[pure-silver] Re: Large Format Tilt
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 30 Nov 2007 03:45:48 -0800
----- Original Message -----
From: "Nicholas O. Lindan" <nolindan@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, November 29, 2007 10:17 AM
Subject: [pure-silver] Re: Large Format Tilt
From: <C.Breukel@xxxxxxx>
My question still stands:
Back focussing is the same as front focussing?
It is, but only at far distances where lens-to-subject >
~10x the focal length.
At near distances the back-subject distance is set first
for the desired magnification and then the lens position
is adjusted for focus. It is easy to remember the order:
it is the same as with an enlarger. For fun, try doing
it backwards...
With cameras that twist into pretzels, like Sinars,
with appropriate twisting there can be quite a difference
between front and back focus - though both will drive you
equally mad.
==
Nicholas O. Lindan
Cleveland Engineering Design, LLC
Cleveland, Ohio 44121
I'm not sure you meant the above. As mentioned in my
earlier post focusing at distances less than about 5 times
the focal length must be done by moving either the back or
the entire camera if it does not have back focusing. If the
lens is moved it changes both the focus and the distance
simultaneously. When this is done there is no point of
focus. The camera must first be set up so that the lens is
the right distance from the object for the desired
magnification ratio. The back is then adjusted for focus. If
the back can not be moved then the camera must be adjusted
by successive approximation by first establishing the lens
position for the desired magnification and then moving the
entire camera back a little exending the lens to maintain
its desired position. After a few tries the correct
adjustment will be found.
The magnification ratio is R = u/v where u = subject
distance and v = film distance. For precision work the
measurements are made to the principal planes of the lens.
For most work the planes can be assumed to be about at the
diaphragm.
The basic lens relationsips are:
1/F = 1/u + 1/v
Where
F = focal length
u = lens to object distance
v = lens to film distance
Image distance will be v = (u * F)/(u-F)
For equal object and image size the distances will be equal
and the total will be 4X the focal length. This
releationship holds for telephoto and reverse telephoto
lenses provided that the measurements are made to the
principal planes.
Its obvious from the above equasion that moving the back
of the camera changes one variable while moving the lens
changes two variables simultaneously.
I've found it helpful to visulize what happens to the
object plane when moving the lens or the back of the camera.
Remember that everything is rotated 180 degrees between the
two planes. Also, that the longitudinal direction in the two
planes is reversed with relation to the lens. So, moving the
lens closer to the image plane (film or ground glass) moves
the plane of focus in the object field further away. In
absolute direction both move the same way. So, if you tilt
the back so the top is closer to the lens than the bottom
the plane of sharp focus in the object field will tilted so
that it is further away at the top and closer at the bottom.
Moving either the front or back in a plane (rise, lower,
shift) selects a different part of the image where the
projection or mapping of the image is no longer symmetrical
about the axis. Where the lens is made to be orthoganal, as
are most cameras lenses, this can be used to "correct"
diminishing perspective. Not all lenses are orthoganal, for
instance a fish-eye lens will not correct perspective when
shifted.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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