RE: [Leica] Re: DOF, thanks Austin

["Austin Franklin" <austin@darkroom.com>]

> > > No, with respect, you're not addressing my point, Austin. DOF
> > > as commonly
> > > used (eg in published DOF tables, lens engravings and
> > > published formulae)
> > > refers to DOF of an image at a particular nominal
> > > magnification and has for
> > > fifty years. Your definition doesn't.
> >
> > I believe DOF (ie, engraved on a lense) is referenced to the film,
> not to
> > any magnification.
>
> Wrong!!!  The depth of field scales on lenses are calculated based on
> the COC that the manufacturer finds acceptable.  The "circle of
> confusion" is typically calculated as the largest on-film circle that
> you see as a point when you make an 8 × 12 print and view it from a
> "normal" viewing distance.

Perhaps it is what 'some' (unstated, unsubstantiated, and unqualified)
manufacturers do, but that doesn't make it correct.  Most criteria that you
claim it is based on is completely amorphous.  The COC can be specified, but
only as it relates to the image projected on the film plane.  The film
choice will change the DOF on the film, the film flatness will change the
DOF on the film, reproducing the image from the film can change what is
viewable on the reproduction as DOF...  To relate something to an amorphous
target is completely absurd, and has no basis in sound engineering or
science.

> I do not understand why some people cannot accept that the generally
> accepted definition of coc and depth of field is in fact tied to film
> size.

I understand that you have cited some unknown, unsubstantiated, reference
(of which I am not saying you don't know the source, and can't substantiate
it) that claims this, but that doesn't make it 'correct'.  It just makes it
some amorphous method that some manufacturer supposedly uses.

As has been pointed out, how DOF is 'determined' should be re-thought, and
possibly standardized, or at least certainly qualified.