A line object 5 mm long is located 50 cm in front of a camera lens. The image is focused on the film plate and is 1 mm long. If the film plate is moved back 1 cm the width of the image blurs to 1 mm wide. What is the F-number of the lens?
Text Solution
Verified by ExpertsThe correct answer is:
B
Substituting u = 50 cm and 
= 
in the Gaussian lens formula
+ 
= 
Fig.
Gives f = 8.33 cm, v = 10 cm. From the similar triangles in Fig. we have
= 
,
or D = 0.1 v = 1 cm. Therefore, F = f/D = 8.33.
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