Depth-of-field adapter
From Wikipedia, the free encyclopedia
A shallow DOF adapter is used in conjunction with digital video camcorders to achieve optical film-like shallow depth of field with manual- or auto-focus lenses on a camera whose CCD size is not adequate enough to achieve it natively. A DOF adapter could theoretically be used on a multitude of platforms, although it is most useful on prosumer cameras like the Canon XL H1 where high resolution is a capability but the CCD size is still small enough to elicit use of the adapter. The name 35mm adapter is perhaps more common, since the current designs use a focusing screen the size of 35mm film, but will not be used here since it is theoretically possible to create an adapter of a different size, such as medium format.
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[edit] History
The 35mm adapter started on a professional level with the development of the Mini35 by P+S Technik in Munich, Germany. Since then, many companies have mimicked the idea, such as Cinemek's Guerilla35, Redrock Microsystems' M2, SGpro, indie35, and the El Da Vinci DiGi35 similar adapters available for a fraction of the price of P+S Technik's product.
The idea of a depth-of-field adapter has also been developed on the consumer level, originally by a man named Agus Casse who posted his progress in building an adapter for his camcorder on DVinfo.net Alternative Imaging forum. His development of the Agus35 led to the consequent development of numerous other adapters of varying designs ranging from spinning plastic CD adapters to static microcrystalline wax adapters, the later being pioneered by Frank Ladner and Jim Lafferty. Richard Mellor was one of a few to post a tutorial and parts list outlining the construction of a basic static ground glass adapter. Today, a few members of the DVinfo.net forum sell their own version of a 35mm adapter, such as Quyen Le and his Letus35 which is based on a vibrating design, and Wayne Kinney and his SGpro which follows the P+S and Redrock design and uses a rotating disc as a focusing screen. Dan Diaconu of Easy Focused Lens has also contributed his MPIC (Mighty Pico Image Converter), another shallow DOF adapter design. Tony Relph's El Da Vinci DiGi35 adapter comprises of a chemically engineered static lens as opposed to the rotating and vibrating options. Tõnis Liivamägi from CINEDOF developed the leaf spring biaxial oscillation principle to reduce visible grain often associated with shallow DOF adapter footage.
[edit] How it works
Possibly one of the most impressive characteristics of a shallow DOF adapter is its apparent simplicity. Since most digital camcorders available today have CCD receptors that are too small to facilitate professional depth of field (an XL2 has three chips just 1/3" in size each), a shallow DOF adapter looks to take the place of the camera's CCD and use a larger focusing plate (in many cases, 35mm) to capture an image. Since this image is focused onto a translucent screen (similar to how one would look at a focused image through an system camera's viewfinder), the camcorder is able to frame this intermediate screen and record it. The principle of one of these adapters 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, as the camcorder's only responsibility at this point is to record what is being projected onto the focusing screen (called backfocus).
[edit] Limitations
A shallow 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 grain from the focusing screen becomes more noticeable.
Visible grain can also be a problem with shallow DOF adapters. 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 camcorder is unable to see the grain. In a non-static solution such as this, grain is only a problem at very high shutter speeds, as the properties of motion blur that the adapter relies on to eliminate grain begin to dissipate. Even then, the speed of the moving focusing screen can be increased to counter this problem.
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.
Image orientation is also an issue. The adapter, like any other lens, flips the image upside-down. However, when used in conjunction with another lens, the image remains upside-down, which must be reoriented in post-production, or flipped through the use of a prism system, which increases light loss. Another common solution to help overcome this issue while filming is to use an external monitor, for example a small LCD-screen, and then mount it upside down. However, the footage must still be flipped in post-production.
[edit] Construction
A basic shallow DOF adapter usually has four components: a macro lens, an optional plano-convex lens (sometimes a fresnel lens), a translucent focusing screen, and finally a photographic lens of the user's choice. On camcorders without sufficient macro capability, a macro lens, preferably an achromatic doublet (to minimize chromatic aberration), is usually attached directly to the camcorder so that the camcorder can zoom in and focus on the focusing screen. Without the macro lens, certain camcorders would be unable to zoom in well enough to frame the entire focusing screen and still achieve sharp focus. The optional plano-convex lens is used to avoid vignetting in the image. The focusing screen is where the photographic lens image is projected. The screen must be placed at precisely the correct distance from the flange focal mount of whatever lens is being used (most popularly the Canon FD or Nikon F mount), referred to as the depth of focus. Finally, the front lens is either a manual or, in the case of the indie35, auto-focus lens that captures the image desired.
[edit] Price
The price of a shallow DOF adapter ranges from tens of thousands of US dollars for a professional solution to less than a hundred by using parts available from a local hardware store. The pricing of a shallow DOF adapter unit also depends on the material and technologies used. For instance, a relay lens system must be added if the adapter is to be directly mounted to a camera's body (bypassing the standard lens, as if on a Canon XL1 or XL2). Also, a prism system that is used on the P+S Technik that correctly reorients the image rightside-up adds to the cost.
