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The defenition of the word APOCHROMAT


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Apochromat

An apochromat, or apo lens, is a photographic or other lens that has a high degree of color correction. Chromatic aberration is the phenomenon of different colors focusing at different distances from a lens. In photography, it leads to fuzzy images, and to color fringes at high-contrast edges, like an edge between black and white. Astronomers face similar problems, particularly with telescopes that use lenses rather than mirrors. General purpose photographic lenses are corrected to bring two wavelengths (red and blue) into critical focus in the same plane. Apochromatic lenses are designed to bring three wavelengths (red, green, and blue) into focus. They also generally correct for spherical aberration at two wavelengths, rather than one as in an achromat. Graphic arts process (copy) cameras generally use APO lenses for this reason. Classically designed process or APO view camera lenses generally have a maximum aperture limited to f/9. Recently high speed APO lenses have been produced for medium format, digital and 35 mm cameras.

Optical engineers can design lenses that bring several colors to a focus at the same distance from the lens. Such lenses can be apochromatic, and can give sharper images than uncorrected optics. However, it requires extra pieces of glass in the optical path, and additional kinds of glass. This makes apochromatic lenses heavier and more expensive than their uncorrected counterparts.

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Wikipedia information about apochromat

Hope this helps with the confusion. :lol:

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"the" definition, or "a" definition?

Getting definitions off Wiki is a bit shoddy... :whip2: :lol:

Gaz

I was talking to the chairman of my AS about Apo's and he used the same definition (and he should know being an engineer and having an observatory worth several thoudsand £££ :shock:)

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Here are 2 more definitions:

Achromat - without colour

Apochromat - completely without colour.

Neither of which help much of course ....

Maybe these should be modified to:

Achromat - without too much colour

Apochromat - with little, if any, colour

I've got a feeling that I'm dipping my toe in shark infested waters with these though !.

John,

North Somerset.

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"the" definition, or "a" definition?

Getting definitions off Wiki is a bit shoddy... :whip2: :lol:

It's just a title Gaz. (Fishing again eh?) :) :)

I agree that Wiki are a bit iffy with some most of their info but in this case they are bang on the money

with that one.

Yeh watch them toes John. :)

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SAYING ALL THAT, I have looked through quite a lot of Apo's and seen colour in them my 7" Starfire had some, my mates tak 128 has some there you go but i think as new glass and techniques come on to the market things get better and better also the bigger the objective lens the more chance of colour.WAITING FOR THE COME BACK .

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