Media 20 - Ch 10
Focal length-Wide Angle (short lenses)
Wide-angle lenses are those with focal lengths shorter than normal lenses. Wide-angle lenses reduce the size of the image and broaden the angle of view, compared to the perspective of the human eye. In the 16mm film format, a 15mm lens is considered slightly wide angle, a 10mm lens is wide angle, and an 8mm lens very wide angle. An extreme wide-angle lens, greater than 180 degrees is also called a fisheye lens.
ND filter (neutral density)
if you cannot control the illumination of the scene (e.g., a sunny exterior shoot), utilizing the camera's built-in neutral density filters to block light or using the electronic shutter control to reduce the light entering the camera. Either way, reducing the light will force your aperture to open up and your DOF will narrow.
Dolly vs Zoom
A dolly shot involves changing the camera-to-subject distance with a dolly. A zoom shot involves changing the focal length - Zooming in means adjusting the optical center away from the focal plane and therefore increasing the magnification power of the lens (telephoto), and zooming out means adjusting the optical center back toward the focal plane, causing the image to become more wide angle.
Focal length-Normal (medium lenses)
A normal lens approximates the same perspective and image size that of the human eye would see if one were to stand in the same spot as the camera (not including peripheral vision).
Aperture and Iris
Another adjustable ring found on all lenses used for film production and on all professional video lenses is the aperture ring (or f-stop ring). The aperture ring controls a slender disk, a diaphragm, inside the lens called the iris, which is made up of flat, matte black, metal blades. These blades overlap in such a way that they create an opening that is nearly circular. This opening is called the aperture and all light gathered by the lens must pass through the aperture before it is registered on the film plane or imaging device. By adjusting the aperture ring, the iris either opens (creating a larger aperture opening) to allow more light or closes (smaller aperture opening) to allow less light to reach the film or CCD chip.
Fast vs Slow Lenses
Lens Speed The ability of a lens to gather light is determined by the largest possible f-stop of that particular lens. We refer to this ability as lens speed. A fast lens can open up to allow more light than a slow lens. The larger the maximum aperture can be, the faster the lens is. What limits the ability of a lens to gather light are the optics—the number and quality of the glass elements. For this reason, it is usually the case that wide-angle lenses are faster than telephoto lenses (they use fewer elements). A lens with a maximum aperture of f/ 1.4 is a very fast lens and can register a readable image with very little light. Zoom lenses tend to be much slower, as their construction requires many more elements. A lens speed of only f/ 3.5 is not uncommon for a zoom lens. The maximum f-stop number is usually etched into the front of the lens barrel and can sometimes fall between the usual numbers found on an f-stop scale.
Focal length-Telephoto (long lenses)
Lenses that have a longer focal length than normal and that enlarge the size of the image and narrow the angle of view are called telephoto lenses. In the 16mm film format, a 75mm lens is slightly telephoto, a 120 lens is telephoto, and a 250 lens is very telephoto.
Zoom vs Prime lenses
Lenses that have one fixed focal length are called prime lenses. These lenses are very common in film production. Many cinematographers favor prime lenses because their simple design allows them to be made with few glass lens elements, which means that there is less chance for loss of light or lens aberrations to occur. However, if you are using primes lenses and decide to change the focal length of a lens from one shot to the next (say change from a 25mm lens to a 120mm lens in order to get in closer to the subject without moving the camera), then you need to change your lens. With prime lenses, you need to change the lens every time you want a new focal length. For this reason, many 16mm cameras are built with a rotating turret. This turret has three lens mounts and will accommodate three prime lenses of various focal lengths. The three lenses mounted on a lens turret usually consist of one normal lens, one telephoto lens, and one wide-angle lens. Turrets allow you to switch between lenses by simply rotating and positioning the desired lens in front of the gate. Zoom lenses, which are also referred to as variable focal length lenses, offer precisely that—a continuous range of focal lengths in one lens housing. Zoom lenses are constructed with movable lens elements that slide forward and backward to physically shift the optical center and therefore change the focal length of the lens. Zooming in means adjusting the optical center away from the focal plane and therefore increasing the magnification power of the lens (telephoto), and zooming out means adjusting the optical center back toward the focal plane, causing the image to become more wide angle. Zooming is accomplished with the adjustable zoom ring, calibrated in millimeters, which allows the filmmaker to manually set the desired focal length. Some zoom lenses, primarily on video cameras, utilize a servo zoom motor, so that you can glide from one focal length to another smoothly during a shot.
Focus assist
Some video camcorders offer a focus assist function, which enlarges a portion of the image to help you find critical focus.
Focal Length
The focal length of a lens determines the degree of magnification or de-magnification of the scene being shot. The longer the focal length, the more the subject is magnified and appears larger and closer to the camera. The shorter the focal length, the smaller the subject is and the farther away objects appear. More specifically, if you double the number of millimeters (say from 50mm to 100mm), you will double the size of the subject in the frame. Also, the longer the focal length, the narrower the angle of view becomes and vice versa.
