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[OM] More on DOF and focal length (by Ctein)

Subject: [OM] More on DOF and focal length (by Ctein)
From: usher99@xxxxxxx
Date: Mon, 29 Jun 2009 19:06:17 -0400
Thanks  Chuck,
Interesting indeed.

Been away at mother's place, just catching up a bit.  -finished 
edging/weeding all beds, mulch applied, condensate central A/C hose 
replaced--asked Home Depot plumber for the "Really Right Stuff-"--vinyl 
tubing that would not kink,  silicone tape on the one junction--repair 
son is 500mi away so no failures permitted.


On surface seems to directly contradict Cambridgeincolour post.

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

But not really on closer inspection.  FL is indeed a minor factor until 
approach hyperfocal distance or high magnification.  Articles 
discussing flip side of the same coin.  Quote of footnote in the 
Camincol article  "

"*Note: We describe depth of field as being virtually constant because 
there are limiting cases where this does not hold true.  For focal 
distances resulting in high magnification, or very near the hyperfocal 
distance, wide angle lenses may provide a greater DoF than telephoto 
lenses.  On the other hand, for situations of high magnification the 
traditional DoF calculation becomes inaccurate due to another factor: 
pupil magnification.  This actually acts to offset the DoF advantage 
for most wide angle lenses, and increase it for telephoto and macro 
lenses.  At the other limiting case, near the hyperfocal distance, the 
increase in DoF arises because the wide angle lens has a greater rear 
DoF, and can thus more easily attain critical sharpness at infinity for 
any given focal distance."


I am not sure about the pupil magnification issue for macro--I've 
always ignored it.  Though it may foul up any dof calculations, I think 
it really only affects the "effective aperture" so the DOF at the same 
true exposure is not really changed--anything else would violate the 
sacrosanct "no free lunch" principle of optics.

Mike









Very timely.  Jan and Mike should find this interesting.  Here's more 
on
DOF and focal length (by Ctein) from The Online Photographer:
<http://theonlinephotographer.typepad.com/the_online_photographer/2009/06
/depth-of-field-hellthe-sequel.html>

Chuck Norcutt


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