Depth-of-field adapter

A depth-of-field adapter (often shortened to DOF adapter) is used to achieve shallow depth of field on a video camera whose fixed lens or interchangeable lens selection is limited or economically prohibitive at providing such effect. A DOF adapter could theoretically be used on a multitude of platforms, although it is most useful on prosumer digital camcorders where high resolution is a capability but the sensor size is still small enough to elicit use of the adapter. The term 35mm adapter is common, since most designs use a focusing screen the size of a 35mm film frame (24×36 mm) and interface with lenses designed for 35mm cameras. The use of adapters has decreased largely due to the video function available on newer DSLR cameras.

A DOF adapter focuses an image onto a translucent screen (similar to how one would look at a focused image through a system camera's viewfinder) located between an external lens and the camera's main lens. The camcorder is able to frame this intermediate screen by focusing in macro mode. The principle is similar to pointing a video camera at a movie screen. The lens attached to the adapter now takes the job of the camcorder's focusing and aperture mechanisms. The camcorder's only responsibility at this point is to record what is being projected onto the focusing screen (called back focus.)

A DOF adapter comes with a few limitations. First is the inherent light loss that comes with attaching the unit to the front of the camera. Using any one of these adapters requires that the scene being shot by the camera be adequately lit.

Static (non-moving) adapters suffer greater image degradation from low-light situations because texture on the focusing screen becomes more noticeable. The camcorder used in conjunction with the adapter must focus on the focusing screen inside the adapter which is used as a projection surface. As a result, the camcorder also picks up the pits, dimples and/or specks in the material that give it its translucent properties. The solution to this problem is to shake, rotate or otherwise move the focusing screen so that the texture of the screen is blurred. In a non-static solution such as this, the texture is only a problem at very high shutter speeds, where blurring is reduced.

Vignetting, barrel distortion and chromatic aberration are also problems that are difficult to eradicate. The optics within the adapter must be of high quality and at the correct distance from each other in order to minimize these issues.

...
Wikipedia