[MF3D.FORUM:1687] Re: Using front shift to simulate camera movements?

[Brian Reynolds <reynolds@xxxxxxxxx>]

Don Lopp wrote:
> I am still at a loss as to how this would work . firstlyb the angke
> covered woulbe quite limmited. macro capabilities quite limmited
> .the presision mountig of the mirrors and lenses would be a prblem
> for even ZEISS and after all of that what would you get that
> couldnot be gotten by normal means.

Tom Hubin has presented a very clever menthod of designing a mirror
system that should not cause vignetting, provided enough is known
about the camera before cutting glass.

I'd like to talk about precision in optics.  Although it seems to go
against common sense, you can get high degrees of precision in optics
through very crude means.

The process of grinding an optical surface is about as crude as you
can get.  Traditionally one piece of glass is placed on a sturdy
support (often a 55 gallon barrel), some water mixed with grit is
sprayed on the glass and another piece of glass is pressed against it
and pushed across it as you walk around the barrel.  Allowing the edge
of one blank to pass over the edge of the other blank gives you a
perfect sphere (concave on one, convex on the other).  After getting
your sphere a careful amount of polishing gives a parabola.  If you're
careful about not allowing the edges to cross you get a flat surface
(which is what we want for mirrors in front of cameras).

Commericial optics firms use machines to do the grinding, but it winds
up that manual grinding is better.  The random errors in manual
grinding cancel out in the long run, whereas the periodic errors in
machine grinding tend to reinforce each other and add up.

High degrees of precision in testing and alignment can also be achived
through crude methods.

When the astronomy club I belong to had an instrument group the head
of the group built a laser interferometer.  It was built from a
supermarket checkout scanner laser, a beam splitting cube and a lens.
I worked on the computer analysis of the results.  We could see errors
on the surface of the mirror down to 1/20 wavelength of the laser
light used to test the mirror.  We were able to correct a 17 inch
diameter mirror down to about 1/16 wavelength before the head of the
group moved out of town.

Amateurs around the world align their optics via the star test,
columating the mirrors while studying an out of focus star.

It is also a mistake to assume that the big commercial names are able
to produce better results than amateurs.  Amateurs that a huge
advantage over the commercial operations, they don't have to show a
profit.  An amateur will work on an optic until he is satisfied.  A
commercial company can only work on it so long as that optic shows a
profit.  It is then either shipped as is (if it is within tolerances)
or discarded.  Zeiss does not sell optics that are as highly corrected
as good amateur optics.  They can't afford to.  Instead they take
short cuts (grinding machines and lots of discards) that let them sell
optics that are good enough and show a profit.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"