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Professional Backgrounders
Our professional backgrounders series is designed to provide people who are relatively new to photography with a concise run down on what you should know about the tools and techniques that are the underpinnings of our craft. |
The Photographic Lens
Today's camera lenses are technological marvels that perform the basic tasks of focus and light control. Some contain micromotors for autofocusing and microcomputers for the delivery of image information to the camera's exposure control systems. Aided by computer design and a long history of optical and electronic development, modern lenses are considerably lighter, shorter, and sharper than lenses were twenty or more years ago. Plus, today's lenses are available in a wider range of focal lengths than ever before. Zooms lenses in a wide variety or ranges and specialty optics for macro and super-telephoto photography are all part of the mix.
By definition, a lens one or more pieces of ground and polished glass that alters the direction of light passing through it. Moving the lens or its components different distances from the film plane can focus that light onto the film plane creating an image of the illuminated objects in front of the lens. Unlike the simple lens of a magnifying glass, lenses used in photography are complex, composed of groups of glass elements that are arranged very carefully within the lens barrel, or housing. Because light is altered when it passes through glass, lenses must both correct any of those alterations, called abberations, and incorporate particular optical formulae for every focal length design. Some lenses work in conjunction with computerized cameras and respond to commands that are input from the camera body itself. 
On a practical level, the lens is our photographic window onto the world. It offers us views that range from super-close-up to super-telephoto; it can expand our peripheral vision, or creatively distort our view. Depending on the lens chosen, subjects at different distances from the camera can be made to appear closer together or further apart.
Lens selection and the creative use of lens controls are two of the key tools in photographic expression. The lens we mount on our camera has a great deal of influence on how we see and often establishes the very relationship we have with our subjects. This is one of the principal benefits of the single lens reflex (SLR) camera. Virtually every point-and-shoot camera, and most low and moderately priced digital cameras, come with a fixed lens. On most models you get a zoom lens and you may be able to choose from a variety of focal lengths, but, for example, if you have a point-and-shoot with a 38-115mm zoom lens, there's no way to remove that lens and mount a super-telephoto lens on your camera. The wide range of photographic images available to the SLR user depends largely on the ability to use different types of lenses on the camera. Lens Terms and Descriptions 
The photographic lens performs two major functions: it focuses the light from the scene onto the recording material, and controls the amount of light that passes through the lens and onto the film plane, thus affecting film exposure. The opening in the lens, the aperture, also has an influence on the depth of field or the zone of sharpness within the picture space. Focusing is accomplished by changing the distance between the elements and groups within the lens and the light's target, the film plane. Light control and depth of field are accomplished by changing the size of the aperture opening, the variable bladed diaphragm within the lens through which the light flows.
Lens Categories Lenses are categorized by focal length which also indicates the angle of coverage offered by the lens and the apparent perspective, or relationship of near to far in the recorded scene. Lenses must be compatible with the camera in use, as each lens has a specific mount that will only fit onto a specific brand or type of camera. Autofocus Lenses Autofocus lenses are those that either have a motor mounted inside or are driven by a motor in the camera to automatically focus upon a targeted subject. Manual focus lenses require focusing by eye and hand. Autofocus lenses will only autofocus if mounted on an autofocus-type camera body. Only one 35mm camera brand--Nikon--allows use of their older manual focus lenses on their autofocus camera bodies. With all other brands distinct lenses must be used for both their autofocus and manual focus cameras. Elements and Groups 
Inside the protective outer shell, or lens barrel, lenses have individual glass elements. These elements are gathered in clusters, known as groups. The reason for the complexity of photographic lenses (some lenses are constructed, for example, with 13 elements in eight groups) is that light goes through a number of changes as it passes through glass (or any other medium, for that matter). Each lens is made so that it will correct some of the faults, or aberrations in this light path, before the light strikes the film.
Aberrations These aberrations can effect color, contrast or even the sharpness of the image itself. In total, a photographic lens is a finely balanced set of compromises, each one playing off against another to correct or redirect the light as it passes. For the first hundred plus years of photography, lens design was based upon the science of optics and a lot of trial-and-error. With the advent of computer-aided design, photographic lenses have become more highly corrected, with the optics more finely tuned and improved lens quality as a result. Front & Rear Element There are two external glass surfaces on a lens, known as the front and rear element. The front element faces the subject, while the rear element is where the light exits on its way to the film. The surface of the front element and back elements are coated with a substance, such as magnesium fluoride, which helps to reduce reflections and a loss of contrast, thus sharpness, in the transmitted image. Repeated cleaning or abrasion can wear off this coating. For that reason, many photographers mount a clear glass filter, such as a UV (ultraviolet blocking) or Skylight 1A, over the front element and leave it there. At NYI, our position has always been that a protective filter should be used in conditions that warrant the protection. In sand, near surf and salt spray, or in dusty conditions, a protective filter is essential. If your work doesn't expose your camera to such risks, then you may not see the need of using a protective filter. Focal Length 
Lenses are described in two ways: by focal length and by maximum aperture. Focal length affects both the angle of coverage and the apparent perspective, or relationship of subjects both near and far. Maximum aperture deals with light control, and defines the largest opening available with the lens. We'll cover focal length first.
Practically, the focal length of a lens is a guide to the angle of view, or field coverage of the lens. The lower the focal length numbers the wider the angle of that coverage. For example, a wide-angle lens, such as a 24mm for 35mm format, encompasses more of a scene than a telephoto lens, such as a 200mm for 35mm format, when the scene is viewed from the same position. Focal length also effects the visual relationship with the subject. Simply put, a telephoto lens "brings subjects closer", while a wide angle lens seems to make subjects appear further away than they actually are (see the discussion on Focal Length that follows.) The result is a compressed and expanded perspective, respectively. Aperature Settings The light and depth of field controls are affected by the aperture settings. The aperture is a variable opening within the lens itself that ranges from very small to wide open; the maximum aperture is the largest opening and the minimum aperture is the smallest opening available on that lens. Apertures are designated by f-numbers and are known as f-stops. When an aperture is expressed it reads f/2.8, f/8, etc. The slash appears in the designation because it is a fractional value or ratio. Because the aperture is variable, it can be used in conjunction with shutter speed as a way to control exposure, or the amount of light reaching the film. 
The f-number is defined as the proportion of the diameter of the opening of the diaphragm in the lens to the focal length of the lens itself. Thus, if the diaphragm opening is 10mm wide, and we're working with a 110mm lens, the aperture is f/11. With any given lens, the lower the f-number, the larger the aperture, or opening. And naturally, the larger the opening, the more light passes through the lens.
Aperture settings also play a role in the depth of field in the picture space. As the aperture narrows, the depth of field, or zone of sharpness increases. This allows for very discrete manipulation of sharpness and unsharpness in a scene. Controlling depth of field is an important creative tool in photography. In the next installment, we'll move from the basic elements common to all lenses that we've covered in this section to discuss various types of specialty lenses, and we'll decode those marketing terms-so you'll know have no doubt what IF, ED, USM and the like actually mean. In the other portions of the series, we'll explore focusing and depth-of-field in detail, and we'll also address the question of lenses made by camera manufacturers as opposed to the often less-expensive lenses offered by manufacturers such as Sigma, Tokina, and Tamron. |
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