Knowing what shutter speed produces the right effect for each picture is a skill you, as a Navy Photographer's Mate, must acquire. Your pictures may easily be spoiled by movement of either the camera or the subject. In some instances, this movement can actually improve your photographs.

Novice photographers often find it hard to believe anything can happen during the brief instant the camera shutter is open. This is not true; images can be blurred when a shutter speed as fast as 1/250 of a second is used; for example, when the camera or subject moves during the fraction of a second the shutter is open, the image may be recorded on the film as a blur. Blurring caused by camera movement is noticeable in all images within the photograph. When blurring is caused by subject movement only, the background or some other part of the scene will be sharp, and the subject blurred. Camera movement blur can be corrected by supporting the camera properly or by using a faster shutter speed. Subject image movement can be reduced by using either a faster shutter speed or by panning the subject.

As explained previously, when a faster shutter speed is used, a wider aperture is required to produce correct exposure. For this reason you should know what minimum shutter speed is required to stop or freeze different actions. You must take into account conditions that exist when taking photographs. Strong winds, vibrations, or a ship rolling from side to side must be considered. There is a general rule you must follow for determining shutter speed when handholding a camera. The slowest shutter speed recommended to prevent camera movement blur is to set the shutter speed so it matches the focal length of the lens. When a shutter speed does not exist for the focal length of the lens, select the next highest shutter speed; for example, 1/30 second for a 25mm lens, 1/50 second for a 50mm lens, 1/125 second for a 100mm lens, 1/250 second for a 200mm lens, and so forth.

When a subject is in motion during exposure, the image on the film also moves. Even though the duration of exposure may only be 1/1000 of a second, the image moves a small fraction of an inch during this time. The problem you encounter is how much image movement

PH3 Tim O'Neill 302.95

Figure 4-17. Panning with a moving object.

can be tolerated before it becomes objectionable and adjust your shutter speed accordingly. To determine what forms an objectionable blurring of the image, you must visualize how the photograph is going to be used. An image of a contact print can be much blurrier than an image that is magnified many times. A print that is viewed up close must be much sharper than a print viewed from a distance.

Once you know how the photograph is to be used, you can determine the shutter speed required to produce an acceptably sharp image. In some situations, it may not be possible to produce an image that is completely sharp. When you want a sharp image of a fast-moving object, use the panning technique. When using the panning technique, you move the camera and follow the action of the subject until you make the exposure. This method may blur the background but can provide a sharp image of a moving object even at relatively slow shutter speeds (fig. 4-17).

There are five factors that determine the distance an image moves on the film during exposure. You must consider these factors each time you photograph a moving object. These five factors are as follows:

1. The lens-to-subject distance

2. The lens focal length

3. The speed of the object perpendicular to the lens axis

4. The direction of movement

5. The exposure time

Whenever one of these five factors change, the distance the image moves during exposure also changes. The first four factors determine the speed that the image moves across the film. The fifth factor limits the time it is allowed to move, thereby limiting the distance of image movement.

Subject movement on the film plane is greatest when the subject is moving across the angle of view of the lens (perpendicular to the lens axis). For example, when the subject is moving straight towards or straight away from the camera, it may appear as though it is hardly moving and a fast shutter speed is not required to produce a sharp image; however, when that same subject moves at the same speed across the field of view of the camera, the speed of the subject appears much faster. A faster shutter speed is required to stop the action in this case.

The camera-to-subject distance also affects the amount of image movement at the film plane; for example, a car moving across your field of view at 55 mph from a distance of 700 yards appears to be moving slowly. The same car moving at 55 mph and only 15 feet away appears to be moving very fast; therefore, the closer a moving object is to the camera, the faster the shutter speed must be to capture a sharp image. When the subject is moving diagonally across your angle of view, movement is more apparent than when moving straight away or toward the camera, but less apparent than when moving straight across the field of view.

Remember, long-focal-length lenses exaggerate the effects of camera and subject movement, and short-focal-length lenses reduce the effect.

Experience and common sense are your best guides for determining shutter speed that will minimize image movement, but the following can be used as a guide to help make these determinations:

Double the shutter speed when the subject speed is doubled.

Halve the speed when the speed of the subject is halved.

Double the shutter speed when the camera-to-subject distance is halved.

Halve the shutter speed when the camera-to-subject distance is doubled.

Double the shutter speed when the focal length is doubled.

Halve the shutter speed when the focal length is halved.

When in doubt, use the next higher shutter speed.

There are mathematical formulas used to determine appropriate shutter speeds for subjects moving at all speeds when photographed with various lenses, but the use of these formulas is not practical. Table 4-2 shows stop motion relationships when a 50mm lens is used. This table is not intended to be memorized but is only intended to provide a better understanding of the relationship of subject motion, distance, and direction.