[pure-silver] Bellows factor and DOF

Hi,

I have this rather specific question about bellos factor and Depth Of
Field (DOF). I find the set of programs in Bob Wheelers Vademecum for
the palm (http://www.bobwheeler.com/photo/Software/software.html) rather
handy for several calculations, there is onne thing I do not fully
understand (and I tried contacting Bob but he does not respond.)

Take this example: a 4*5 camera with a 240 mm lens, physical diameter
aperature is 30mm, so 240/30 = (f)8.

For a 1:1 magnification the bellows are extended twice (240+240)=480mm,
so the same physical diameter results in 480/30= f16.

Which fits ofcourse: the program (Dofu) gives a Bellows factor of 4,
meaning a 4 times longer exposure times (or 2 stops more ).

The program also outputs the DOF at the 1:1 magnification distance (480
mm from the lens) which is 1.6mm in front of and 1.6mm behind the focal
plane at 480mm. So far so good and I understand that.

But the program also outputs an Adjusted N (read F stop) of 4, and the
manual says that to realy know the DOF you should calculate again the
DOF, this time not using the original f8 but the adjusted F4: resulting
in: 0.8mm in front and 0.8mm behind the focal plane.

This is what I do not understand: on the one hand means extending the
bellow that your f-stop changes from f8 to f16 (INCREASING dof), OTOH
the program tells that the for proper DOF caculation you should use 2
f-stops wider.

Does that mean that the longer focal length has a much bigger effect on
DOF that the smaller F stop?..

I hope you can still follow me, and insight appreciated!

Best,

Cor
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