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What are the advantages of ED glass?

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Chris Farris View Drop Down

Joined: October/01/2003
Location: Texas
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    Posted: March/15/2004 at 14:23
ED glass (also called FL for fluoride glass, APO for apochromatically corrected and HD for High Definition) can be found in ultra high end spotting scopes and is deemed to be the ultimate glass for true color representation, clarity and brightness.  But will you see a difference when looking through a spotting scope with ED glass?  Are they worth the extra money?....sometimes as much as $500.00 more.  Lets take a look at what makes them better and different than standard spotting scopes.

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 binoculars, 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 binoculars with high magnification and spotting scopes, has successfully passed on this proven technology from extreme photo lenses. 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 binoculars and 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: 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 binoculars and spotting scopes with high magnification is improved significantly. This glass provides an 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.
There is no doubt that binoculars and spotting scopes constructed with ED glass are better than non-ED binoculars or spotting scopes, only the intended user can determine if the ED glass warrants the extra money.  A potential buyer should ask them selves a few question.
1.  Is weight a non-contributing factor in your decision making process?
2.  Is the color of the object viewed extremely important?
3.  Will minute details of the object need to be viewed?
4.  Will a camera be used in conjunction with the spotting scope?
If you answered YES to at least two of the questions above, you should take a better look at ED glass and your budget.
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