Apochromat - Wikipedia

An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better ... Classically designed apochromatic process camera lenses generally have a ... Apochromat FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Thisarticleneedsadditionalcitationsforverification.Pleasehelpimprovethisarticlebyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.Findsources: "Apochromat" – news ·newspapers ·books ·scholar ·JSTOR(October2009)(Learnhowandwhentoremovethistemplatemessage) Chromaticaberrationofasinglelenscausesdifferentwavelengthsoflighttohavedifferingfocallengths. Anapochromat,orapochromaticlens(apo),isaphotographicorotherlensthathasbettercorrectionofchromaticandsphericalaberrationthanthemuchmorecommonachromatlenses. Contents 1Explanation 2Usageinphotography 3Seealso 4References Explanation[edit] Chromaticaberrationisthephenomenonofdifferentcolorsfocusingatdifferentdistancesfromalens.Inphotography,chromaticaberrationproducessoftoverallimages,andcolorfringingathigh-contrastedges,likeanedgebetweenblackandwhite.Astronomersfacesimilarproblems,particularlywithtelescopesthatuselensesratherthanmirrors.Achromaticlensesarecorrectedtobringtwowavelengthsintofocusinthesameplane–typicallyred(~0.590 µm)andblue(~0.495 µm).Apochromaticlensesaredesignedtobringthreecolorsintofocusinthesameplane–typicallyred(~0.620 µm),green(~0.530 µm),andblue(~0.465 µm).[1]Theresidualcolorerror(secondaryspectrum)canbeuptoanorderofmagnitudelessthanforanachromaticlensofequivalentapertureandfocallength.Apochromatsarealsocorrectedforsphericalaberrationattwowavelengths,ratherthanoneasinanachromat. Apochromaticlensbringsthreecolorstoacommonfocalplane.Noticethatthislensisdesignedforastronomy,notviewing,sinceoneofthewavelengths(~0.780 µm)isinthenearinfrared,outsidethevisiblespectrum. Telescopeobjectivelensesforwide-banddigitalimaginginastronomymusthaveapochromaticcorrection,astheopticalsensitivityoftypicalCCDimagingarrayscanextendfromtheultravioletthroughthevisiblespectrumandintothenearinfraredwavelengthrange.Apochromaticlensesforastrophotographyinthe60–150 mmaperturerangehavebeendevelopedandmarketedbyseveralfirms,withfocalratiosrangingfromf/5tof/7.Focusedandguidedproperlyduringtheexposure,theseapochromaticobjectivesarecapableofproducingthesharpestwide-fieldastrophotographsopticallypossibleforthegivenaperturesizes. Graphicartsprocess(copy)camerasgenerallyuseapochromaticlensesforsharpestpossibleimageryaswell.Classicallydesignedapochromaticprocesscameralensesgenerallyhaveamaximumaperturelimitedtoaboutf/9.Morerecently,higher-speedapochromaticlenseshavebeenproducedformediumformat,digitaland35 mmcameras. TheApochromaticlensusuallycomprisesthreeelementsthatbringlightofthreedistinctcolorstoacommonfocus Apochromaticdesignsrequireopticalglasseswithspecialdispersivepropertiestoachievethreecolorcrossings.Thisisusuallyachievedusingcostlyfluoro-crownglasses,abnormalflintglasses,andevenopticallytransparentliquidswithhighlyunusualdispersivepropertiesinthethinspacesbetweenglasselements.Thetemperaturedependenceofglassandliquidindexofrefractionanddispersionmustbeaccountedforduringapochromatdesigntoassuregoodopticalperformanceoverreasonabletemperaturerangeswithonlyslightre-focusing.Insomecases,apochromaticdesignswithoutanomalousdispersionglassesarepossible. Usageinphotography[edit] Independenttestscanbeusedtodemonstratethatthe"APO"designationisusedratherlooselybysomephotographiclensmanufacturerstodescribethecoloraccuracyoftheirlenses,ascomparablelenseshaveshownsuperiorcoloraccuracyeventhoughtheydidnotcarrythe"APO"designation.[2][3] Also,whenconsideringlensdesign,the"APO"designationisusedmoreconservativelyinastronomy-relatedoptics(e.g.telescopes)andmicroscopythaninphotography.Forexample,telescopesthataremarked"APO"arespecialized,fixedfocallengthlensesthatareoptimisedforinfinity-likedistanceswhereasinphotography,evencertainrelativelylow-pricedgeneral-purposezoomlensesaregiventheAPOdesignation.[4] Seealso[edit] Focuserrorforfourtypesoflens,overthevisibleandnearinfraredspectrum. Achromat Superachromat Fluoritelens Listoftelescopetypes References[edit] ^"WhatdoAPOandApochromaticmean?".stason.org.LensesFAQ. ^A300mmf/2.8photographiclenswiththe"APO"designation,testedforchromaticaberration ^A300mmf/2.8photographiclenswithoutthe"APO"designation,testedforchromaticaberration ^"Aninexpensivephotographiczoomlenswiththe"APO"designation".Archivedfromtheoriginalon2011-03-10.Retrieved2011-03-25. 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