Why Do Close Objects Need More Exposure?
Case of Where Object is Far Away
When the object is far away from the camera, the magnification of the system given by :-
is approximately zero. S2 is
the focal length of the pinhole camera. If it is increased the light
falling on
the focal plane decreases in accordance with the inverse square
law. The proportion of light falling on
the focal plane is given by
:-
This explains why, for the same exposure, longer focal
length lens require larger diamter of aperture opening
given the same f
stop number.
Case of Where Object is Near (Macrophotography)
As the object is brought closer to the camera, the magnification m is no longer approximately zero. It begins to take on small values eg 0.1, 0.25, 0.33 etc In this case, given a fixed focal length, various photo sources say as the magnification increases, the exposure is required to be increased - by a factor of (1 + m) or (1+m)2. (Not sure which is correct.)
However, on an intuitive level, when the object is brought closer to the camera, more light is being brought closer to the focal plane, so the exposure should need to be decreased, not increased. Hence I'm puzzled.
Usuff
June 30
2006