Minox ED-GLAS DATASHEET

INNOVATIONS
IN BINOCULAR ENGINEERING (II)

W

HAT’S THE BENEFIT OF ED-GLASS IN A TELESCOPE

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MINOX GmbH •Walter-Zapp-Str. 4 •D-35578 Wetzlar •Tel.: [phone] / 917-0 •Fax: [phone] / 917-612 •www.minox.com

Minox information order number: 99 199

The experience gathered over decades in the
production of top quality optical systems has
brought forth MINOX products with a perfor­mance reaching the limits of technical feasibi­lity.The profound knowledge in the production
of top-class lenses led to stringent quality
standards which apply to MINOX binoculars
and telescopes as well. Continuous innovation
at MINOX means a clear advantage for the ob­server in image sharpness and brightness.With
ED glass (also called FL for fluoride glass and
APO for apochromatically corrected) MINOX is
setting new standards in nature observation.

What are the advantages
of ED glass in telescopes?

White light is split into its component colors
when it passes through a prism or a lens. The
reason for this, as with other transparent ma­terials, is the change in the refractive index for
the different colors, known as the dispersion of
the glass in question.The amount of dispersion
in the different types of optical glass depends
on the glass composition, i.e. dispersion is a
material constant of the glass used.
This color dispersion of incident light means
that the image of an object imaged by a sim­ple lens is split into many colored partial ima­ges which are not only positioned at different
locations behind the lens, but also differ in
size. If you pick up such an image on a screen
or look at it with a magnifying glass, which is
what the eyepiece represents, instead of sharp
details and contours you will see, depending
on the focusing, a more or less sharp core in
one color, e.g. blue-green,which is surrounded
by a more or less blurred corona in a different
color, e.g. red.Altogether the image is unsharp
due to this chromatic aberration which, in prin­ciple, cannot be eliminated by a single lens.
As the specific dispersion of the various types
of optical glass materials are – fortunately –
different, by combining at least 2 lenses of dif­ferent glass types it is possible to correct their
chromatic aberration. Every optical system of
any real use, whether a photo lens or binocu-

lars, therefore features at least 2 lenses of dif­ferent glass types.
By designing the correct radius of the lens ele­ments and selecting the right types of glass
materials, the optics engineer can position at
least two of the colored partial images at the
same place and match their size. The remai­ning residual aberration for the other colors is
usually so slight that the image already ap­pears free from chromatic error. This is then re­ferred to as a chromatically corrected or achro­matic system.
The remaining residual aberration of an achro­matic lens, however small, will show up if the
intermediate image is subsequently magnified
or looked at through a strong magnifying
glass, i.e. eyepiece. Particular attention must be
paid in photo lenses with long focal lengths or
binoculars with high magnification > 10x and
spotting scopes to ensure that the residual
aberration is reduced as much as possible.
Fortunately, chemists and glass manufacturers
have been able to melt special glass types
containing fluoride which either feature parti­cularly low dispersion, known as Extra-Low
Dispersion glass types, or glass where the re­fractive index changes in a different way to the
majority of "normal" optical glass, these being
referred to as glass types with anomalous par­tial dispersion. For the sake of simplicity, these
two types are usually referred to in combined
form as ED glass. This glass type was first used
in the photographic sector for telephoto lenses
with long focal lengths and high speeds, for a
particularly extensive color correction.
By using this type of glass it is possible to po­sition the color partial images at the same pla­ce for the entire visible light (across the visible
spectrum) and perfectly match their size. Un­wanted color contours are thus effectively eli­minated. This is then referred to as apochro­matic color correction.
The use of special glass containing fluoride in
the new MINOX binoculars with high magnifi­cation and MINOX spotting scopes, has suc­cessfully passed on this proven technology

from extreme photo lenses to binoculars. In
spite of the high magnification and light gat­hering power of the lenses, the residual chro­matic aberrations have been significantly re­duced by these means.The image provided by
the new MINOX binoculars and MINOX spot­ting scopes with ED glass is free from any di­sturbing color fringes or glare, i.e. it is rich in
contrast with pin-sharp definition.
There are, however, some negative aspects
with ED glass types, which should not remain
unmentioned: They are considerably more ex­pensive than normal glass types, the proces­sing method is more difficult and is therefore
more costly, the specific gravity and conse­quently the weight of the lenses is frequently
higher. Nevertheless, this excess in expenditu­re does pay off.With ED glass the optical per­formance of the MINOX binoculars and MINOX
spotting scopes with high magnification is im­proved significantly. This glass provides a pla­stic image exceptionally rich in detail and with
high color rendition throughout the visual
field. The observer is able to see a true-to-life
image, even under critical light conditions.

Simple lens element, not color corrected.

Two lens element with achromatic color correction.

Two lens element using ED/ fluorid glass with
apochromatic color correction.

Heussinger Grafik Design, Solms