You are contemplating the purchase of a \"telescopic\" lens for your reflex came

Question

You are contemplating the purchase of a "telescopic" lens for your reflex camera so that you can take pictures of birds from a distance that does not disturb them. The lens you are looking at is specified to be able to zoom to a focal length of 600.0 mm. With this lens from what distance could you photograph a bird that is 140.00 mm tall so that the height of its image would just fill the 35.00 mm dimension of a standard 25.00 x 35.00 mm film negative? Give the distance of the bird as its distance from the lens, in meters.

Explanation / Answer

Object/Image = Di/Do = (Di - f)/f = f/(Do - f) 35/115 = 450/(Do - 450) 0,3043 = 450/(Do - 450) 0,3043(Do - 450) = 450 0,3043Do = 450 + 0,3043*450 Do = 1928,8 mm = 1,928 m --------------------------------------… Control: Di = 1,928*35/115 = 587 mm 1/f = 1/Do + 1/Di 1/450 = 1/1928 + 1/587 0,0022222 = 0,0005186 + 0,0017035 = 0,0022222

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