[pure-silver] Re: Large Format Tilt
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Thu, 29 Nov 2007 12:30:20 -0800
----- Original Message -----
From: <C.Breukel@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, November 29, 2007 8:04 AM
Subject: [pure-silver] Re: Large Format Tilt
It does, but to use these 4 knobs for focussing would be
extremly
akward, besides that there is no ruler to measure the
focussing depth.
My question still stands:
Back focussing is the same as front focussing?
I guess so,
Best,
Cor
Not quite. If one is far from the object the two are
pretty much the same but if you are working close they are
not. Focusing the front changes the distance of the lens
from the object. If one is working closer than about 5 times
the focal length the two interact noticably. At unity
magnification one can not focus by moving the lens only.
Focusing must be done by moving the back or, when a camera
has only front focusing, by moving the entire camera to keep
the lens to object distance constant.
For swings and tilts its helpful to visualize the image
field as projected by the lens. Moving either changes the
part of the image field recorded by the film, however, front
and back movements change the field in different ways. When
moving the back the optical axis of the projected image
remains the same, when moving the lens it changes. Movement
of either front or back when parallel is equivalent when far
from the object but front movement will change the effective
position of the camera when close. Tilting the lens changes
the angle of the optical center of the image, tilting the
back does not.
One can buy cheap protractors and affix them to the
camera so that some measurement of angle can be made and
recorded. This may be helpful where there is no means of
calibration on the camera.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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