Re: Re: [Leica] (apo)logic

["Mike Boylan" <kindrman@istar.ca>]

Dear Gary and List,

Forgive me if I haven't read all the posts pertaining to this topic.

The definitive answer you seek for scientific proof of Apo superiority
is available in the form of MTF charts available from many
manufacturer's. Although I don't believe MTF's to be totally indicative
of a lens visual performance for normal subjective photography they
are quite useful for making some comparisons of lens performance. The
MTF results are quite acceptable for enlarging lenses since we are
primarily concerned with the imaging of a flat field. The ability to
accurately resolve the detail of the original, with high definition and
contrast are clearly indicated by the MTF's.

I refer to the Rodenstock enlarging lenses since we are the Canadian
distributor for them. If we look at the charts for the Apo-Rodagon 50mm
f/2.8, at f/2.8 and for 10x magnification. The lens is able to resolve
a frequency of 40 line pairs per mm at above 50 % across the entire
image diagonal. Compared to the value for a 50mm Rodagon lens which is
about 10% less capable. The curves for the Apo-Rodagon are also much
flatter out to edge of the film's diagonal (43mm). this demonstrates a
lens which is already performing very highly without the aid of
stopping
down. 

When stopped down by 2 stops this difference becomes less noticeable,
between the two lenses, but this is limited by diffraction. The
performance at the edge is
still better for the Apo-Rodagon. Its lens chart also shows an
additional frequency for 80 LPmm which is only applicable for a lens
that is designed for an extended enlarging range, or for examination of
details at a closer than normal viewing distance. The range for the
Apo-Rodagon 50mm lens is 20x (mag.) and 15x for the Rodagon.

The issue of longitudinal colour aberrations is less important for B&W
enlarging, however a less well corrected lens can exhibit a focus
shift,
within the applicable wavelengths involved, for the material in use.
The
bigger in enlargement size that you go-- and the further from the
optical axis you go-- the more visible this effect becomes. It is not
gross but a slight blurring of detail.

Of course all this becomes a moot point if your enlarging system is not
of equal value. The tolerances for maintaining precise focus require
optimum film flatness, and plane parallel alignment of the negative
carrier, lens stage, and the easel. Most consumer systems are not very
well aligned for critical performance. This will be painfully obvious
if you use an optical alignment device.

Lens design is a balance of trade-offs. The use of low dispersion glass
permits the use of more extreme lens designs to optimally correct
imaging errors. Individually we are unaware of their miniscule
properties. When we are viewing an image that is not only resolving the
small details, but the small transitions in tone -- unimpaired by
aberrations -- we see their cumulative effect as an outstandingly
brilliant optical image.

In my opinion it is such factors that also make the Leica image
distinctly different
in it's visual impact. When other manufacturer's lenses begin to smear
as you enlarge, the fine optics are only just beginning to show what
they are all about.


Kind regards, 

Mike Boylan
Professional Products Division
Kindermann (Canada) Inc.

> 
> ------------------------------
> 
> Date: Sat, 04 Mar 2000 09:13:44 -0500
> From: "Gary D. Whalen" <whalen@circle.net>
> Subject: Re: [Leica] (apo)logic
> 
> Erwin,
> 
>     To those of us within this newsgroup that are totally stupid
(maybe I'm
> the only one) concerning the advantages, or lack thereof, of APO
enlarging
> lenses I would certainly appreciate a definitive answer.  I have read
all
> of the responses concerning this topic (and appreciate them all) but
I
> would gleefully accept some direct information supporting the claim
of APO
> superiority.  Hey, the bottom line - I just want to know if there is
> scientific fact supporting the need to purchase an APO enlarging
lense for
> B&W photography.
> 
> Thanks
> 
> Gary D. Whalen
>