T3D Re: TECH-3D digest 382

[Eric Goldstein <egoldste@xxxxxx>]

Bob Maxey looks at the DOF marking on his various lenses and says:

> Also, please remember that different lenses have different DOF.

According to Kingslake, the only way to increase the DOF for two lenses
with identical FLs is to introduce perfromance-degrading aberrations:

"Sperical aberration, chromatic aberration, field curvature, and
astigmatism all cause a longitudinal displacement of the best image from
the paraxial focal plane, and thus the depth of field may be aided for
objects situated closer than the focused plane and reduced for objects
lying beyond the focused plane, and vise versa." - "Optics in
Photography" page 102.

I think Bob M. is noticing that different manufacturers use different
CoC criterial for their markings and thus have us tolerate varying
degrees of sharpness when hyperfocusing. Unless the lens suffers from
significant aberrations (as the prewar classic lenses which had a
reputation of increased DOF did!) it is FL, f/stop and focusing distance
which govern DOF, not lens design.

Eric G.


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