342CHAPTER 25 25.5The exposure time is being reduced by a factor of 211256 s1132 s8tt==Thus, to maintain correct exposure, the intensity of the light reaching the film should be increased by a factor of 8. This is done by increasing the area of the aperture by a factor of 8, so in terms of the diameter, ( )2221484DDπ=or 8=. The new f-number will be ()( )1221-number4.0-number1.4888ffffDD===or 1.4f25.6(a) The intensity is a measure of the rate at which energy is received by the film per unit area of the image, or 1imageIA∝and x. Consider an object with horizontal and vertical dimensions yhhas shown at the right. If the vertical dimension intercepts angle θ, the vertical dimension of the image is yhq′ =, or y′ ∝. Similarly for the horizontal dimension, x′ ∝, and the area of the image is 2imagexyA′′=q∝. Assuming a very distant object,
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