[pure-silver] Re: Large Format Tilt
----- 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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