Shutter speed and motion blur

After having learned about camera exposure in general and aperture, shutter speed is the next element we are going to discuss. Here, the naming itself is a bit confusing. The term doesn't refer to any speed at all.

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Understanding shutter speed

The setting denotes the duration, or the time, of an exposure. This is the time during which the medium in your camera, e.g. the sensor or the film, will be exposed to light.

The term is inasmuch misleading, as increasing the speed usually implies reducing the time. "Double shutter speed", however, might actually mean "twice the exposure time". Pay special attention to the context to avoid surprises.

The amount of light contributing to the exposure value increases proportionally to the time. Thus, doubling the time will result in the double exposure (increasing it by 1 EV, or 1 stop), and halving it will reduce the exposure by a factor of 2 (or by 1 EV, or 1 stop). The common settings include:

1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s, 2s, 4s

You can easily extend the sequence on both ends. Latest camera models are capable of exposure times down to 1/8000s.

As with apertures, it is common to show fraction numbers as inverted values, e.g. 125 for 1/125s. In such cases, durations over 1s are additionally accompanied by a sign for seconds ("). Thus, 4 will refer to 1/4s, while 4" will denote an exposure time of 4 seconds.

For a hand-held camera, the time of exposure is understandably limited by your ability to keep the camera stable. As a rule of thumb, this time in seconds is approximately equal to the inverted focal length in mm of the lens attached to the camera. For example, a 50mm lens without image stabilisation will still produce sharp images at 1/60s, while a 200mm lens will "keep quiet" just up to 1/250s. IS adds about 2-3 stops to this estimate, depending on the lens, allowing for times around 1/8s in the former and 1/30s in the latter case (verify if I'm correct!)

Time and blur

Exposure time has the most prominent effect on the appearance of moving subjects in the image. The faster your motif moves during the exposure and the closer it is to the camera, the more prone it is to motion blur.

This is quite easy to explain. A racing car moving at 250km/h (around 160mi/h) covers almost 70m every second. It means over 55cm (22") during an exposure with 1/125s. An image of the car taken by a steady camera with this setting will turn out rather blurry. Choosing a (much) shorter exposure and additionally following the car with the camera while actually taking a picture will ensure its sharp rendering.

Moving the camera during the exposure will also add blur to the background and additional dynamic to your image.

4s long exposure made the pace of time visible in this image
Wikimedia Commons

Sport, action and industrial photography are the most common areas where "frozen" movement is the usual form. By contrast, many landscape photographers seek to introduce motion blur to their images of moving clouds, flowing water, or falling snow. Slow shutter speeds aka long exposure times required to achieve this effect call for a tripod under your camera. If you'd rather freeze your waterfalls, choose exposure times below 1/250s.