The camcorder is a video camera and a video recorder (VCR) in one. It has three major components: the image sensor, the lens system and the recording part.
Before the camcorder all cameras were installed with a special camera tube, such as the Vidicon, the Saticon and the Newvicon. This meant they were big and heavy and consumed a lot of power. They could not cope with intense light sources as these caused 'streaks' on the image. Glaring lights or clearly lit objects became part of the image, and remained there into the next shot.
Charge-Coupled Device (CCD)
In the modern camcorder the tube has been replaced by a special chip; a semiconductor image sensor. This image sensor is called a CCD (Charge-Coupled Device). The CCD chip is a round plate of about 1/2" or 2/3" diameter in small camcorders. The CCD image sensor is made up of between 320,000 microscopically small light-sensitive elements in standard camcorders and 500,000 in high-end and professional camcorders. These elements are placed on a surface of no more than one square centimeter. The image for recording is projected via the lens on the CCD chip, then converted to an electric signal, which is the video signal. The image projected on the CCD chip charges the cells electrically. The brighter the incoming light, the more powerful the charge which is how a charge image is created. This charge image is then converted into an electric current or signal which corresponds to the light and color information.
To gain a better understanding of this process, we will now have a look at the process in the old camera tube. In this tube, the conversion takes place by means of a fast-moving electron beam which scans the entire image field of each cell (in the same way as a TV tube). In the camera tube the electron beam picks up only as many electrons for each cell as it needs to neutralize the charge caused by the light. In this way the original constant electron power changes into a current that varies accurately along with the information on the photo conductive coating. Thus, a video signal is generated.
For CCD chips, this process is a little different. Instead of a flashing electron beam, the process takes place via a second coating. Every 1/50th of a second the complete charge image is transferred to the second layer in an instant. Meanwhile, in the next 1/50th of a second the next image is built up in the charge carriers, the cells of the second field transmit their charges one by one, resulting in a continuous electric current. Power and current direction correspond accurately to the charge, and thus the light information generating a video signal. The video signal consists of black and white and color information (Y and C signal). The black and white information (luminance signal Y), consists of the three primary colors: 30% red, 59% green and 11% blue. (For more information see Chapter 2: the TV Set.)
In professional tube cameras these colors are generated via prism systems, after which three camera tubes or recording elements take care of the rest of the processing steps. In standard camcorders only one recording element is used which is the CCD chip. Color filters are installed in front of the CCD chip, which separate the image into the three primary colors. A complicated matrix circuit generates two color-difference signals from the three primary colors, which are ultimately interwoven with the black and white information into the final video signal. To achieve better color reproduction, some professional camcorders have three CCDs, one for each primary color.
Color separation in professional cameras.
The main advantages of the CCD image sensor is that it keeps the camcorder light and small. The CCD consumes only little power, is always ready to use and offers excellent image quality. The picture sharpness is high, giving good color images on conventional TVs. It is also more shock resistant than a camera tube, and so CCD cameras can stand rough handling. CCD image sensors have a high light sensitivity: they offer good image quality at a light power of 10 to 15 Lux, which equals light that is emitted by a single candle at a distance of 20 centimeters. An additional advantage of the CCD chip is that it is not sensitive to streaks, blurring or burning. A CCD camera can be directed towards a heavy light source, such as the sun, without burning spots, comet-like streaks or color stains on the images.
Color separation by means of a color filter as used in camcorders.
The CCD chip is the heart of the camera, but the lens is the key to the performance of the camcorder. The quality and price of a camcorder are largely determined by the quality and characteristics of the lens. Lens-related subjects which are covered here are diaphragm, focal distance, lens types, depth-of-field and autofocus.
As in a photo camera, the lens of a videocamera is provided with a diaphragm control which regulates the amount of light to go through the lens by adjusting the lens opening. In a way, it works in the same way as the pupil in the human eye. If too much light comes through the lens, a picture will be overexposed. When too little light comes through, a picture becomes underexposed and therefore too dark. For example, if one makes a video outdoors in the bright sunlight, there is so much light that it does not require the whole lens to be open. The lens opening can be adjusted, so that less light comes through. In the opposite case, for example, indoors on a gloomy day, the incoming light can be increased by opening the lens wider.
