The thing that changes with focal length is the rate at which focus changes, i.e. the amount of blur outside the DOF-zone.
But at the same magnification and f-stop, DOF itself is the same, no matter what focal length.
People keep using formulae derived from hyperfocal distance formulae.
In which the focal length figures twice (as F(ocal length) and as M(agnification), though noone can explain why.
They may work for hyperfocal distance calculations, i.e. when DOF is infinite. I don't know. But not for DOF
Every bit of 'proof' offered for the assumption that DOF is different when focal length changes, yet scale remains the same makes use of those same old formulae.
None (not a single one) of the proofs offered on the site linked to holds any water.
Last edited by Q.G.; 07-08-2009 at 10:21 AM. Click to view previous post history.