[MF3D.FORUM:1698] Re: Using front shift to simulate camera movements?

["Don Lopp" <dlopp@xxxxxxxxxxxxxxx>]

Have you ever seen pictures take with a500mm Zeiss Mirotar they are
sharp -good grinding , good centering , excellent mounting- far sharper than
any thing I have seen  made by Nikon, Canon, Minolta etc. I am speaking of
the f/4.5 500mm Mirotar Incidentally I was involved  in the grinding  of
12inch mirror lens foa a Gregoian telescope about 53 years ago using
electric phonograph motors to guide it for photo use-I was not the brains
involeved but I was of some use ?  DON
----- Original Message -----
From: "Brian Reynolds" <reynolds@xxxxxxxxx>
To: "Medium Format 3D Photography" <MF3D.Forum@xxxxxxxxxxxxx>
Sent: Friday, April 13, 2001 10:50 AM
Subject: [MF3D.FORUM:1687] Re: Using front shift to simulate camera
movements?


> 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"
>
>