The diaphragm can be adjusted in different 'steps' or 'stops'. These steps in lens opening are called the diaphragm values standardized internationally as follows:1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22
The diaphragm values represent the ratio between the diameter of the lens and the focal distance. The higher the diaphragm value, the smaller the lens opening. With diaphragm value 1 the lens is completely open. With value 22 the lens is almost closed. Every next number is 2 times larger than the previous number, causing every higher or lower step to have a light absorption that is precisely twice as small or big. In photo cameras these diaphragm values are usually indicated on the lens, but this is not the case on videocameras. This is because a videocamera is provided with an automatic diaphragm controller, something which in practice is not only a welcome tool, but may even be considered to be a prerequisite for trouble-free video recording in changing light conditions. In many cases the automatic controller can be switched off, by pressing the button IRIS.
In camcorders, the diaphragm automatically adjusts itself to the light situation. Left: open diaphragm (value 1). Right: almost closed diaphragm (value 22).
Being able to switch off the automatic diaphragm controller can be of use in case of backlight recordings, which, with normal diaphragm would be too dark and non-transparent. In this case the lens needs to be opened one step further, so that more light shines on the recording element. To facilitate this backlight compensation many camcorders are provided with a special button for backlight correction (Back Light).
Sometimes a negative rather than a positive correction is required, for example when filming through a small gate. Without correction the gate would be recorded with all its details, whereas the view behind it, which you really wanted to record, would be overexposed. In this case the diaphragm needs to be reduced one stop. This form of backlight compensation is called High Light correction.
A manual diaphragm has great value if you want to fade in and out at the beginning and ending of a recording. In the opening shots the image will grow out of the dark and at the end it will disappear in the dark again, to give a professional impression. Some camcorders have a special FADER feature that automatically fades in and out in this way.
Every lens has a focal distance (f). It is the point at which light beams emitted by the lens come together and project a clear image on the image sensor. The focal distance is always mentioned on the lens, e.g., f = 50 mm. The larger the focal distance, the larger the images will be displayed. In a videocamera we talk of much smaller images (1/2" or 2/3"), than in the early plate cameras. As a result, videocameras have a relatively small focal distance, something like 9 to 12 mm, whereas the early 9x12 cm plate camera had a standard focal distance of 12 cm (120 mm), the 6x6-camera 75 mm and the miniature camera 50 mm.
Focal distance is the distance between the lens and the focal point.
Showing the relationship between focal distance and image angle. The smaller an image angle, the longer the focal distance.
The Telephoto Lens
A normal focal distance is approximately equal to the diagonal of the projection field. This projection field is in a photo camera the negative and in a camcorder the CCD-chip. By taking a lens with more focal distance than the diagonal of the projection field, the same field will contain a smaller part of the image. This is what a telephoto lens does: it decreases the image angle. The effect on the image is that it seems as if faraway objects get closer, but simultaneously you get the impression that everything is compressed and the perspective fades away.
By using a lens with a shorter focal distance, much more is displayed on the image field. The image angle gets bigger, as we work with a wide-angle lens. When the angle is extremely wide, say 180 degrees, we call it a fish-eye objective, which leads to a typical circular image.
To be able to work with lenses with various focal distances, some camcorders are provided with a so-called C-Mount-vatting (24.4 mm diameter - a common feature on photo cameras) that allows you to change lenses on your camera. Most camcorders, however, are equipped with a zoom lens.
The Zoom Lens
The zoom lens is a lens that allows you to change the focal distance of the lens. This type of lens is installed in almost all camcorders. The zoom lens is a simple way to go from tele-(far) to wide-angle (close-up to macro), making use of all the stops. This can be done manually via a handle or with a built-in electrical motor (motor zoom). There are zoom lenses with a range of 6x, 8x, 12x, but sometimes they do not go any further than 3x.
The zoom range is determined by the ratio between the two outermost focal distances. If they are, for example, 9 and 54 mm, then the zoom range is 6x. If they are 12 and 36 mm, then the range is 3x. This last one is rather limited to be of use in practice. Basically, a zoom range of 6x is the lowest useful zoom, whilst a 12x range only makes sense if you attach a very stable stand to shoot rock-solid pictures in tele mode. Applying a good stand when filming is always advisable, but when using 12x zoom-in it is a necessity. Practically all zoom lenses have a special macro mode. To use it, the object you want to film needs to be very close (at a distance of 5 cm of the lens) to be recorded sharply. This feature enables you to make screen-wide recordings of very small objects such as stamps, or insects.
