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