KODAK CUSTOMER SERVICE PAMPHLET - AA-3 Some Questions and Answers about Camera Lenses Your camera lens functions in much the same way as the lens of the human eye. Basically, the lens collects light rays reflected from a subject and, unlike a pinhole, focuses these rays to form a sharp image. What makes a lens work? A camera lens can be made from a piece of glass or plastic which has two opposite regular surfaces, either both curved or one curved and the other flat. Most camera lenses are actually made up of a number of lens "elements" which function together and are simply referred to as a "lens". As the light rays that are reflected from the subject pass through the camera lens, they are bent. The extent to which they bend is controlled by the composition of the lens and the curves of the lens surface. In a properly designed lens, all the light rays from the same part of a subject will meet at a point behind the lens. A sharp image of the subject is formed at the point where these light rays meet. The film must be located at this point for the picture to be in sharp focus. What is the focal length of a lens and what does it do? In very simple terms, focal length is the distance between the optical center of the lens and the film when the lens is focused at infinity. The focal length of the lens of most adjustable cameras is marked on the lens mount. The focal length is usually given in millimetres or inches. There is a direct relationship between the focal length of a lens and the image size of the subject record ed on film: the longer the focal length, the larger the image on the film. For example, if Lens A has a focal length of 50 mm, and Lens B has a focal length of 150 mm. The subject is the same size and the same distance from the lens in both situations. In this case, the image produced by Lens B, which has a focal length of 150 mm, is three times as large as the image produced by Lens A, which has a focal length of 50 mm. A lens of long focal length produces a larger image than one of short focal length. What are lens openings, and how are they determined? When we talk about a lens opening, we mean the opening (usually called aperture) that lets the light through the lens to expose the film. The sizes of lens openings are usu- ally expressed in terms of f-numbers, for example, 1/2.8 and f/4. The f-numbers are determined simply by dividing the focal length of the lens by the diameter of the aperture. For example, if the focal length of the lens is 100 mm and the aperture is 25 mm, the f-number is f/4 (100/25 = 4). So when you have a lens opening of 1/4, you know that the aperture is only 1/4 of the focal length. Similarly, f/8 means that the aperture is 1/8 of the focal length, f/11 means the aperture is 1/11 th of the focal length, etc. When you understand that f-numbers indicate the size of the aperture as a fraction of the focal length, it's easier to understand that the smaller the f-number, the larger the lens opening. Some typical f-numbers used in expressing a series of lens openings, from large to small, are f/2, f/2.8, f/4, f/5.6, f/li, f/16, and f/22. What does relative aperture mean? Let's look at two lenses, each with a 9 mm aperture and focused on the same person. Both lenses transmit the same amount of light, but Lens A has a focal length of 50 mm and Lens B has a focal length of 150 mm. With the 150 mm lens, the image of the subject produced on the film will be three times as large as the image produced with the 50 mm lens. While both lenses transmit the same amount of light reflected from the subject, the light is spread over an area nine times as large with the 150 mm lens. For this reason the image made on the film by the 150 mm lens is less bright. So while both lenses have a physical aperture of 9 mm, the longer-focal-length lens has a smaller relative aperture. It would be difficult to take properly exposed pictures if, for example, f/8 on a long-focal-length lens and f/8 on a short focal length lens didn't mean the same thing from an exposure point of view. Fortunately, they do but for this to be true, the f/8 on the long focal length lens must be a physically larger opening than the opening of f/8 on the shorter focal length lens. Both lenses have the same _relative_ aperture but their _physical_ apertures are of different sizes. Why must I use a larger-than-normal lens opening when I use a lens-extension device? For making extreme close-up pictures with some advanced cameras, you can use an extension tube or bellows to extend the camera lens. These devices allow you to get close to your subject and still get a sharp picture. However, since the f-numbers on the camera are based on a normal lens-to-film distance, the marked f-numbers are not a true indication of the image brightness reaching the film when the lens-to-film distance is increased by a lens- extension device. Since the image on the film is less bright when you use a lens-extension device, you should make an exposure compensation by using a larger lens opening or slower shutter speed. If your camera has a built-in meter it will automatically indicate the correct