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Case Studies
LENS BASICS
By Gordon Tubbs, Assistant Director,
Broadcast and Communications Division at Canon U.S.A., Inc
First in a series of three articles
One of the greatest mysteries of the video production world is the camera lens. While it is clearly one of the most essential parts of the production chain, the lens is also one of the least understood. While many videographers and cinematographers have an understanding of how cameras work and how they operate, they still tend to think of optics as the “magic” they know works - somehow.
It’s important for camera operators to understand not only what happens when they zoom a lens but why it happens that way. Here’s one reason why. Consider that many of today’s lenses now include highly advanced electronics that offer built-in menu systems. These systems can help camera operators perform many functions with the lens they never could accomplish in the past. However, camera operators with a better understanding of lenses will be able to benefit more from these powerful digital controls than other users with less knowledge of lenses.
Before delving into their functionality, though, let’s take a look at some of the different categories of lenses. These include:
· Broadcast and Professional grade – Broadcast-grade represents the state-of-the-art lens with functionality for HDTV and SDTV. This type of lens also offers full feature sets that include digital controls and 16:9/4:3 switchability. Today’s professional-grade lens can offer extremely high quality but is made for applications where more expensive optical and mechanical features are not needed, such as long telephoto and very wide-angle capabilities.
· Studio/field, ENG and EFP lenses – The studio/field lens, also known as the “box-style” lens, is designed for use with studio or portable cameras, and typically has a larger housing. A lens with a longer telephoto capability, such as Canon’s 100xs or 86xs, falls into this category. An Electronic News Gathering (ENG lens) is lighter, more compact and designed to be fully portable for camera operators who have to be mobile and ready to shoot at a moment’s notice. Meanwhile, an EFP lens, while still portable, has greater telephoto capabilities than an ENG lens but is generally too heavy for an operator to rest on his/her shoulder for long (hence the benefit of using of a tripod).
Virtually all of the lenses manufactured for use in broadcast or professional applications are zoom lenses. A zoom lens can be changed in focal length continuously without losing focus. The name “zoom” comes from the strong visual impression it creates. A zoom lens will be identified with two key numbers in a specification that may read 100x9.3 or 17x7.7. The first number is the zoom ratio, or the degree to which the focal length can be changed from a wide shot to a close-up during a zoom. The second number is the lens’ focal length at the wide end, which is the distance from the optical center of the lens to the front surface of the camera imaging device at which the image appears in focus (with the lens set at infinity). The higher the zoom ratio, the more telephoto it is; the lower the focal length, the greater amount of wide angle it offers.
Here’s something else to consider. If a camera operator changes the distance from the lens to the object he is shooting, the image size will also change. The position where the image is formed also changes, meaning that the image has to be refocused each time the lens is moved. But if two lenses or lens groups are combined by moving them in synch, it is possible to change the magnification without destroying the focus. In other words, move one part of the lens system to change the size of the image, move another part to keep it in focus.
A zoom lens has at least two moving parts: the variator and the compensator. The part of the lens system that moves to change the image size is called the variator. The part that moves to maintain focus is called the compensator. A typical hand-held zoom lens requires another group of lenses on the front end called the focusing group. The image then passes from the focusing group through a stationary lens relay group, sending the image to a beam-splitting prism. In order to keep the image in the same position as the variator and compensator move, the lens group must move along precise curves that are determined by the laws of geometric optics, controlled by the grooves of the lens’ barrel cam mechanism. The cam grooves are essential for maintaining focus and are machined to micron tolerances.
Along the way, a zoom lens must also correct optical aberrations so that the image will stay sharp when zoomed. These aberrations are caused when the path of the light rays undergo complex changes during zooming. Therefore, it’s important for a high-quality lens to minimize aberrations at each lens group, which are also carefully balanced to correct each other.
There are several other key specifications within a zoom lens. The first item to check is image size. The CCD on a professional or broadcast lens is ½ or 2/3 inch, so the lens must be the same. Since the image formed by the lens is round (not rectangular like the shape of a TV screen), the range of the image is called the image circle. In a TV camera, the CCD sensor occupies a rectangular area inside and touching the image circle, and its size is what defines the image size.
In addition to the aforementioned focal length, an equally important specification is the F-number. This indicates the brightness of the image that a lens forms. The smaller the F-number, the brighter the image. The F-number is closely related to a lens’ depth of field since, for a given focal length, the larger the aperture of the lens, the smaller its F-number. The stop ring of the lens is marked with a series of numbers, such as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. It is all part of a complex formula. Ultimately, though, each time the ring is turned one number up the F scale, it decreases the brightness by half.
Another aspect of lens basics is the minimum object distance (MOD), which is the closest you can get to your subject. In broadcast and professional lenses, this is measured from the front vertex of the lens, which is the front-most surface of the focusing group. Because of the small amount of space in a studio, studio lenses need to have a short MOD. Telephoto zoom lenses, on the other hand, don’t need such a short MOD. If you need to get closer than your lens’ MOD, a macro mechanism makes it possible. If there is no macro function, switch to a close-up lens.
A key property of a lens is flange-back. This is the distance from the flange surface of the lens mount to the image plane. It must precisely match the distance from the camera flange surface to the surface of the CCD. Since each camera has a specific flange-back, the user must choose a lens with the same flange-back to go along with it. Related to that, back focus is the distance from the back-most surface of the lens to the image plane, and it is designed to avoid contact with the camera’s filter and dust-protection glass.
When using a lens, it helps if the camera operator is familiar with its resolving power. This indicates its imaging performance. Resolving power is determined by shooting a chart with lines of various widths, then seeing how far down the lens can still separate and reproduce the lines. Note, however, that a lens with high-resolving power doesn’t necessarily deliver good image quality – it only expresses the limit value of the lens. Therefore, another way to evaluate the total performance of a lens is by its modulation transfer function, or MTF. MTF expresses the reproducibility of contrast, shown by a graph with spatial frequency on the horizontal axis and contrast reproducibility on the vertical axis.
The last basic aspect of lenses camera operators should know about is chromatic aberration. This arises from dispersion, when the refractive index of glass differs with wavelength. The first kind of chromatic aberration is longitudinal aberration, a focus error wherein all RGB information is not focused on the same point. The second kind is lateral aberration, a registration error of the RGB information. The third and final kind of chromatic aberration is geometric distortion caused by the bending of straight lines. Canon lenses, for example, correct chromatic aberration by using fluorite crystal. This crystal has a different dispersion from ordinary optical glass in the focusing groups of zoom lenses.
Operators should remember that what goes on inside the lens isn’t magic or an unsolved mystery; it’s simply physics and the focus of constant research and development. Anyone who wants to know more can contact a lens professional and ask! We’re always happy to talk about our favorite subject, which is lenses, and how to apply them for maximum image quality.
Next month: Where are lenses going? We’ll examine the latest technological advances in lenses.
Gordon Tubbs is Assistant Director, Broadcast and Communications Division at Canon U.S.A., Inc. He can be reached at gtubbs@cusa.canon.com
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