T3D Re: Medium format macro needs 2 more stops?

[john bercovitz <bercov@xxxxxxxxxxx>]

object space     lens   image space
      
     -
        -
           -                -|
              -          -   |
                 -    - |    |
                   ()   |    |
                 -    - |    |
              -          -   |
           -                -|
        -
     -

Above is a lens.  The subject is on the left and the film is
on the right.  There are actually two different lenses - one
is 35 mm format and the other is medium format.  But I have
overlapped them on the same drawing just as I have overlapped
the two different sizes of film.  

Note that the medium format film is twice as far away from its 
lens.  If weren't twice as far away, it wouldn't have the same 
coverage and we would be talking apples and oranges.

Since it is twice as far away, the light falls off by a factor
of four.  Another way of saying it is that the same light cone
has to cover a piece of film which is twice as big in both
directions hence has four times as much area.

Too bad we don't have images.  I could tell you so much more 
with images.  Like why linear aperture determines depth of
field, not relative aperture (f/stop).  I could instead suggest 
you all go out and buy Kingslake but then we wouldn't have these 
discussions anymore.
8-)

John B


------------------------------