Clinical Research Associates

GRID DISTORTION PICTURE

A picture of a square grid gives a relative indication of distortion. It also shows areas of poor focus. Some cameras have permanently blurry perimeters, others have blur spots that come & go depending on the focus setting (position of the lens elements or CCD). Some cameras have mis-aligned CCD’s which result in rotated images on-screen.

ANGLE OF VIEW

Angle of view, expressed in degrees, defines the direction a camera "looks" relative to the long axis of the handpiece. A viewing angle greater than 90° (called "retro-view") makes its easier to get intraoral images, particularly in the posterior region. However, too large of retro-view can hamper extraoral imaging because visual alignment of the handpiece does not correlate with the image alignment.

HORIZONTAL & VERTICAL FIELD OF VIEW

The field of view, expressed as an angle, defines how wide an area the camera "sees". A large field of view allows the patient & operator to see more & stay oriented, however, there can be greater distortion. In the posterior region, a large field of view is an advantage since the camera is often forced close to the teeth due to the surrounding tissue.

DEPTH OF FIELD

Depth of field, expressed as a distance, is the range over which objects are in sharp focus. With a large depth of field, a camera is less sensitive to position & requires fewer focus adjustments. Fixed focus cameras usually have large a large depth of field to accommodate a variety of shots, but if an image is out of focus nothing can be done to correct it. Variable focus cameras usually have a shorter depth of field at any given focus setting, requiring the operator to adjust the focus more often, but most images can be brought into sharp focus. Depth of field is dependent on focus setting for variable & autofocus cameras.

MAGNIFICATION

Magnification, expressed as a multiplication factor, is how many times larger or smaller an object’s image appears on-screen compared to the real object. It is expressed as a range, dependent on the depth of field, over which objects are in sharp focus. Objects in the near field appear larger than objects in the far field. Variables which effect magnification include: 1) How close the camera is positioned to the object; 2) Focus setting; & 3) Size of monitor. Each camera was tested with the monitor supplied. If none was supplied, a 13 inch monitor was used.