It depends on the size of the object that is 15 ft. away. A large truck may fill the frame at the 35mm zoom setting, but even 300mm will not be telephoto enough to fill the frame with a robin. You need to use the angle of view provided by the lens. A 35mm lens provides an angle of view of about 60 degrees. Using trigonometry, tangent of 60 deg = 1.7321, so at a distance of 15' the frame will be filled by an object that is 26' (15' x 1.7321) long. A 300mm lens provides an angle of view of about 8 degrees. tan 8 deg = 0.1405. At a distance of 15' the frame will be filled by an object that is 2' long (15' x 0.1405). To determine the lens needed for a specific subject you'd work with the inverse tangent (TAN1). An object 5' tall at 15' away gives a ratio of 0.3333 (size/distance = 5/15 = 0.3333). The inverse tangent of 0.3333 is 18 degrees, which is about the field of view given by a 135mm lens.
9/25/2002 6:14:44 PM
