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TECHNOLOGY FEATURE VIDEO GUIDE


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Through The Looking Glass


The promise of 4K production and imagery has put attention squarely on lenses, with new models designed specifically for the technology appearing on the market. But that often-overlooked lump of glass on the front of a camera has always been an important tool, as Kevin Hilton explains.


THE CAMERA is the most obvious and recognisable piece of equipment on a film or television shoot but it is what sits on the front of this that is the most crucial and creative component in acquiring images. The basic workings of a lens can be explained dryly in the language of schoolboy physics, which is necessary as it lays down how light from the object or person being filmed form an image that can be filtered on set during filming or manipulated in post-production to create a particular look. The practical and creative come together in the fact that


GLOSSARY


Lens A formation of transparent materials, typically made of glass, which, when brought together, receives and refracts light rays. In this configuration a lens is able to form an image to a set distance between the lens and the capture medium, known as the focal length. The focal length dictates the type and size of image created. The lens sits in a housing on the front of the camera, either as a single unit using a single-screw or bayonet mounting, or as part of a rotating turret of several lenses, with light rays converging through it from different points on the subject to create a latent image.


56 September 2013


lenses with different focal lengths can be manufactured, giving narrow or wide angles for shooting as required. The starting point with any lens is the capture of rays of light from whatever is being shot. Beams from different parts of the subject travel to the lens in a cone formation and are then bent as they pass through it. This causes a convergence of the rays to a single point on the capture medium, be it film, tape, optical disc, hard disk or solid state memory. A latent image is formed as rays from all points on the subject converge.


The components of a lens


are housed in a barrel-like construction. This lens barrel is fitted with a calibrated ring for focusing; when adjusted it moves the lens nearer to or further away from the capture medium depending on how far away the camera is from the subject. The barrel also has an iris diaphragm. Like the membrane of the same name in the human eye, the iris in a camera controls the amount of light reaching the recording medium. The iris diaphragm in a camera is made up of a circle of overlapping metal leaves within the lens barrel, which are opened and closed manually or automatically using the f-stop – or t-stop – ring to allow more or less light through. If an image is overly bright the iris is closed, or stopped or irised down. If more light is needed it is opened wider, or stopped or irised up. Lenses can be described in


general terms according to the type of angle produced. A standard lens is analogous to the angle of vision found in the normal human eye. Wide-angle relates to any


field of view wider than standard, while long-focus and telephoto are able to focus over distances and magnify a far-away subject. These lenses generally have a fixed focal distance and are also known as prime lenses. Zoom lenses come under the heading of variable focus. These are single units that can be adjusted during shooting to give standard, wide-angle or telephoto views, doing away with the need to change lenses on a regular basis, which is why zooms are used commonly in TV news and documentary production.


FOCAL LENGTH Lenses can also be described by their focal lengths, usually given in millimetres. A lens that has a focal length approximately twice the diagonal of a frame produces an image that is similar to what the eye would experience. For 35mm film a lens of that type would have a focal length of 50mm, with a horizontal angle of view of around 25 degrees. For 16mm stock an equivalent lens


would have a 25mm focal length, while in 65mm film it would be 125mm. The focal length of long- focus and telephoto lenses is generally above 25mm, with 50, 75 and 100mm being popular; over the last two years demand for 130mm has grown. Zoom lenses vary considerably in focal length, depending on make. Some can deliver 17 to 85mm, others 12 to 120mm and 12 to 240mm. These optics are occasionally classified by the ratio of the maximum focal length to that of the minimum (12 to 120mm and 12 to 240mm would be 10:1 and 20:1 respectively). Among the leading lens


manufacturers are Zeiss, Canon, Panavision, Fujinon, Arri, Schneider Kreuznach and Cooke Optics. Many lens types can also be used to create particular looks and effects. Depth of field – the area of acceptable focus both in front of and behind the main plane of focus in a shot – can be exploited by using wide-angle lenses. Even if the action in a scene extends some way before and after


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