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 500.0 mm.  With this lens from what distance could you photograph a bird that is 135.00 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

Start with M = h'/h = q/p

35/135 = q/p

.259p = q

Now apply 1/f =1/p + 1/q

1/500 = 1/p + 1/.259p

1/500 = 1.259/.259p

629.5 = .259p

p = 2430 mm

That is 2.43 m

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