Various people wrote:
"- --- "C.H.Ling" <chling@xxxxxxxxxxxx> wrote:
> I have no experience on the Zuiko 20mm macro, but I
> have seen a macro
> book, the writer uses a lot of different gears,
> ranging from Leica to
> Nikon to Olympus. He said the Zuiko 20mm's
> diffraction problem was
> quite high which limited the use of smaller
> apertures.
>
> C.H.Ling
>
> "Dean C. Hansen" wrote:
> >
>
> > I have a lot to learn about optics--and the
> ways of this 20mm macro
> > lens. Anyone have any suggestions for using this
> guy?
> > Many thanks,
> > Dean
>"
Doing some quick Friday afternoon calculations (which I haven't checked through
but I think they look sensible) for this combination gives the following:
At f/8 , and a 270mm extension the 20mm macro lens will be at a magnification
of 12.5.
I'm ignoring flange to nodal point distances etc. i.e. assuming the lens is
thin.
The object distance will be 22mm.
The angular resolution assuming no lens aberrations (diffraction limited
performance) is 2.5e-4 rad, in other words an object resolution of 5 microns.
This is 100 line pairs/mm.
This sounds good, except we have to take the magnification into account at the
image. This reduces the resolution to 100/12.5 or about 8 line pairs/mm.
So, the 20mm lens will at best only resolve features on the object which are
bigger than 5 microns (0.0002"), and this will be represented on the negative
by a relatively soft looking image.
The answer is to use a bigger aperture. At f/4 the resolution will be 2x better
etc.
This is why in microscopy, numerical aperture is always kept as large as
possible, and depth of field is virtually non existent.
Hope this helps.
Chris Barrett
Malvern
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|