"Bob AZ" wrote in message
...
On Jan 1, 4:39 pm, Anton Shepelev
wrote:
Hello all,

Anton et al

FWIW

From the book "The Focal Encyclopedia of Photography"

"the focal point of the lens is the point at which the lens
can be
revolved such that the focus plane remains stationery. The
focus
point is not always within the body of the glass/lens. Also
the
revolvement to try this is limited by the field of view of
the lens.

Bob AZ

Optical benches are equipped with what is called a focal
slide. This allows small fore and aft movement of the lens
while wiggling it. The image of a very distant object, such
as the target in a collimator, is viewed and the slide
adjusted until there is no motion of the image as the lens
is wiggled. The axis or rotation is now exactly at the
principle point. I just posted another way of finding the
principle points but this is the academically correct one. A
view camera is actually a simple optical bench but without
the calibrations for all movements and without a microscope
(or telescope depending the distance of the virtual image)
to examine the aerial image. A view camera can give you very
useful data on longer lenses. For short focal length lenses
one really needs the precision of a true optical bench.


--
--
Richard Knoppow
Los Angeles, CA, USA