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Parfocal distance refers to the distance between the mounting position of an objective and the focal plane of the specimen. In theory, if objectives of different magnifications have the same parfocal distance, focus is maintained when swithching from one magnification to the next. Most microscope companies, includig Olympus and Zeiss, manufacture objectives with the parfocal distance of 45 mm. Nikon recently introduced CF160 objectives that have a longer parfocal distance of 60 mm. Although manufacturers try to produce objectives with the same parfocal distance across different magnifications, it is difficult to obtain perfect parfocality. High magnification objectives (above 40X) have a depth of field that is less than 5 µm, meaning that parfocal distance accuracty with 5 µm for perfect parfocality between objectives. This is a challenging feat when considering the precision required to manufaccture an objective lens, and parfocality deviations across different objective lenses are normal.
Automated microscope systems iwth a motorized Z drive offer the convenience of automated parfocality correction. The CELENA® S Digital Imaging System from Logos Biosystems has a parfocality correction feature that automatically adjusts for the parfocal deviations that normally occur across different objectives. The CELENA® S has a motoried Z stage fitted with a precision ball screw and stepper motor system for repetitive positioning accuracy and the intuitive user interface makes setting up parfocality correction quick and easy.
See the Tech Note: Automated Parfocality Correction with the CELENA® S
Learn More: CELENA® S