T3D PS: Where?

[john bercovitz <bercov@xxxxxxxxxxx>]

Of course when you focus out a long ways or you put an extension between 
your lens and your camera body, the principal point moves out away from 
the camera body.  The reason is that the principal point is tied to the 
glass.

There's even an exception to that: My Nikon 55 mm macro lens has a fixed 
position for the principal points but my Nikon 60 mm macro does not because 
it focusses partially by changing focal length.  The reason Nikon did this 
has mostly to do with autofocus. 

Are you doing something in the range of 1:1 or are you more in the range
of 10:1?  1:1 is where all this stuff becomes more critical.  At 10:1, I
wouldn't worry too much about the exact position of the principal point.

John B


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