Depth of Field - shallow & deep controlling it
The primary factor in determining depth of field is the size of the image format. The smaller the format, the deeper the depth of field tends to be. It is easier to get a deep DOF in 16mm film than it is in 35mm film. And consumer video cameras, with very small CCD chips, tend to have very deep depths of field. However, as one of the controllable variables, production format is not especially flexible because it is usually chosen for reasons more pressing than its depth of field potential. There are three other variables that determine the actual range of DOF over which we have some control: ■ The aperture opening. The larger the aperture opening (smaller f-stop numbers), the shallower the DOF will be, and the smaller the aperture opening (larger f-stop numbers), the deeper the DOF will be. That is why scenes shot in very low light situations have such a shallow depth of field that we sometimes can see an eye in focus, but the ear, just a few inches back, is out of focus. Conversely, scenes shot in brightly lit environments can have a DOF so deep that it appears that everything in the background, as far as we can see, is in focus. ■ The focal length of the lens. The longer the focal length of the lens, the shallower the DOF will be, and the shorter our lens, the deeper our depth of field will be. Wide-angle lenses create deeper depth of field than telephoto lenses. ■ The focus point setting (distance of the critical plane of focus). The closer to the camera the focus setting, the shallower the depth of field will be, and the farther away we place the plane of critical focus, the deeper the DOF will be.
Focus - Rack focus, pulling focus
Usually, focus is something you set and leave for the duration of a shot. However, there are times when you may need to change the plane of critical focus during a take, while the camera is running. This is called pulling focus and it is common practice in film production. The person who does the actual adjustments to the focus ring is called the focus puller. There are two kinds of focus pulling. Rack focus means shifting the plane of critical focus between two static subjects along the z-axis. For example, in this shot from Tim Burton's 1994 film Ed Wood (Figure 10-13), the focus begins on the background subject Dolores, Ed's girlfriend, as she looks through her closet. When she wonders out loud where her lost angora sweater is, a precisely timed rack focus shifts the visual emphasis to the foreground, and to Ed's knowing reaction, providing a humorous punch line for the scene because we know he has been secretly wearing his girlfriend's clothes. In these cases, you must find each focus point ahead of time and mark them—either on the focus ring of the lens with paper tape or on a follow focus ring if you have one (Figure 10-14); this allows you to rack focus smoothly and precisely without hunting for focus.
X, Y - axis Field of View FOV
X-axis= horizontal Y-axis= vertical There are two ways to affect the size of a subject in the frame. The first is to change the camera-to-subject distance—moving the camera itself closer or farther from the subject. The other is to alter the magnification of the scene by changing the focal length of the lens you use. There are significant compositional differences between these two options. First, let's consider the x-axis and y-axis differences. When you change only the camera-to-subject distance, the subject indeed gets larger or smaller, but you maintain the same horizontal (x-axis) and vertical (y-axis) vista -called the field of view (or angle of view). However, leaving the camera stationary and changing focal length (longer or shorter) to change the size of the subject in the frame alters the field of view, narrower or wider. This significantly changes the amount of background information contained in the frame. Compare the two long shot examples. The subjects in both shots are nearly identical in size, and the horizontal center of both frames is the same (no left-to-right angle adjustment); however, the shot taken with the wide-angle lens and camera moved closer includes more background (to the left of the frame), and the shot taken by leaving the camera stationary and using a longer focal length (shot b) has narrowed the field of view to exclude them. Notice also how much more we see of the space above and below the two subjects in the wide angle/ close camera frame.
Z-axis and Depth
Z-axis = depth (or relative distances of objects along the z-axis) The other perspective dimension that is important to consider is that of the perception of depth, or relative distances of objects along the z-axis. A normal lens replicates the same perception of depth that our eyes see. For example, if we use a normal lens to frame a subject in a medium shot with another object five feet behind, that object will indeed seem like it is five feet behind the subject in the shot. Wide-angle lenses tend to exaggerate the depth along the z-axis, especially when close to the subject. The space between objects appears to be greater because of the relative distances between the camera and the objects along the z-axis. For example, look at the three images taken with a wide-angle lens and moving the camera (Figure 10-3, a1, b1, and c1). The two subjects are, in reality, about six feet apart and the z-axis perspective in shot a1 doesn't seem exaggerated; but as we move closer with the wide-angle lens (b1 and c1), the distance between them seems to grow wider and wider. This is because the distance between the subjects, relative to the distance of the camera to the foreground subject, becomes greater. Put another way, the distance between the camera and foreground subject is much shorter than the distance between that subject and the background objects (background guy and other pillars). In shot c1, the camera is only about a foot and a half away from the guy in the foreground, making the distance between the subjects four times greater, causing these objects to appear far from each other. The more wide angle our lens, the more it will exaggerate depth in this way. Wide-angle lenses are often used to exaggerate, for example, the space between a person (near foreground) and a destination (background), to stress the idea that they have a long way ahead of them before reaching their destination.
