A portrait photographer uses a mm lens set to to photograph a subject
PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Geometrical optics and imaging · Geometrical optics and imaging
Optics and Photonics, Ch Stops, pupils and optical instruments
Problem
A portrait photographer uses a 50 mm lens set to f/4 to photograph a subject, using an exposure time t=1/125 s that correctly exposes the scene. She then closes the aperture to f/8, doubling the f-number, to increase the depth of field.
((a)) Before calculating anything, predict whether the new exposure time must be longer or shorter than 1/125 s, and by roughly what factor; predict also whether the depth of field becomes larger or smaller.
((b)) Compute the new exposure time needed for the same total exposure, and the factor by which the tolerated defocus (depth of field) has changed.