> I think we agree to disagree, because this is off topic here. I'll
leave
> it up to anyone interested to do the test or the math and draw their
own
> conclusions. Formulae for DOF can be found at
>
> http://www.photo.net/photo/optics/lensFAQ
OK
derivative time ... showdown
Using the exact formulae shows you to be correct after all (but IMO to
a small extent and nothing like the magnitude of the popular
statement).
Even at 1:20 magnification the 20mm lens only give 1.5 times the DOF
of a 500mm ... and those are extremes of F for most people.
These formulae are in Langford's Advanced Photography but they are
straightforward to derive by geometry.
Near focus: F.u.(F + ç.N)/(F^2 + u.ç.N)
Far focus: F.u.(F - ç.N)/(F^2 - u.ç.N)
DOF = Far - Near
Where F = focal length
u = object distance from front node
ç = diameter of circle of confusion (here taken as 0.025 micron)
N = Aperture (f-number) (below taken as f22)
Anyway: for various example magnifications at f22 with object distance
adjusted for equal magnification ... compare the differences in DOF
for a 500mm and 20 mm lens ...
M 500mm 20mm
5:1 0.2640 0.2643
1:1 2.200 2.207
1:5 33.001 33.924
1:20 462.247 693.180
I guess I get too bogged down with macro where F has a vanishingly
small effect on DOF and where indeed the increased field of view is
the overwhelming consideration. But 99% of the time this statement
is made people are referring, mentally, to orders of magnitude
difference not the small percentages calculated above.
I admit it ... I wuz wrong (ish ;o)
Bob
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
