I can make several comments on these discussions.
For the enlarger, and doubling the size of the print. Here our light source
is the light falling on the negative. As the image size is increased the
lens comes closer to the negative, hence effectively more light will fall on
the aperture of the lens. All this light will be focussed on the paper,
hence effectively there will be a (slight) increase in intensity. However,
as this light falls on four times the area (for 2 times magnification) there
will be a four times increase in exposure time.
Normally the lens movement will be small so the change on light falling on
it will be close to the same for both enlargements.
Also we may need to consider any reciprocity failure for the paper, so a
longer exposure may be needed.
If you compensate by opening the lens 2 stops, you avoid reciprocity
failure, but presume that the aperture settings of your enlarging lens are
themselves accurate.
For the laser beam, all lasers have an angle of divergence of the beam.
While we may not be able to assume the centre of the laser as equivalent to
a point source, from the diameter of the spot and known convergence angle we
can determine an effective point souorce, and from that the inverse square
law will be obeyed.
Jim Thyer