exposure settings, no additional compensation is necessary. If you're using a 35 mm camera without a built-in meter, you can use the following table as a guide to determine the amount of exposure increase necessary with an extended lens. Exposure Increase for Extended Lens 35 mm Cameras Width of Subject Area Open Lens by Or Multiply Exposure (inches) (f-stops) Time by 11 1/3 1.3 5 1/8 2/3 1.6 3 1/4 1 2 2 1/4 1 1/3 2.5 2 1 1/2 2.8 1 1/4 1 2/3 3.2 1 1/8 2 4 1 2 1/2 5.7 3/4 3 8 For cameras with built-in meters, use the exposure recommended by the meter. What is depth of field? The distance range within which objects in a picture look sharp is called depth of field. From a practical point of view. depth of field varies with the size of the lens opening, the distance of the subject focused upon, and the focal length of the lens. Depth of field becomes greater as 1. The size of the lens opening decreases. 2. The subject distance increases. 3. The focal length of the lens decreases (and subject distance remains unchanged). What is an aspheric lens? The surface of the majority of lenses made today is a segment of a sphere. An aspheric lens, on the other hand, has a curved surface which is not part of a sphere. Aspheric lenses are more difficult to manufacture than normal lenses and are not as common. An aspheric lens will correct various lens aberrations that would ordinarily require several more lens elements to correct. In addition to having fewer elements, a lens with an aspheric element is lighter and usually more expensive than a normal lens. What Is a color-corrected lens? A color-corrected lens is one that brings light rays of different colors reflected from the same part of the subject into focus at the same point behind the lens. A short history lesson will help you understand why color-corrected lenses are important. At one time, only black-and-white film was available. At first, black- and-white film was sensitive only to blue light. Later it was orthochromatic, that is, sensitive to both blue and green light. With many of the lenses used at that time, light rays of different colors that came from the same part of the subject did not come to focus at the same point behind the lens. But as long as the film could see only blue and green light, it didnÕt make too much diffeence when red light rays didnÕt come to focus at the same point as the blue and the green light rays. However, orthochromatic films weren't ideal. For example, since these films couldn't see red light, red objects (like lips) registered as black on the prints. For this reason, panchromatic films were developed. Because panchromatic films are sensitive to all colors, they are capable of rendering colors in proper degrees of black and gray. Now that the new films could see red and other light rays, it was necessary to make a lens that would bring all the light rays into focus at the same point. Different lens formulas and different types of glass were developed to provide proper, sharp register of all the light rays. These color-corrected lenses are essential for color photography but were needed long before there was any such thing as color film. What is a coated lens? A coated lens is one coated with a thin layer of special material that reduces light reflections from the air-glass surfaces of the lens. Most photographic lenses available today are coated. LetÕs see what happens when the lens is uncoated. When light strikes any air-glass surface of an uncoated lens, a small percentage of that light will reflect back from the surface and not go through the lens. In a multiple-element lens, each air-glass surface will reflect some of the image-forming light that should reach the film. Most of this wasted light will just be reflected from the surface back out through the front of the lens and will be lost. However, some of these light rays may be reflected a second time from the surface element of the lens so that they do reach the film. Because of the several angles of reflection, such light rays will not reach the film at the point where they should and will degrade the quality of the image. Since the lens coating reduces the amount of light that is reflected from an air-glass surface, it contributes to a clearer, crisper image and makes the lens more efficient by reducing light loss. If you look at a coated lens from an angle, the surface will appear colored. However, the coating on a lens does not affect the color of pictures taken through the lens. If you look through the lens, the coating is colorless. LENS-SUBJECT-IMAGE POSITIONS. APPROXIMATE POSITION OF SUBJECT AND IMAGE m = magnification F = focal length F = f-number x = distance of image from focal point or distance that lens is extended from infinity setting u = subject distance v = image distance h = height of subject hÕ= height of imag& All dimensions must be expressed in the same unit of measure. To convert dimension in divide by millimetres to metres 1000 centimetres to metres 100 inches to metres 39.4 feet to metres 3.28 millimetres to inches 25.4 Measuring u and v from a point midway between the front element and the rear