The diaphragm does not only determine the amount of light that is absorbed, but also the amount of depth-of-field. If the lens is fully opened, only the object on which the camera is focused is reproduced sharply. The more the diaphragm is opened, the more depth-of-field extends forward and backward. This means that in the case there is little light, and the lens is wide open, only limited depth-of-field can be expected. Depth-of-field should not be confused with sharpness! An image is always sharp (if you have focused accurately on the object), whether or not there is acceptable depth-of-field. In case of extended depth-of-field, there is increasing sharpness from the front areas to the back areas. The center of sharpness, i.e., the focus, is not situated precisely in the middle of the sharpness area. It is situated at 1/3 of the front sharpness border. As a result it is much more difficult to get greater depth-of-field in recordings of close situations than of faraway objects. Therefore, the closer the recording, the more accurately one needs to focus.
Depth-of-field is also related to the focal distance. The longer the focal distance, the smaller the depth-of-field and vice versa. (In other words, the wider the image angle, the greater the depth-of-field). In the telephoto mode the depth-of-field is smallest and must therefore be focused most accurately. Because depth-of-field increases when shifting towards wide-angle, the image will remain sharp during the whole process. Only in the outermost wide-angle situation might it be necessary to do some adjusting, but this has to do with lens quality rather than the focusing.
In summary, the greatest depth-of-field occurs at wide-angle and a small lens opening.
Every camcorder features automatic focusing, or autofocus. Autofocus works almost like the human eye, but not as fast, although some advanced systems can almost compete. How does autofocus work? How can a lens determine what to focus on? Not only is it necessary to precisely measure the distance to the main object, but it also needs to determine what the main object is. Then, information needs to be transferred instantaneously to a motor which needs to bring the lens into the correct position just as fast. To achieve this, there are two basic systems, active and passive.
Active Autofocus Systems
Active systems are called active because they send out a signal in order to measure on what to focus. Until recently, many camcorders came with some sort of infrared (IR) autofocus system, and (less applied) an ultrasonic system. Both systems use the principle of sending out signals in pulses. These signal are reflected back by objects and registered by the receiver. Then, the time difference is measured between the outgoing and incoming signal, which allows the system to calculate the precise distance. It is a kind of radar system and as far as the ultrasonic system is concerned, it is the same principle that is used by bats. Because sound waves travel slower than infrared light waves, this means that the ultrasonic system functions slightly slower than the infrared system.
I do not know of any electronic IR time-of-flight systems used in consumer camcorders. Timing to a fraction of a nanosecond would be required even today. Most used an IR triangulation system whereby a beam of an IR LED is emitted from an aperture on one side of the camcorder projects a spot onto the "scene". The spot was tracked by an IR detector on the other side of the camcorder. The angle between the emitter and detector then depends on distance, so a mechanical linkage could be used to adjust the lens focus without additional electronics.
Infrared system with time measurement. The time difference between the outgoing and incoming beam determines the distance. IR measuring is not influenced by windows, but sometimes it is by shiny and dim spots and other IR sources.
The advantage of the IR autofocus is that it can also be used when there is little light or no light at all and that it is rather fast. But there are some disadvantages. The IR autofocus system is sensitive to infrared light, so also to other sources that emit infrared light, such as open (camp) fires. infrared light simply goes through glass. This might be welcome if you want to film through glass, but a shiny part of the window can reflect the IR beam, so that it will focus on the window. The IR beam could be absorbed by black dim surfaces, causing the lens to be set automatically to infinity. When the distance is calculated, the lens is adjusted to the right position by the lens motor.
Passive Autofocus Systems
All camcorders currently manufactured have some sort of passive autofocus system which has replaced the infrared and ultrasonic systems. These camcorders do not transmit any light or sound themselves, but only react to incoming light. The basic principle is that the CCD image sensor will try to create an image as sharp as possible. Because the sharpest image possible requires the highest possible frequency, the system tries to find the highest frequency. Either the lens or the CCD chip moves swiftly back and forth to find out at what position it can register the highest possible frequency.
