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Chapter 3 Lenses

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Chapter 3 Lenses This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved. Chapter 3 Lenses Objectives Information in this chapter will enable you to: • List two simple rules of light and optics. • Defi ne transmitted light and tell why it is important to photography. • Describe the types of lenses and how each transmits light. • Explain focal length, aperture, and f-numbers. • Describe different types of lens mounts. • Describe the proper method of cleaning a lens. • Describe how to change lenses while minimizing the amount of dust getting in the camera. Technical Terms aberrations incident light angle of view lens bayonet lens mount lens aperture compound lens lens mount convergent lens magnifi cation crop factor negative lens divergent lens normal lens element optics fast lens positive lens f-number refraction f-stop screw mount lens focal length single-touch zoom lens focal plane stopping down focus two-touch zoom lens image point zoom lens image size 31 This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved. 32 Exploring Photography Chapter 3 Lenses 33 A lens is a transparent material that has Simple Rules of Light and Light rays as it passes through an object is known as at least one curved surface. See Figure 3-1. refraction. The principle of refraction makes When light enters the camera, it passes Optics it possible to design lenses that can make an through a lens. The lens collects and bends object appear larger or smaller. To understand how a lens works, some Light source light rays. The rays are bent so they form a basic rules of optics, the physics of light, must sharp image on the fi lm or digital sensor. be known. Light usually travels in straight The Pinhole Camera See Figure 3-2. A lens can be made of special lines, called rays. See Figure 3-3. If there is A simple pinhole camera can make pho- optical glass or plastic. nothing in the path of a light ray, it will con- Figure 3-3. Light rays usually travel in straight lines, tographs without a lens. See Figure 3-5. Light tinue in the same direction. Light rays can but they are not always traveling parallel to one refl ected from the subject enters through be transmitted, absorbed, and refl ected. The another. the pinhole. It travels through the empty shape and properties of a material will deter- space inside the camera to form an image on mine what happens to the light ray after it Incident light the fi lm. The tiny pinhole allows light rays contacts the material. See Figure 3-4. to travel in a straight line from the subject In the study of camera lenses, we are Reflected being photographed to the fi lm. An inverted mostly concerned with transmitted light. rays (upside-down) image strikes the fi lm. See This is the light that passes through the lens. Figure 3-6. Absorbed If the pinhole is made larger it admits Transmitted Light rays more light, but rays passing through the pin- hole scatter. The resulting image is not very Light rays can travel through a trans- Transmitted Figure 3-1. A lens is a transparent glass or plastic parent or clear material such as water, glass, rays sharp. Figure 3-7 shows photographs taken disc with at least one curved surface. Light passes and plastic. The light rays, however, are bent with a pinhole camera. through it. Figure 3-4. Light rays can be transmitted, absorbed, as they pass through. The bending of light and refl ected. Such cameras are not very practical for general photography. The results are poor, Light from the sun or an artificial source lights the subject The reflected light is gathered by the lens which focuses it on the film or digital sensor to make the exposure Light is reflected from the subject Figure 3-2. The lens collects and bends light rays so they form a sharp image on the fi lm or digital sensor. Figure 3-5. These pinhole cameras can make photographs without using a lens. This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved. 34 Exploring Photography Chapter 3 Lenses 35 requirements for good photographs. Several Plate glass lenses that refract light differently may be necessary to correct defects. Ray bent The Compound Lens Light ray inside glass A compound lens is made up of two or more simple lenses, as shown in Figure 3-13. Figure 3-8. A simple lens is more practical than a Each simple lens in a compound lens is called pinhole. The lens allows more light to strike the fi lm. Ray parallel to original path an element. Individual lenses are classifi ed into two Figure 3-6. A pinhole camera produces an upside- The shutter is placed in or behind the lens, major groups: down image on the fi lm. and can be opened for carefully measured lengths of time. Light falling on the fi lm or • A positive lens, also known as a especially if there is any movement. Exposure digital sensor can be controlled to fractions Figure 3-9. Light rays passing through a sheet of convergent lens is thicker at the center and times are long. But, the pinhole camera dem- of a second. glass bend, but exit traveling parallel to the incident gathers light. See Figure 3-14. Convergent onstrates how simple photography can be. Light striking the surface of an object is light. means it bends the light inward. • A negative lens, also known as a Web site called incident light. See Figure 3-4. A light divergent lens is thicker at the edges and www.pinholeday.org ray passing through a sheet of glass changes its path as it enters the glass. However, the Ray emerging spreads light. See Figure 3-15. It is called On the last Sunday of April each year light ray will exit, traveling in the same direc- from other divergent because it bends light outward. photographers take pictures with a pinhole tion as when it entered. See Figure 3-9. surface Incident ray camera. You can post one of these photos to This only happens when the glass sur- Lens Aberrations the website to share with visitors all over the striking one faces are parallel. If the surfaces are not par- surface Defects or fl aws in a lens are called world. Worldwide Pinhole Photography Day allel, as in a prism, the rays will be refracted was created to promote and celebrate the art aberrations. These defects are not, in most or bent at each surface. See Figure 3-10. cases, the result of a manufacturer’s careless- of pinhole photography. Most inexpensive cameras have simple Figure 3-10. Light rays passing through a prism are bent at each surface. ness. They are caused by the behavior of light. lenses. The simple lens can he compared to two A lens maker can correct for most aberrations Kinds of Lenses prisms placed base-to-base. See Figure 3-11. by using many elements. Notice how the light rays pass through the The pinhole can be replaced with a simple prisms. The rays do not converge (come Coated Lenses lens. See Figure 3-8. Since more light can enter together) at the same point. the camera, exposure time is reduced. However, However, by rounding the prisms, the Modern lenses are also coated to protect a shutter is now needed to control the light. rays passing through the lens are made to against stray light. A thin layer of a chemical substance, usually magnesium fl uoride, may eliminate all but about one percent of these refl ections. The amber, blue, or magenta tints seen on a lens indicates the presence of this coating. Figure 3-11. A simple lens acts like two prisms Lens Quality placed base-to-base. Lens quality depends upon the char- acteristics of the optical glass used and the converge or focus at the same spot. This com- lens design. Making one batch of glass often mon spot is called the image point. The fi lm requires weeks. Even with rigid controls, most or sensor in the camera is at this point. See of the batch cannot be used to make quality Figure 3-12. lenses. The glass that can be used is remelted A simple lens is made of a single piece Exposure using a small opening Exposure after pinhole has been enlarged and cast in molds. The rough glass casting is of glass or plastic. It may not meet all of the ground and polished into the fi nished product. Figure 3-7. Pictures made with a pinhole camera. This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved. 36 Exploring Photography Chapter 3 Lenses 37 Models of lenses are drawn on computers Wide angle and evaluated before they are manufactured. 14 mm Lens In the past, the calculations were made by hand. It often took years to design a lens. 20 mm Finished lens elements are precisely fi t- 24 mm Image point ted into the lens barrel. The lens barrel is 28 mm carefully fi tted to the camera body. 35 mm 50 mm Focal Length of a Lens 85 mm All lenses have a focal length. The focal 100 mm length of a lens is the distance between the 135 mm lens and the focal plane when the lens is set 200 mm to infi nity.
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