LENS ASSEMBLY AND AUTO-FOCUS
Normally the lens assembly on the front of the camera has two functions – zoom and focus. The zoom function is carried out by a complex assembly of individual lenses, moved along the axis of the barrel at different rates when the zoom lever is rotated. A small electric motor, driving through a clutch, is incorporated to zoom the lens through its range of typically 20:1.
Focusing is manually adjusted by a ring at the front of the barrel or an ‘electronic’ control linked to a drive motor, and ranges from infinity down to about 0.5 m. Many cameras have a macro facility available in a click-stop at one end of the zoom control, permitting focusing down to a few millimetres.
All domestic cameras have an auto-focus system in which the focus ring is driven by a small electric motor built into the lens barrel assembly. The motor and lens form part of a servo system, a closed loop in which the image at picture centre is continually checked and adjusted for correct focus. Older home-use and some specialised cameras use infra-red or ultrasonic beams to measure the distance between camera and subject. A better system of automatic focus control is outlined in Fig. 6.12. The incoming light is split by a semi- transparent prism, and a small percentage is passed to an array of sensors which examine the centre portion of the picture. Twenty- four such sensors are present, each consisting of a micro-lens containing two CCD photodiodes. The information from these is analysed by a microprocessor which produces ‘drive lens in’ and ‘drive lens out’ commands to the focus ring drive motor depending on whether the focus plane is before or behind the subject. This technique, which includes the subject in the servo loop will work through closed windows
and via mirrors. Not all scenes are amenable to auto-focusing; sometimes the main picture feature is not central in the frame, and sometimes a degree of defocusing of some or all of the televised scene is required for production or artistic effects. For these reasons, and to conserve battery power where applicable, the auto-focus facility can be switched off.
The current trend is to use the video signal itself as reference for the auto-focus system as shown in Fig. 6.11. It generally gives more accurate results, and permits a choice of zone sizes for focus sampling.
VIEWFINDERS
The camera’s electronic viewfinder (EVF) has three main functions. It frames the shot for the operator and checks optical focus during shooting; it relays information from the camera’s system-control section on settings, status and operational mode; and it acts as picture monitor during in-the-field playback in the case of a camcorder. It is difficult to manufacture a very small colour screen with good enough colour fidelity to accurately judge the picture hue, or with sufficient definition to permit accurate optical focusing, especially with high- band and digital cameras.
The norm, then, is a black-and-white viewfinder tube of about 2.5 cm diagonal mounted in a ‘chicken-leg’ housing hinged at its back end on the top surface of the camera. It has an eyepiece and viewing lens with focus adjustment. The display tube is necessarily a low- energy device with small deflection angle. As in larger picture tubes, magnetic deflection and electrostatic beam-focus systems are used. The VF tube is driven by what amounts to a complete monitor circuit, including video amplifier and output stage; sync separator; time- bases; and high-voltage supplies for the picture-tube, the whole being miniaturised and designed to operate from its own (typically 5 V) supply rail, derived from the camera’s own power supply via a stabiliser/regulator circuit. No external controls are needed, though brightness and beam-focus controls may be provided as semi- accessible presets. This complete independence of the EVF system is necessary to enable the viewfinder to perform its role as a video monitor during tape replay when the camera section is switched off. Although physically very small, the components and techniques of the EVF are just the same as are used in the TV receiver and monitors covered in the first half of this book.
Colour EVF
The fact that colour viewfinders struggle to do justice to the performance of a good video camera has not prevented manufacturers incorporating them in home-movie camcorders! A few models offer the best of both worlds, with a conventional black-and-white VF tube plus a small (4–10 cm diagonal) colour LCD display in a fold-out panel on one side of the camcorder body. In the Viewcam the rear panel of the camcorder consists of a colour LCD panel of 8–10 cm diagonal. While suffering from the above-mentioned shortcomings as a camera viewfinder, it does have the advantages of not needing to be held to the eye while shooting, and of affording more than one viewer (with difficulty!) to watch the playback on location. A third class of camcorder sports a conventional viewfinder housing, but it contains a mini-LCD panel in place of the little monochrome tube, again with an eyecup and lens. This stretches the cost versus perform- ance of LCD technology tight, and the ‘chicken-wire overlay’ effect on the VF image can be very obtrusive.