But how does an autofocus system determine at what point to focus? In general it will take the object that is situated in or near the center of the image field. In active systems this is realized by focusing the light or sound beam as sharply as possible, and in the case of passive systems, to focus the receiving optics as accurately as possible. In case of doubt, for example when two adjacent objects are near the image center, the autofocus will always take the nearest object. If the video film maker wants to choose the other object, he or she can always switch off the autofocus and focus manually.
The recording part of a camcorder works basically in the same way as the recording/playing part of the VCR. The main difference lies in the fact that a camcorder has smaller components and double the number of heads. For further information on how the recording part works, see Chapter 3: the Video Recorder. At present, there are five types of camcorders that use different recording systems and a sixth one will be coming soon.
Currently, there are five non-compatible camcorder types that use different recording systems. They are divided into two families: VHS and 8 mm.
The VHS movie or VHS camcorder is designed to be used with standard VHS-cassettes, and therefore is part of the large-sized camcorder category. This camcorder allows for the longest playing times, up to four hours when applying the E-240 cassette, although shorter tapes (E-180, E-120, or E-60) might turn out to be more practical. Including the battery and the cassette, the weight of this type of camcorder is over two kilos.
Smaller camcorders weigh much less, something like 1 to 1.5 kilos, but lighter is not always better. It is much more difficult to make stable recordings with a small, light camcorder, than with a heavier type that is carried on your shoulder. However, if you want to take your camcorder along on a trip, a small, lighter device is easier to transport. As a normal VHS cassette is used, the recordings can be played back immediately via any VHS recorder. Due to their size and weight, VHS movie camcorders are hardly used amongst consumers in Europe anymore.
To be able to use smaller cameras and still apply the world's most used recording system, a smaller VHS-version was designed: VHS-Compact (VHS-C). The width of the videotape remained the same (1/2" or 12.5 mm), as did the tape speed and the tape recording speed: only the size of the cassette became smaller. As a result a VHS-C tape can be played back in any conventional VHS-VCR, using a special adapter. There are also video recorders with a compatible loading system in which both cassette types, VHS and VHS-C, can be applied without having to use adapters. As the cassette is much smaller and the tape much shorter, the maximum playing time is 45 minutes in SP-mode (Standard Play). It is also possible to record at half the speed (LP - Long Play, or EP - Extended Play), making the maximum playing time 60 minutes. However, this can only be done at the expense of image and sound quality.
The Super-VHS(C) system is an improved VHS-C system. As the resolution now is 400 lines, compared to 240 lines for the VHS and VHS-C systems, the image is much more detailed. It is now possible to edit and copy tapes without visible quality loss. Only after copying from a second copy recorded tape, do slight quality reductions become noticeable. A disadvantage of this system is that it cannot be played back via conventional VHS recorders. It is only possible to play back via S-VHS domestic recorders, using a VHS-C adapter. If there is no S-VHS domestic recorder available, S-VHS(C) tapes can only be played back via your camcorder or copied on VHS cassette, which means loss of the high quality. S-VHS(C) camcorders produce high quality recordings, especially the hi-fi stereo sound versions.
Video-8 is a completely different system which uses an 8 mm videotape. The small tape is considered to be a prerequisite to achieve a truly small and handy product. Although 8 mm tapes cannot be played back on a VHS-VCR, they can be copied to VHS and S-VHS format tapes. The Video-8 system, which was developed specifically for portability purposes, offers better image and sound quality than the VHS-C system. The Video-8 tape also has longer playing times than the VHS-C tapes, 90 minutes compared to 45 minutes (VHS-C). In LP mode, it even offers 180 minutes, although a loss of quality results.
Just as the Super-VHS(C) system is an improvement on VHS-C, Hi8 is an improvement on the Video-8 system. As in the Super-VHS system, all improvements relate to increased bandwidth and better separation of color and brightness signals, Hi8 goes further by applying a specially developed Metal Evaporated (Metal-E) tape, ensuring that the image quality is even better than in Super-VHS(C). However, Hi8 recordings cannot be played back via standard Video-8 equipment.
DVC - Digital Videocassette
A digital videocassette standard is, at the time of writing, being developed and to be introduced as a new standard for home video. The tape size will be small, and the system will have the quality features of digital recording: high quality performance in sound and image.
