Originally posted by troyz The bellows method would decrease depth of field, but would not decrease depth of focus (see wikipedia for the distinction) so it won't
Could you be more specific please? I have found the depth of field (DoFld) and depth of focus (DoFoc) articles on Wikipedia but I am not sure if I understand you correctly. My knowledge about optics is (at best) very limited.
First, according to Wikipedia:
The image distances are measured from the camera's image plane to the lens's image nodal plane, which is not always easy to locate. (
Depth of field - Wikipedia, the free encyclopedia). If I understand correctly, the nodal plane (whatever it is
) is a constant. When using bellows, the distance ("v") between this constant and an image plane is increased greatly. So,
DoFld = sf^2/(f^2+-X), where
X = Nc(s-f)
1/DoFld = (f^2+-Nc(s-f))/(sf^2) = 1/s +-Nc/(f^2)-+Nc/(sf) = +-Nc/(f^2)+(1-+Nc/f)*(1/s) = a+b/s
hence
DoFld = s/(as+b)
(a=+-Nc/(f^2), b=(1-+Nc/f))
s - distance of "ideal" focus
f - focal length
N - "f-number"
c - circle of confusion (c=0.02 for Pentax K7)
DoFoc = 2Ncv/f
From all of this, both DoFld and DoFoc do depend linearly on the use of bellows (in DoFld "a" is small, "b" is almost 1 and, as "s" is also small, DoFld is approximately DoFld=s/b). Obviously, the question is "how does "s" relate to "v"...
So, to sum it all up -
The bellows method [...] would not decrease depth of focus is not true (or I am missing something).