element of the lens is accurate enough for practical use with a normal lens (not telephoto or wide-angle). The formulas that do not include v are valid for telephoto lenses and wide-angle lenses when u is large enough so that any inaccuracy in measuring u from the center of the lens is insignificant. The fundamental relationship between focal length, image distance, and subject distance is 1 1 1 - = - + - F v u Formulas that are more directly useful and some examples follow: Magnification: hÕ v v-f f M = - = - = --- = --- H u F u-F Lens-to-Image Distance: Fu v = ----- = mu = (m+1)F u-F Subject-to-Image Distance: (m +1)squared u + v = --------------- F m Lens-to-Subject Distance: Fv v | 1 | u = ---- = - = | - + 1 | F v-F m | m | Example 1: How long must a room be for you to photograph groups 10 feet wide when you use a lens with a focal length of 8 inches on a 4 x 5 inch camera? Solution: Allow 41/2 inches for image on horizontal axis of negative. Work in inches, so 10 feet = 120 inches. h' 4.5 Then m = ----- - .038 h 120 | 1 | and u = | --- + 1 | F = |.038 | (26.3 + 1) F = 27.3 x 8 = 218 inches = 18+ feet This answer gives the lens-to-subject distance. You will also need to add at least 7 feet to allow space for the camera, photographer. background separation, etc. The minimum room length is therefore 25 feet. The room width must be at least 15 feet in order to accommodate the group and lights. Focal Length: u v F = -------- = --------- | 1 | m + 1 | - + 1 | | m | Example 2: For a room 20 x 32 feet and a 21/4 X 21/4 inch camera, what is the longest focal-length lens feasible for photographing a scene 10 feet wide? Solution: Since you need about 7 feet of room length for working space, the maximum lens-to-subject distance available is 25 feet (32-7) or 300 inches; u = 300. You should allow at least 1/8 inch of space on either side of the negative. The usable width of the negative is then 2 inches. Since the width of the subject is 10 feet (120 inches), the magnification (m) equals 2 divided by 120, or .017 The formula now reads: 300 300 F = ----------- = -------- = 1/.017 + 1 59 + 1 300 --- = 5 60 Answer: 5 inches (127 mm) is the maximum usable focal length. Lens Movement from Infinity Position: F squared x = --------- u - F Field Size (front-element focusing lenses): u Field width = negative width x --- F Effective f-Number for Lens Extension: The effective f-number is greater than the indicated f-number because of the increased image distance (lens-to-film distance). When the subject distance u is less than 8 times the focal length of the camera lens, use one of the following formulas to determine the required exposure compensation. The formulas are valid for any subject distance. v x f Effective f-number = ------- = f (m + 1) F Where v = lens-to-film distance or focal length plus lens extension from infinity focus, f = f-number indicated on lens-opening scale and F = focal length. For close-up pictures with lens extension, use the effective f-number obtained from the first formula when determining your exposure, or compensate your exposure time directly by using the second formula. Note: Exposure compensation is made automatically with some cameras through the lens exposure meters. Correction may or may not be necessary with flash. Fixed Circle of Confusion Camera Most Widely Used (in inches) 8 mm movie .0005 Super 8 movie .00065 16 mm movie .001 110 (13 X 17 mm) .0012 126 (28 x 28 mm) .002 135 (24 X 36 mm) .002 Roll film .005 4 x 5-inch and F/1720 critical use larger or F/10OO liberal use DEPTH OF FIELD Depth-of-field computations are made on the basis of a fixed circle of confusion or on a circle of confusion equal to a fraction of the focal length. Lenses of different focal lengths used at the same f-number have the same depth of field for equal image sizes. As a general rule, one-third of the depth of field is in front of the subject and two-thirds is behind the subject. An exception to this rule is extreme close-up lenses, including those made with close-up lenses, where depth of field is about equal on both sides of the subject. Method A, Fixed Circle of Confusion: F = focal length of lens f = f-number setting H = hyperfocal distance u = distance for which camera is focused d = diameter of circle of confusion H x u Near limit of depth of field (measured from camera lens) = ---------- H + (u-F) H x u Far limit of depth of field (measured from camera lens) = ---------- H - (u-F) Hyperfocal Distance (near limit of depth of field when lens is set at infinity): F squared H= ------------- f x d Method B, Circle of Confusion a Fraction of the Focal Length of the Lens: u = distance focused upon. 0 = angular size of circle of confusion. For critical definition, 0 is 2 minutes of arc and the linear size of the circle of confusion is approximately F/1720 (tan 2Õ=.00058). F L = effective diameter of lens = --- f LDF = limit of depth of field u squared tan 0 Near LDF: (measured from plane focused on) = ----------------- L + u tan 0 u squared tan 0 Far LDF: (measured from plane focused on) = ----------------- L - u tan 0 KODAK CUSTOMER SERVICE PAMPHLET - AA-3 |