= Microscope parfocal distance
The <microscope parfocal distance> is the specified axial distance from an objective's mechanical mounting reference to the focused specimen plane in its intended optical arrangement. Matching this distance helps objectives on a common mount place their focused object planes near the same location. It is a mechanical/optical interface dimension, rather than the <microscope working distance> between the front end and its specified near surface.
A primary manufacturer example shows two objectives with the same45mm parfocal distance and mounting-thread specification but visibly different body diameters and access geometries. The shared distance therefore does not imply identical front-end clearance.<Labeling of Objectives by Leica Microsystems>[“Diameter of Objectives”, Figure8, common45mm parfocal distance with different bodies]{ref}
For an original coordinate example, put the mounting reference at $z=0$ and the focused specimen plane at $z=-45\,\mathrm{mm}$. An objective front surface at−35mm leaves10mm clearance to an uncovered flat specimen; a different objective front surface at−44.4mm leaves0.6mm. Both have45mm mounting-reference-to-focus separation in this stipulated arrangement. Comparing only their parfocal distances would conceal the clearance difference.
Nominal agreement also does not establish perfect focus after every exchange. If actual focused-plane separations are45.00 and45.03mm, keeping the specimen and mounting plane fixed leaves30µm of focus mismatch. Under a stipulated total acceptable <microscope depth of field> of5µm centered at focus, that mismatch exceeds the axial tolerance. A fine focus adjustment or a characterized offset is then needed to compare sharply focused images.
The <focal length> is another length with another reference: the relevant <principal plane>, rather than the mounting shoulder. Magnification, tube-lens arrangement, cover correction and immersion conditions also need to match the instrument design. A shared thread or parfocal distance alone does not establish all those optical compatibilities.
For metrology, identify the mounting reference, focused specimen plane and intended optical configuration before comparing distances. A focus test on a flat reference at the same stage setting can characterize interchange offsets. The resulting axial-offset information is distinct from an image-scale calibration made with a <stage micrometer>.
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