Sound Recording - The Camera Microphone
All camcorders come with a built-in microphone, to allow you to record the sound together with the image recording. As a result, sound and image are perfectly synchronized. The built-in microphone is usually surround sensitive and records surrounding sounds, which sometimes results in the recording of unwanted disturbance. A separate direction-sensitive microphone can be used to avoid this from happening. Sometimes this comes as a built-on microphone, but in most cases there is a microphone socket available to use for an additional microphone. A telescopic microphone is a microphone that can be enlarged enabling the microphone element to get closer to the source. Even though such a microphone leads to better results than the conventional built-in microphones, it is far from perfect. Recording images and sounds are two different things. Filming usually does not take place too close to the object, whereas the microphone needs to be positioned as near to the source as possible. Consequently, professional video making always requires (especially when you are, for example, interviewing people) a separate microphone. It is for this reason that the camcorder is provided with a separate microphone connection. This can be used to either plug in the built-on microphone or separate microphone with connection cord. This cord should not be too long, as it will cause loss of quality and additional humming and noises. To be able to hear what is recorded, the camera is supplied with a separate ear telephone connection.
Camcorders come with a wide range of extra features to make filming easier, better and more creative. As the list would be almost unending, only the more common features are described here in alphabetical order:
Audio dubbing makes it possible to add sound afterwards to any recording, without affecting the original image. This takes away the worries of recording in places with a lot of noise or any unwanted noises occurring during recording. After the image recording you can make the sound recording. Only the special soundtrack (mono) on the tape can be used for dubbing and not a hi-fi recording. The mono track is also present on hi-fi tapes.
Electronic Sensitivity Control
An electronic sensitivity control makes it possible to film even when there is hardly any light. However, when there is only little light, the resulting color quality will not be optimal. Besides standard High and Low modes, this sensitivity control often features automatic control, which is comparable to automatic diaphragm control.
High Speed Shutter
The High Speed Shutter enables the film maker to make recordings at much shorter intervals, e.g., 1/1000 second compared to a conventional 1/50 second. Thus, objects that move at high speeds can be frozen on a still picture. When played back with the still playback option, the image will be very sharp, without the usual blurred side effects. As a result of short scan times, the image will only become jumpy when played back in normal mode. As in conventional photography, short scan times require much light. In many cases the available light will turn out to be insufficient. Nevertheless, there are camcorders available with a wide range of short shutter times, from 1/150 to 1/10,000 second.
The Insert feature allows you to insert additional images in an existing recording. Thanks to the so-called flying erase head, it is possible to add some variation to a long-winded scene. Some frames will be lost to allow this insertion, because for every new recording the tape is rewound just a little bit and erased over that same area. In order to ensure the relevant images are not lost, one should continue recording for a couple of seconds when making the original film.
By pressing a button the recorder part plays back the last couple of recorded images via the viewfinder, after which the camera is automatically switched to record pause. The next recording will then follow on seamlessly.
The stand-by switch saves a lot of power. In stand-by mode it is not possible to film, as the viewfinder does not function. However, the tape is in place, the camera is ready to start and one press of the button will activate the camera.
As camcorders have become smaller and lighter in weight, it has become more difficult to shoot a steady picture. Steady Shot is a feature that allows you to make recordings which result in a more steady picture. Optical Steady Shot works on the principle of counter movement of the camera optics to the camera itself. Electronic Steady Shot works on the principle of extra field on the CCD chip for vertical corrections, and a special memory module for horizontal correction.
As one would like to see what is being recorded, every camcorder comes with a viewfinder. Most commonly used is the electronic viewfinder. This is a complete, small image monitor, a mini-black and white or color tube/LCD screen, with a screen diameter of approximately 2 cm. The viewfinder also gives information, via LEDs or in digital figures, on start and stop, under- and over-exposure, white balance, battery status, recording duration, recording time and date, and sometimes on titles that can be entered and recorded on tape. Immediately after the recording session the viewfinder can be used as a monitor to display what is recorded. Sometimes the viewfinder can be removed from the camera. In that case the recordings can be watched via an extension cord. Often there is a an eyecup attached to the viewfinder and for film makers that wear glasses there is a dioptrical eye adjustment (Eyepiece Corrector Control). Another way to see recordings is via the TV set, which then serves as a monitor.
The viewfinder with on-screen information.