the camera on a tripod, but motion blur will always be a problem. It can affect trees in a landscape or people walking, so you have to increase the shutter speed to compensate. It’s surprising how fast the shutter speed needs to be. For fashion photography, I try to use 320 or faster.

Camera shake is also affected by the lens. The longer the focal length the more it magnifies the shake. So the minimum shutter speed you can use is related to this focal length.

There used to be something called the reciprocal rule, which said that your shutter speed should be at least as fast as the focal length of your lens. Meaning that if you are using a 50mm lens, you should have a shutter speed of 1/50th second or faster.

But this was back when we were shooting on film and were less fussy about image quality. Even then Ansel Adams found that he needed a speed of 250 to get sharp shots.

The reciprocal rule also assumes you are shooting with a standard full-frame camera. If you are using a cropped sensor you need to apply the crop factor. A 50mm lens on an APS-C body needs a shutter speed 1.5 times the focal length, a 75th or faster.

The weight of the camera also makes a difference. Lighter cameras have less inertia to damp out movement.

At least we now have image stabilisation. Cameras detect shake and try to correct it by moving lens elements or the sensor. Some Olympus cameras allow shooting handheld at amazingly low shutter speeds.

The upshot of all this is that you need to do some tests to see what shutter speeds work for you. You should try some shots at slow shutter speeds to see how the result looks. That way you’ll know when the problem is happening to your images. This kind of blur looks different to focusing problems.

Shutter speed can be used creatively. A long exposure time will smooth out a waterfall or the surface of the sea. Or impart a ghost-like quality to moving people.

If you leave the shutter open long enough, people on a street can disappear completely. You can even eliminate tourists from an interesting monument. If you try it out you might find that even with the narrowest aperture available on your lens the shutter speed is too fast. So you can put something called a neutral density filter over the lens to reduce the light. Effectively it’s sunglasses for your camera.

For homework try shooting the same scene at different shutter speeds. See how low you can go before camera shake becomes a problem. If you have a continuous shooting mode try its fastest setting. Take a burst of pictures and see if any of them are sharper than the others

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That’s it for now.

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