Wilt wrote in post #14690341
We're now debating semantics. Find anyone who denies, "Large format has shallower DOF than 135 format". The DOF difference is associated with the format size in the minds of most. The FL difference in lenses used for same FOV at same camera position is not commonly the logical link.
I think the real answer to the debate is whether the equation which represents to DOF difference includes FL or includes some other format-size-dependent factor (such as mag factor to make final print). I don't claim to really know, but will attempt to look this up.
I found this information within Wikipedia http://en.wikipedia.org/wiki/Depth_of_field :
"“Same picture” for both formats
The results of the comparison depend on what is assumed. One approach is to assume that essentially the same picture is taken with each format and enlarged to produce the same size final image, so the subject distance remains the same, the focal length is adjusted to maintain the same angle of view, and to a first approximation, magnification is in direct proportion to some characteristic dimension of each format. If both pictures are enlarged to give the same size final images with the same sharpness criteria, the circle of confusion is also in direct proportion to the format size. Thus if
is the characteristic dimension of the format,
With the same f
-number, the DOF ratio is then
so the DOF ratio is in inverse proportion to the format size.
This ratio is approximate, and breaks down in the macro range of the larger format (the value of
in the numerator is no longer negligible) or as distance approaches the hyperfocal distance for the smaller format (the DOF of the smaller format approaches infinity).
If the formats have approximately the same aspect ratios, the characteristic dimensions can be the format diagonals; if the aspect ratios differ considerably (e.g., 4×5 vs. 6×17), the dimensions must be chosen more carefully, and the DOF comparison may not even be meaningful.
If the DOF is to be the same for both formats the required f
-number is in direct proportion to the format size:
Adjusting the f
-number in proportion to format size is equivalent to using the same absolute aperture diameter for both formats, discussed in detail below in Use of absolute aperture diameter
Notice that neither FL nor aperture diameter are parameters within any of the three equations, but
, the characteristic dimension of the format format size is in all three!