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Blur is often caused, not by movement of the subject, but by movement of the camera at the time of exposure. This movement, generally referred to as camera shake, cannot be classified as neatly as subject movement. It depends on a number of variable factors: the human element; the camera; the focal length of lens; the shutter speed.
Easily recognizable characteristics of camera shake (distinguishing it from other types of unsharpness) are an equal degree of blurring of all parts of the picture, and frequently the appearance of two slightly displaced and somewhat unsharp images. This also covers the whole of the negative area; the image appears as if it had been slightly smeared over the film or plate.
The above applies to cameras with a diaphragm shutter. Camera shake with a focal plane shutter used at the slower speeds appears similar. At faster speeds image positions may look discontinuous or distorted across the picture area. The Human Element. Some people can hold a camera steady at shutter speeds as slow as 1/10 second, others are troubled by camera shake even at a shutter speed of 1/100 second.
Part of the cause undoubtedly lies in the way the camera is held and how the shutter is released. The standard advice on holding the camera is to stand with the feet apart and steady the camera against the chest or face (depending upon the type of viewfinder). Before making the exposure some photographers find it helpful to take a breath, hold it and at the same time squeeze the shutter release steadily, rather than trying to click it erratically.
While this advice will do much to reduce serious camera shake it is always wise to avoid shake by taking advantage of any support available and using the fastest shutter speed possible.
The design of the camera makes it more or less prone to suffer from shake. Generally, large, heavy, and square cameras give the least shake, and small, light, and streamlined cameras, the most. But the position, angle, and release pressure of the shutter control may have as much influence as the size and shape of the camera, and a camera that is free from shake when used at waist level may be impossible to hold steady when using the eye-level finder.
Generally, the longer the focal length of the lens the greater the risk of camera shake. For the same amount of angular movement of the camera, the image formed by a 2x telephoto lens shows twice as much blur as the image formed by a normal angle lens. The Shutter Speed. Photographs taken at fast shutter speeds are less likely to show blur than those taken at slower speeds. But the amount of blur is not necessarily in proportion to the shutter speed. If the blur is produced (as it often is) by a jerk that takes effect towards the end of the exposure, it may be just as pronounced at 1/40 second or faster as at 1/20 second.
Avoiding Camera Shake
Camera shake is very often to blame for blur that is put down to misfocusing, subject movement, or poor lens definition. Many photographers habitually accept a lower standard of sharpness than the camera is capable of, because they do not realize that camera shake is to blame.
The remedy is to make a number of exposures with the camera rigidly clamped, say, to the top of a heavy table.
This will show the standard of definition that the camera will give when there is no shake. The photographer should then experiment to find the shutter speed and methods of holding the camera and releasing the shutter that will give this standard of definition for hand-held exposures.
Trials carried out with a large number of operators show that the slowest shutter speed that the average photographer can use without undue fear of camera shake is about 1/125 second.