Question on lens mounting methods

I used to work in a Motion picture rental company. We spent great time
and effort checking and collimating our lenses with laser optical
benches. Frequently we would dismantle a lens and add shims of 1/1000
inch to the lens to adjust it. Sometimes an element would shift ever so
slightly and we would return the lens to manufacturer to re-align.

Thus I find the idea of un-screwing a lens from the shutter and
re-attaching it after mounting on a board to be suspect. Surely the
exact distance from the front cells to the rear cells is critical. I
have even heard tell of photographers that use the same shutter with
different lenses.

Is there some standard on how the threads on the shutters and cells are
positioned ? How can one be sure that you are getting it back in the
optimal alignment ?

"James Dunn" wrote

Thus I find the idea of un-screwing a lens from the shutter and
re-attaching it after mounting on a board to be suspect.

No problem. Only if you lose a shim, and there usually isn't one.

Surely the exact distance from the front cells to the rear cells
is critical.

Everything has an allowable tolerance. For semiconductor equipment
a 1/1000th of an inch adjustment in lens position would be held to
be laughably coarse, after all those folks are comfortable working with
a feature size of 18nM, about 1 millionth of an inch.

For a view camera lens I doubt a double blind study would
show a noticeable difference on the print if cell spacing was 1/50th of an
inch off.

I have even heard tell of photographers that use the same shutter with
different lenses.

Yeah, we're cheap^H^H^H^Hpoor.

Is there some standard on how the threads on the shutters and cells are
positioned?

Lots of them.

How can one be sure that you are getting it back in the optimal alignment?

Looks good on the ground glass, shoot it.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.

James Dunn wrote:
.........

Thus I find the idea of un-screwing a lens from the shutter and
re-attaching it after mounting on a board to be suspect. Surely the
exact distance from the front cells to the rear cells is critical.
.......

Is there some standard on how the threads on the shutters and cells are
positioned ? How can one be sure that you are getting it back in the
optimal alignment ?


The spacing between the cells of the lens is NOT determined by the threads.
Setting the in/out location of a lens cell by having the threads tighten against
each other would be imprecise. A different amount of torque applied by the
user, or thread wear, would cause a different spacing. The method used is
different. There is a shoulder on the cell behind the threads that butts
against a flat area on the shutter surrounding the lens. If you feel the
resistant against turning, you should feel it suddenly rise when the shoulder
hits the stop.

The distance between the flat area (stop) on the front and the flat on the back
of the shutter is specified by the manufacturer of the shutter with a tolerance.
For example, the brochure that came with a Copal 1 shutter gives the "front
and rear lens mounting length" as 20 +/- 0.025 mm. (Probably some old standard
was 0.001 inch, thereby explaining the figure of 0.025 mm). The clever optical
designer will design so that their lens design will perform with the desired
imaging quality despite 0.025 mm variations in the spacing of the cells. If the
spacing needs to be controlled more precisely, extra work (= higher price) will
have to be done, e.g., measuring and correcting with a shim. (However, my
guess is that lenses with shims are probably correcting manufacturing variations
in the optical components rather than in the shutter.)

--Michael

Thanks Michael,
I suspected there was more than I was seeing.
Is it also safe to presume that diferent manufacturers,
ie. Copal, Compur and Seiko have different ideas of
the correct/optimal distance from seating flange on the
front of the shutter to the same on back ?
Is it also different from say Copal #3 to Copal #1 ?

That is, if a lens was designed to use a Copal #1 would
it be spaced correctly in a Copal #3 ?

I guess I should also ask if the thread sizes are the same
from manufacturer to manufacturer ?

"James Dunn" wrote in message
m...
Thanks Michael,
I suspected there was more than I was seeing.
Is it also safe to presume that diferent manufacturers,
ie. Copal, Compur and Seiko have different ideas of
the correct/optimal distance from seating flange on the
front of the shutter to the same on back ?
Is it also different from say Copal #3 to Copal #1 ?

That is, if a lens was designed to use a Copal #1 would
it be spaced correctly in a Copal #3 ?

I guess I should also ask if the thread sizes are the same
from manufacturer to manufacturer ?

There is no standardization of the thickness of shutters.
Each make is different and often different shutters from the
same manufacturer are different. The lens cell or cell and
adaptor are designed to space the cells the correct distance
apart.
The criticalness of this distance depends on the lens
design but, in general, the better the lens the more
critical it is of spacing.
Lens elements in a lens cell are located by the structure
of the cell or by inserted collars. Some lenses have spacing
shims. Element spacing can be very critical. In general,
individual adjustment is expensive so not too many lenses
offer it. However some very high quality lenses are
individually spaced and any shims must be accounted for and
replaced when disassembling them.
Centering is generally done when the element is
manufactured but not always. The usual arrangement for
elements with spherical surfaces is to clamp them between
rings. The ring clamp will automatically center the lens
provided it has been ground correctly. Cemented elements are
centered before cementing by precision edge grinding. they
are also automatically centered by the mount. Aspherical
surfaces are not automatically centered and some mountings,
Leitz for instance, mount the elements in rotatable rings
which are hand centered in the mount. The centering of these
lenses can be lost when disassembled.


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