A 80 mm focal-length lens is used to focus an image on the sensor of a camera. T

Question

A 80 mm focal-length lens is used to focus an image on the sensor of a camera. The maximum distance allowed between the lens and the sensor plane is 120 mm.

(a) How far ahead of the film should the lens be if the object to be photographed is 13.0 m away? ______ mm

(b) How far ahead of the film should the lens be if the object to be photographed is 3.0 m away? ______ mm

(c) How far ahead of the film should the lens be if the object to be photographed is 1.0 m away? ______ mm

(d) What is the closest object this lens could photograph sharply? ______ cm (centimeters!!)

Explanation / Answer

Here f = 8.0cm

a) do = 1300cm so 1/f = 1/do + 1/di => 1/di = 1/f - 1/do solving we get di = f*do/(do - f)

so di = (8.0cm*1300cm)/(1300 - 8.0) = 8.04cm = 80.4mm

b) do = 300cm so 1/f = 1/do + 1/di => 1/di = 1/f - 1/do solving we get di = f*do/(do - f)

so di = (8.0cm*300cm)/(300 - 8.0) = 8.21cm = 82.1 mm

c) do = 100cm so 1/f = 1/do + 1/di => 1/di = 1/f - 1/do solving we get di = f*do/(do - f)

so di = (8.0cm*100cm)/(100 - 8.0) = 8.69cm = 86.9 mm

d) Now di = 12.0cm so 1/f = 1/do + 1/di => 1/do = 1/f - 1/di solving we get do = f*di/(di - f)

so di = (8.0cm*12cm)/(12- 8.0) = 24cm = 240mm

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