A Viewfinder Darkly

Photography tips and tutorials

Shutter Speed Explained

March 6 2017

by Philip Northeast

The shutter speed settings on a camera do more than alter the photograph’s exposure. It also influences the effect of camera shake and the look of moving subjects.

Shutter speed is how long the shutter is open allowing light through to the digital sensor (or film).  It ranges from fractions of a second through to minutes,  a typical shutter speed is 1/125, indicating the shutter is open for that part of a second.

camera shake

In this example, a fast shutter speed helped prevent any blur due to camera shake.

Shutter Speed and Exposure

Shutter speed is one element of the exposure triangle, along with the size of the lens aperture, and light sensitivity of the digital sensor or film.

The traditional shutter speed settings are in steps to double, or halve,  the time the shutter is open. For example, 1/250 of a second becomes 1/125 second for a slow shutter speed or 1/500 second for the next fastest speed. This is based on doubling or halving the amount of light reaching the light sensitive material. The standard range of aperture steps also halves or doubles the amount of light coming through the lens.

Moving subject

Using a slow shutter speed and the moving hand is blurred.

This relationship made it easy for photographers to calculate exposure before the introduction of computerised automatic metering systems in cameras.  Photographers could balance the effects of aperture and shutter speed on the image for the same exposure.  Current cameras where control is electronic rather than purely mechanical, photographers use fractional steps for more precise control of exposure.

How to avoid camera shake

If anything in the photo moves while the shutter is open the movement is recorded by the camera. The moving object is blurry depending on how far it moves while the photo is being taken.   Photographers can minimise  Blurred Motion in Photography by using a fast camera shutter speed. This shutter speed has to be fast enough so that the distance the object moves, while the shutter is open, is not discernible. The faster the object is moving then the faster the camera shutter speed needed to reduce any blur.

The right shutter speed varies for moving subjects. It depends on the speed of the subject, and the distance between the camera and the subject. The general principle is faster subjects need faster (or shorter open time) shutter speeds.   This is the technique for a sharp photograph of the subject stopped in time.

Portraying motion

Shutter speed choice is part of the photographic technique for showing a moving subject in a still photograph.  The problem with using a fast shutter speed to eliminate motion blur is that sometimes this does not convey the impression of a moving subject. For example, a racing car photographed with a fast shutter speed can look as though it is parked. Ironically, in this case, photographers use a slower shutter speed and move the camera to follow the subject.  Naturally, moving the camera produces blur in the images. But if the camera is kept pointed at the moving subject it will be sharp with a blurry background, suggesting movement.

sharp subject

Here the camera is moving to keep the subject sharp while the rest is blurry

Moving subjects are difficult to capture because there are so many variables. The answer is practice, practice and more practice

What is  Camera Shake

Camera movement while the shutter is open results in a blurry photograph.  Movement is inevitable when hand holding the camera. One way to minimise the blurriness in a photograph is to use a faster shutter speed.  Reducing the time the shutter is open reduces the effect of any camera movement.  This becomes a problem when using a telephoto lens due to extra weight and narrower angle of view. The general rule is the minimum shutter speed is 1/focal length of the lens. For a 250mm telephoto lens, the suggested shutter speed is  1/250 second or faster. This is not always achievable considering restrictions of aperture and ISO.  Image stabilisation systems reduce this restriction, and a common aid is a physical support such as a monopod or tripod.

blurry photo

This is an example of severe camera shake

In low light conditions, the restrictions imposed by aperture and ISO mean long exposure times or slow shutter speeds. This makes it preferable to use extra support such as a tripod to minimise camera movement.

Types of shutters

The characteristics of different types of shutters influence their operating speeds.

Focal Plane

This is the common shutter type in DSLRs and similar cameras and sits just in front of the focal plane, this the surface where the image is recorded. It is a two-part mechanical system that uses two moving “curtains”. The first curtain opens to allow light through to the light sensitive material (digital sensor or film) and the second curtain closes to complete taking the photograph. They are called curtains because they originally were made from fabric and worked like curtains. Now they are metal blades but still work the same way. At higher speeds, the shutter is never fully open. The closing curtain starts while the opening curtain is still moving. Effectively it becomes a moving slit. In available light conditions, the exposure system ensures an even exposure over the focal plane.

camera shutter

A closed Focal Plane shutter on a film SLR

This moving slit presents problems for flash photography. Electronic flash is a very bright and instantaneous. If the focal plane shutter is not fully open when the flash fires only part of the scene will lit by the flash, and the rest of the image will only receive the available light.  The fastest speed where the shutter is fully open is the shutter synchronisation speed. The camera waits until the first curtain is fully open before it fires the flash.

Leaf

Leaf shutters are similar to the aperture mechanism and are usually part of lens, rather than the camera body.  All the elements open and close at the same time, so there is no partial or slit effect of the focal plane shutter. This makes them preferred for cameras where flash is important as the higher shutter speeds are possible with this type of shutter.

Electronic

In film days all the shutters were mechanical. In the digital age it is possible to turn the pixels in the sensor on and off for the required time to act as a shutter. The problem is the speed of reading and processing the image data. As processing power increases this type of shutter is becoming more practical for larger sensors.  The normal restriction at this stage of development is that electronic shutters in DSLRs tend to act the same way as focal plane shutters. That is they can only read some of the information at any one time,  so they have that moving slit characteristic.    This is the shutter used by cameras for video and the moving slit results in a “rolling shutter” effect. For example, the rotating blades of a helicopter appear bent.  Despite these minor drawbacks, electronic shutters are the future.

Exposure

Shutter speed is still an important factor in taking a correctly exposed photograph. However, photography is the art of compromise and photographers must balance the demands of exposure against the need to control blur and moving subjects.

 

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