Good question, see this link:
http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/
zuiko/htmls/100mm.htm
...and just quoting a small section:
" Focusing features floating rear elements to counter and suppress
distortion to absolute minimal level. While Extra-Low Dispersion (ED) glass
and special dispersion glass with high refractive index and low dispersion
optical properties are used in the front lense elements to provide extremely
high resolution and contrast images. The extra-low dispersion glass ("ED")
with superior chromatic-aberration-correcting properties, enables secondary
colour spectrum in particular, blue and red light rays be brought to focus
at the same plane to offer images with crisps, sharp details and faithful
colour reproduction. There are only TWO known Zuiko lenses below 100mm focal
length which use ED glasses thus far in their optical composition, other
than the Zuiko AUTO-T 100mm f/2.0 discussed here, another ZUIKO lense was
actually a very fast speed Zoom lense, the ZUIKO 35-80mm f/2.8ED which
incidentally, was introduced at a very late stage back in 1996.
Incidentally, the Zuiko AUTO MACRO 90mm f/2.0 which offers as alternative to
this lense as it has the same lense speed at f/2.0 but its main strength is
being a true MACRO lense which can focus from infinity to 1:2 life size
reproduction at its closest working range."
So perhaps the floating rear elements have some effect on the DOF ?
...Wayne
>
> Do either the 100/2.8 or 100/2 use a component of internal focussing ?
>
> ...Wayne
>
> >
> > Before digging into full wave dof calculations vs thin lens
> > approximation and correspondence with Nasse at Zeiss,
> > would not have thought such a dof discrepancy
> > could be real. It seems very possible now. Real FL differences also
> > change the distribution of the dof, but don't know
> > the actual measured FL for each of the Zuikos and doubt they differ
> by
> > very much. First thought the differences
> > might be due to focus shift----see the demonstration below with the
> > Zeiss 85/1.4. Fast lenses of certain designs w/o floating elements
> > often shift focus stopping down. Dr. Nasse seems to get pinged on
> this
> > issue a bunch.
> >
> > http://www.the-digital-picture.com/Reviews/Zeiss-85mm-f-1.4-ZE-
> Planar-
> > Lens-Review.aspx
> >
> > Then AG said he had focused at the shooting aperture, which
> > ruled that out.
> >
> > Still, a similar set up with the Zuikos might easily resolve the
> > question unequivocally.
> > Would need to be cautious with the experimental design. Dr. Focus
> > appears to be around to help.
> > Anyway enjoyed the post.
> >
> >
> > Mike
> >
> >
> >
> > When shot at exactly the same apertures, and carefully
> > > selecting the same focus point, the 100/2.8 always seems to have a
> > > touch
> > > more DoF on the near-side. At this time, I'm calling this a human
> > > error, yet
> > > time and again, when focused in exactly the same manner on exactly
> > > the same
> > > point, the 100/2.8 is sharper closer in. I've even focused the
> lenses
> > > stopped down and the same thing occurs. Before letting this go,
> > > totally,
> > > what I'm wondering is whether or not the rate of defocus either
> side
> > > of the
> > > plane of focus is assymitrical between the two lenses.
> >
> > --
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