Re: Image diameter ....?

"Art Reitsch" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> I can't find a reference on this, so am I thinking
straight?  A 4x5
> film, taking the longer dimension, is 5inches = 127mm.  So
if  a lens
> has an image diameter of 127mm or more it will cover 4x5,
right?  And
> the larger it is, the more movements permitted?  So on,
say, a 75mm lens
> what would be a reasonable minimum image diameter for some
modest
> movements for landscapes?
> Thanks.
> Art
>
  A "normal" lens has a focal length equal to the diagonal
of the format. For 4x5 _film_ that is about 152mm (6
inches). Sheet film of 4x5 and larger is slightly smaller
than the "nominal" size to allow its being used in adaptors
in glass plate holders.
  Many 4x5 press cameras have 127mm or 135mm lenses because
press photographers prefered to have slightly wide angle
lenses. Since the lenses were used well stopped down for
depth of field anyway a normal lens of the Tessar type would
provide this coverage. However, they are working right at
their limit of coverage. If you want movements you must use
either a longer lens or a true wide angle lens, that is, one
with coverage substantially larger than the format diagonal.
One reason that Dagors were so popular for view camera use
is that they will cover nearly 90degrees. Modern Plasmat
type lenses, like the Rodenstock Sirronar or Schneider
Symmar, cover a minimum of 75 degrees and the newer ones
more, so they are, if not true wide angle lenses, certainly
wide field.
  Dagors are inferior in correction to Plasmats but are much
smaller and lighter, probably not a problem for 4x5. Tessars
will cover about 60 degrees maximum but it varies a little.
Generally, the faster the lens the less it covers.
  Wider angles are gotten with other designs of lenses. Most
modern true wide angle lenses for press and view cameras are
of a type based on a lens by the Russian designer Roosinov.
This type has less fall off of light than the standard
design of lens.
  A general answer to your question is that the maximum
coverage angle of a lens depends on its design. Some lenses
have much larger image circles for a given focal length than
others. On the small side one has lenses like the Goerz
Artar or Dogmar with about 48 degree maximum coverage and on
the other hand a lens like the current Super-Angulon with
perhaps 105degrees. Obviously, the shortest focal length
that can be used with a particular format size depends on
the type of lens.
"Normal" coverage is about 53 degrees. You can calculate the
angle a lens covers for a given format dimension from:

Angle = 2*arctan [(size/FL)/2]

 Where size is any image dimension of interest, normally the
diagonal but can be length or width if you want to know the
viewing angle.

   Note that perspective is dependant exclusively on the
camera position with regard to the subject. Once the camera
is placed the focal length does not affect the perspective
only the size of the image on the film.

-- 
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Richard Knoppow
Los Angeles, CA, USA
[EMAIL PROTECTED]