Field curvatureThe effect of field

Field curvatureThe effect of field curvature means that a flat structureis imaged on a curved surface.This image error can be completely rectified bymaking a suitable choice of the lens curvatures in theobjective. Objectives with a flattened field of viewcontain the word ‘Plan’ in their name. Objectives witha completely flat field of view were invented atCarl Zeiss in 1938 by Hans Boegehold. Depending onthe color correction, the following Plan objectivesare available: Plan-ACHROMAT, Plan-Fluorite andPlan-APOCHROMAT.In practice, field curvature is particularly disruptivein the case of flat specimens such as blood smearsor histological sections. Modern objectives of thePlan-NEOFLUAR and Plan-APOCHROMAT class areCover glasses and mounting mediaCover glasses have a decisive influence on the imagingquality of a microscope as they form an opticalcomponent of the objective. Many objectives are calculatedfor a cover glass thickness of 0.17 mm exactly.For special purposes, e.g. smears, there are also objectivesfor uncovered objects (cover glass thickness = 0).If, however, the thickness of the cover glasses useddeviates from the calculated value, the result is aclearly perceptible deterioration of the image causedby spherical aberration. It should also be taken intoconsideration that the thickness of the mountingmedium also has an impact on the effective coverglass thickness. In practice, a deviating effective coverglass thickness becomes noticeable above a numericalaperture of 0.35. Above a numerical aperture of0.7, even extremely small deviations (+/- 0.01 mm)from the specified cover glass thickness have a significanteffect on the image. For this reason, manyhigh-aperture objectives are equipped with a correctioncollar.fully flattened up to a field of view of at least 25 mm.