Critical underfocus in low‐magnification electron microscopy: Rationale and practice
Identifieur interne : 000430 ( Istex/Corpus ); précédent : 000429; suivant : 000431Critical underfocus in low‐magnification electron microscopy: Rationale and practice
Auteurs : Timothy Fahrenbach ; John H. LuftSource :
- Journal of Electron Microscopy Technique [ 0741-0581 ] ; 1988-04.
English descriptors
Abstract
Focus in transmission electron microscopy, especially at low‐to‐moderate powers, is usually achieved by an empirical, though largely haphazard, process of defocusing the objective below what is known to be perfect focus as defined by the absence of Fresnel diffraction fringes at the edges of holes. In this article we provide a precise, empirical method of focus, whose rationale resides in the less‐than‐perfect image transfer from the image space of the objective to the negative, i.e., the modulation transfer function. In practice, “perfect focus,” as defined above, is established with a beam deflector (“wobbler”), to which underfocus is then applied routinely by reference to a table. Such “critical underfocus” values have to be calibrated, as described here, for each microscope and depend on all factors that influence contrast, that is, accelerating voltage, condenser defocus and coherence of illumination, focal length of the objective, aperture sizes, combination of intermediate and projection lenses, and the properties of the photographic emulsion.
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DOI: 10.1002/jemt.1060080411
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In this article we provide a precise, empirical method of focus, whose rationale resides in the less‐than‐perfect image transfer from the image space of the objective to the negative, i.e., the modulation transfer function. In practice, “perfect focus,” as defined above, is established with a beam deflector (“wobbler”), to which underfocus is then applied routinely by reference to a table. Such “critical underfocus” values have to be calibrated, as described here, for each microscope and depend on all factors that influence contrast, that is, accelerating voltage, condenser defocus and coherence of illumination, focal length of the objective, aperture sizes, combination of intermediate and projection lenses, and the properties of the photographic emulsion.</div></front></TEI><istex><corpusName>wiley</corpusName><author><json:item><name>Dr. Fahrenbach</name><affiliations><json:string>Laboratory of Electron Microscopy, Oregon Regional Primate Research Center, Beaverton, Oregon 97006</json:string></affiliations></json:item><json:item><name>John H. 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In this article we provide a precise, empirical method of focus, whose rationale resides in the less‐than‐perfect image transfer from the image space of the objective to the negative, i.e., the modulation transfer function. In practice, “perfect focus,” as defined above, is established with a beam deflector (“wobbler”), to which underfocus is then applied routinely by reference to a table. 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Such “critical underfocus” values have to be calibrated, as described here, for each microscope and depend on all factors that influence contrast, that is, accelerating voltage, condenser defocus and coherence of illumination, focal length of the objective, aperture sizes, combination of intermediate and projection lenses, and the properties of the photographic emulsion.</p></abstract></abstractGroup></contentMeta></header></component></istex:document></istex:metadataXml><mods version="3.6"><titleInfo lang="en"><title>Critical underfocus in low‐magnification electron microscopy: Rationale and practice</title></titleInfo><titleInfo type="abbreviated" lang="en"><title>CRITICAL UNDERFOCUS</title></titleInfo><titleInfo type="alternative" contentType="CDATA" lang="en"><title>Critical underfocus in low‐magnification electron microscopy: Rationale and practice</title></titleInfo><name type="personal"><namePart type="termsOfAddress">Dr.</namePart><namePart type="family">Fahrenbach</namePart><affiliation>Laboratory of Electron Microscopy, Oregon Regional Primate Research Center, Beaverton, Oregon 97006</affiliation><description>Correspondence: Laboratory of Electron Microscopy, Oregon Regional Primate Research Center, Beaverton OR 97006</description><role><roleTerm type="text">author</roleTerm></role></name><name type="personal"><namePart type="given">John H.</namePart><namePart type="family">Luft</namePart><affiliation>Department of Biological Structure, University of Washington School of Medicine, Seattle, Washington 98195</affiliation><role><roleTerm type="text">author</roleTerm></role></name><typeOfResource>text</typeOfResource><genre type="article" displayLabel="article"></genre><originInfo><publisher>Wiley Subscription Services, Inc., A Wiley Company</publisher><place><placeTerm type="text">Hoboken</placeTerm></place><dateIssued encoding="w3cdtf">1988-04</dateIssued><dateCaptured encoding="w3cdtf">1987-05-15</dateCaptured><dateValid encoding="w3cdtf">1987-10-30</dateValid><copyrightDate encoding="w3cdtf">1988</copyrightDate></originInfo><language><languageTerm type="code" authority="rfc3066">en</languageTerm><languageTerm type="code" authority="iso639-2b">eng</languageTerm></language><physicalDescription><internetMediaType>text/html</internetMediaType><extent unit="figures">4</extent><extent unit="tables">1</extent><extent unit="references">14</extent></physicalDescription><abstract lang="en">Focus in transmission electron microscopy, especially at low‐to‐moderate powers, is usually achieved by an empirical, though largely haphazard, process of defocusing the objective below what is known to be perfect focus as defined by the absence of Fresnel diffraction fringes at the edges of holes. 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