Television Cameras: The lens system
Published by Emil Abraham,
A camera lens is an optical body that features a single lens or an assembly of lenses that mounts to a camera body. Some lenses and interchangeable, while others are built into the construction of the camera body. Modern lenses attempt to set the angle of incidence and angle of refraction to equal values to decrease the amount of aberration, and feature a focus element that allows the operator to dictate which portions of the image are acceptably sharp, and which portions are blurred.
Engraved on the front of every lens are two important numbers:
- The lens’ focal length—or in the case of zoom lenses, its range of focal lengths. This gives you a clue to the variations in shot sizes the lens will provide.
- The lens’ largest aperture or f-stop (e.g., f/2)— the smaller this f-stop number, the larger the lens’ maximum aperture, so the better its performance under dim lighting (low- light) conditions.
The term focal length is simply an optical measurement—the distance between the optical center of the lens and the image sensor (CCD or CMOS) when you are focused at a great distance such as infinity. It is generally mea- sured in millimeters (mm).
A lens designed to have a long focal length (long focus) behaves as a narrow angle or telephoto system. The subject appears much closer than normal, but you can only see a smaller part of the scene. Depth and distance can look unnaturally compressed in the shot.
When the lens has a short focal length (short focus) this wide-angle sys- tem takes in correspondingly more of the scene. But now subjects will look much farther away; depth and distance appear exagger- ated. The exact coverage of any lens depends on its focal length rela- tive to the size of a camera CCD’s image size.
Zoom range
The zoom range is the degree to which you can change the focal length of the lens (and thereby the angle of view, or vista) during the zoom. The zoom range of a lens is often stated as a ratio, such as 10:1 or 40:1. A 10:1 zoom means that you can increase the shortest focal length ten times; a 40:1, forty times. To make things easier, these ratios are usually listed as lOx (ten times) or 40x (forty times), referring to the maximum magnification of the image of which the lens is capable.
Zoom
Zoom is framing movement, in which the focal length of the zoom lens is altered to make the subject appear closer to, or further away from the camera. Note that this effect is similar, but not the same as moving the camera itself closer to or further away from the subject.
Digital zoom vs. optical zoom
There are two types of zoom on a video camera — digital zoom and optical zoom. A camera can have either or both types. The two different types are very different and the unwary buyer can get caught out badly by not understanding how they work.
Digital zoom works by magnifying a part of the captured image using digital manipulation. This is the same as how a graphics program resizes an image to a larger size. The process involves taking a certain number of pixels and creating a larger image, but because the new image is based on the same number of pixels, the image loses quality. At small zooms (up to 20x) the loss may not be too noticeable. At large zooms (up to 100x or more) the quality becomes absolutely terrible. Optical zoom is provided by the lens (i.e. the optics) and does not lose image quality. The zoom is provided by a telephoto lens.
Lens Types
There are two fundamental types of lenses on video cameras:
Prime lenses (primary lenses or fixed focal length lenses) can’t zoom in or zoom out. Therefore, every time the filmmaker wants to get tighter on his composition, he can either (1) physically move the camera closer to his subject or (2) change the camera lens for one with a longer focal length and narrower field of view – the telephoto lens. Accordingly, if the filmmaker decides for a wider composition, he can either (1) physically move the camera away from the subject or (2) change the camera lens for one with a shorter focal length and wider field of view – the wide-angle lens.
As a trade-off for the inconvenience of switching lenses, the image quality is far superior with prime lenses, offering more pristine, clearer recordings. Professional narrative filmmakers always prefer prime lenses.
Zoom lenses (or varied focal length lenses) are by far the most common type of lenses known to the average consumer. Prosumer camcorders and professional video cameras often come with it right from the box. This is so because with zoom lenses you can go telephoto or wide-angle at the push of a button. It’s a no-brainer; if you can zoom in or zoom out, the lens is called a zoom lens.
The advantage of zoom lenses over prime lenses is that you don’t have to change lenses to get to a tighter or a wider composition; it saves time. Plus, if the cameraperson is not switching lenses, they don’t have to worry about carrying them or constant cleaning. This choice of lens is ideal for documentary and newsgathering since, in both situations, the operator doesn’t have the luxury of pre-planning, rehearsals, and second takes. With zoom lenses, the operator can record, for instance, a podium with several people on it, or go for tight close-ups and show each one’s faces.
The disadvantage of zoom lenses is the loss in image quality. Narrative filmmakers should always use prime lenses, unless they are going for a specific camera effect or trick, such as the dolly counter zoom.
