13246_CAI_Feedback_Mar2018_COMPLETE_v1

only a quarter as much light through it, making the camera only one quarter as sensitive. F stops range from 1.2 (for “fast” wide angle lenses) to 4.5 (for telephoto lenses). Some lenses have a variable iris which changes its usable diameter or aperture, and some have an automatic iris which increases the aperture as the scene changes from day to night. The aperture also affects the depth of field (the range of distances from the camera over which the image is in focus), larger apertures having a smaller depth of field. The angle of view is determined by the focal length of the lens. For 1/3” CCD cameras, the relationship is approximately as follows: Lenses with short and long focal lengths are known as wide angle and telephoto lenses respectively. Alternatively, the lens can be selected in relation to the required horizontal angle of view, as shown alongside. A lens must be compatible with the CCD size. Never use a smaller CCD than the lens image format! An optical zoom lens has an adjustable focal length, sometimes with a range of up to 20:1. Some cameras also have a digital zoom facility. The optical focus adjustment on a zoom lens is carried out with the lens fully zoomed in. Picture resolution is sometimes quoted as “TV lines”. This is a horizontal measurement of the amount of detail in a scene that can be viewed on a TV display. It is derived from analogue TV, being the number of distinct vertical lines, alternately black and white, visible across the width of the viewing screen. The greater the number of lines, the higher the resolution. Where F = Focal length in mm D = Object distance in metres W = Scene width in metres 43 CAI Feedback Spring 2018