Vari-angle prism image stabilization delivers sharp images in portable HD lenses.
Creating stable and sharp images with an HD camera/lens combination requires that the optical image projected on the camera-image sensor remain spatially stable during exposure. Shooting video with a handheld or shoulder-mounted camera/lens can easily produce shaky pictures due to any number of inadvertent physical factors (i.e., shooting while walking or running, being jostled by a crowd, shooting from a motor vehicle, etc.). Shaky video can also result from plain old fatigue by the camera operator as well.
These visible tremors typically manifest themselves as a vibration frequency in the neighborhood of 1Hz. They will be compounded in amplitude, and extended in frequency, if the camera operator is walking or running while shooting. Even a tripod-mounted camera/lens can produce a shaky image if it's mounted on flooring, or a tower, that is subject to vibration, or if the lens-camera system is subject to a blowing wind. Shooting handheld while riding on a motorcycle's back seat, from within an automobile or from within aircraft or boats, can all introduce variations in vibration amplitudes and frequencies.
The basis of image stabilization is to restore the image — in real-time — to its correct spatial location on the camera image sensor system. Today, one can find three basic approaches to this: mechanical gyro-stabilized housings for the lens-camera system; electronic systems within the camera that move sensor readout or digital sampling of the video to counter the inadvertent displacement of the optical image on the camera sensor; and optical correction within the lens itself.
Optical stabilization for a portable HDTV lens
There were three motivations behind Canon's consideration of incorporating a built-in optical stabilization system into a portable HDTV production lens:
The very nature of HDTV — namely, large-screen portrayal of high-resolution imagery — will benefit from elimination of even a small degree of image shake that can blur that imagery.
The 2/3in HDTV image format is an internationally standardized lens-camera interface. Thus, any 2/3in HD lens can mount on any of the 2/3in HD cameras made by all of the world's professional camera manufacturers. A 2/3in HD lens with built-in image stabilization will remove image unsteadiness from any associated camera regardless of manufacturer. Using appropriate lens mount converters, the same 2/3in lens can also benefit the professional 1/2in and 1/3in camcorders.
By making the correction in the lens, both the main camera video output and the separate video portrayed in the camera viewfinder are stabilized.
The company has pioneered two primary approaches to built-in optical stabilization in a lens. In both, special sensors detect any physical motion of the lens, and, in turn, this electronic information is used to control an optical element so placed that it introduces a counter deflection to the original disturbed light rays, thereby restoring the lens output optical image to its intended spatial location on the camera image sensors. The two optical technologies are quite different and are respectively known as vari-angle prism image stabilization (VAP-IS) and shift-lens image stabilization (Shift-IS).
The new 2/3in HJ15e×8.5B portable HDTV lens incorporates optical stabilization based upon vari-angle prism technology. This article describes that technology.
Principle of vari-angle prism optical stabilization
The central concept underlying in-lens optical stabilization entails the placement of an optical element within the main light path of the overall lens system. This optical element has the ability to dynamically deflect light rays in a manner that counters the inadvertent deflection of those light rays caused by physical disturbances and vibrations. A wedge prism constitutes such an optical element.
Wedge prisms come in many forms. Figure 1 shows two classic variants on a fixed wedge prism.
A light ray passing through these glass prisms incurs a deflection of amount Theta degrees, which is proportional to the wedge angle Alpha. At the top of the figure, the design is such that the entrant light ray will be deflected down by a fixed angle Theta. The bottom shows a horizontally-oriented wedge prism that will deflect the light ray in the horizontal direction.
If the angle Alpha can be made variable — in both the horizontal and vertical directions — then a dynamic control of the main light ray deflection can be implemented. Designing a wedge prism that can be adjusted both horizontally and vertically in real time constituted the core design task in the development of the VAP-IS system. Before examining details of the entailed technologies, it is useful to first understand the basic correction mechanism offered by a variable wedge prism.
The correcting action of the VAP-IS system
The implementation of VAP-IS technology places the variable wedge prism at the optical input port of the lens system — directly in front of the focusing element group. Thus, the prism directly intercepts all of the light rays passing through the “entry pupil” of the lens. The manner in which the prism corrects for image shake or vibration is simply explained with reference to three sequential steps outlined in Figure 2.
The top image illustrates conditions when the lens-camera system is stable in terms of any physical motion. The Vari-Angle Prism is not moving, so it imparts zero deflection to these light rays. Thus, all of the light rays entering the lens system are focused and accurately positioned on the camera image sensor. The drawing shows the central light ray correctly positioned on the center of the imager.
The center drawing in Figure 2 shows the momentary condition at the instant there is a physical jolt, jiggle, or vibration to the lens-camera system. Immediately, the light rays projecting from the lens are displaced on the image sensor. Compensating for this displacement, however, are three core miniature electronic components that are built into the lens itself. They include motion detectors, a microcomputer and actuators. All are central to the VAP-IS control system.
When the light rays are bent, the motion detectors in the lens instantaneously signal the microcomputer that such a physical disturbance has taken place, and between them, they describe the nature of that disturbance. The microcomputer, in turn, makes high-speed calculations under control of a sophisticated algorithm and transmits a related control signal (via driver circuits) to a pair of actuators that are gripping the vari-angle prism.
As indicated in the bottom drawing of Figure 2, these actuators physically “squeeze” the variable prism with high rapidity. This action implements the desired variation in the prism wedge angle that introduces the instantaneous adjustment of the angle of refraction of the light rays passing through the prism. Optical position sensors continuously sense and report back to the microcomputer on the correction angle of the prism, thus closing a feedback loop that drives toward zero error.
Larry Thorpe is the national marketing executive of the broadcast and communications division of Canon.