Other types of lenses
Telephoto lenses are longer than the average. In comparison to normal lenses or the human eye, telephoto can get really tight on the subject. It correlates to a zoom in with zoom lenses. The image is optically enlarged to show fine details from a subject that is far away.
Telephoto lenses compress space, making people and objects that are far apart appear really near one another. If a filmmaker is shooting a scene on the freeway where actors have to run among speeding vehicles, telephoto lenses can be a safe method to keep actors distant from danger while still selling the illusion that the talent is just a few feet away from moving cars.
One drawback of telephoto lenses is that shakiness and movements are more pronounced and therefore noticeable. Mathematically speaking, even though the camera itself may be moving only a fraction of an inch, the framing may be moving several feet. With ultra telephoto lenses, pans and tilts may be nightmarish to accomplish since the moves could be too fast or shaky. Use a tripod! Shallow depth of field is another inherent characteristic of telephoto lenses. Ergo, telephoto lenses are ideal to rack focus.
By comparison, wide-angle lenses are the opposite of telephoto. Wide-angle lenses have a broader field of view, and therefore they can show sweeping panoramas of mountains, oceans, and forests. Since camera jiggle is negligible with wide-angle, this choice of lens is ideal for handheld work.
Wide-angle lenses also exaggerate depth, making people and objects appear further apart from one another. Extreme wide-angle lenses should not be used too close to actors unless for a specific purpose or effect since wide-angle bends the image, which is totally unflattering.
Normal lens are halfway between telephoto and wide-angle lenses. Normal lenses mimics what the human eye sees, without neither getting too close to the subject, nor distorting it, nor compressing the distance among the plains.
Lenses by function/camera type
Studio and Field lenses
Note that a 20x studio lens becomes a field lens if it is used "in the field," that is, for a production that happens outside the studio. Generally, however, field lenses have a much greater zoom range (from 40x to 70x) than studio cameras. Some field lenses have even a greater zoom range, allowing the camera operator to zoom from a wide establishing shot of the football stadium to a tight close-up of the quarterback's face. Despite the great zoom range, these lenses deliver high-quality pictures even in relatively low light levels. For studio use such a zoom range would be unnecessary and often counterproductive.
ENG/EFP Lenses
These lenses are much smaller, to EFP' fit the portable cameras. Their normal zoom range varies between l l x and 20x. A 15x zoom lens would be sufficient for most ENG/EFP assignments, but sometimes you might want a closer view of an event that is relatively far away. You would then need to exchange the 15x zoom lens for one with a higher zoom range—such as 20x or even 30x. You can also use a range extender (discussed later in this chapter), which would let you zoom beyond the normal zoom range into a tighter shot.
A more important consideration for ENG/EFP lenses is whether they have a wide enough angle of view (a very short focal length), which would allow you to shoot in highly cramped quarters, such as in a car, a small room, or an airplane. Also, the wide-angle view is important for shooting in the wide-screen 16 x 9 format.
Many lenses have digital or mechanical stabilizers that absorb at least some of the picture wiggles resulting from operating the camera, especially when in a narrow-angle (zoomed-in) position. Realize, however, that such stabiliz ers cause an additional drain on the battery. Use this feature only if you don't have a tripod or are unable to stabilize the camera in any other way.
Consumer Camcorder Lenses
These zoom lenses generally have an optical zoom range of 10x to 18x. You may have noticed that the problem with zoom lenses on consumer camcorders is that the maximum wide-angle position is often not wide enough, despite their good zoom range. Most camcorders have some sort of image stabilization. Some high-end prosumer models, which have a built-in lens, let you attach elements that allow a wider angle or tighter close-ups.
Range extenders If a zoom lens does not get you close enough to a scene from where the camera is located, you can use an additional lens element called a range extender, or simply an extender. This optical element, usually avail able only for lenses on professional cameras, does not actually extend the range of the zoom but rather shifts the magnification—the telephoto power—of the lens toward the narrow-angle end of the zoom range. Most lenses have 2x extenders, which means that they double the zoom range in the narrow-angle position, but they also reduce the wide-angle lens position by two times. With such an extender, you can zoom in to a closer shot, but you cannot zoom back out as wide as you could without the extender. There is another disadvantage to range extenders: they cut down considerably the light entering the camera, which can be problematic in low-light conditions.
References:
Herbert Zettl, Television Production Handbook, New York, Wadsworth
Millerson, Gerald, Video Production Handbook