Parfocal | Parfocal
Key Words: working distance, finite optical system, Infinity optical system, CFI60 optics, parcentric
Definition:Microscope objectives that can be changed with minimal or no refocusing are termed 'parfocal'
TECHNOLOGY:
Parfocal microscope objectives have an identical distance (the parfocal distance) from the nosepiece objective mounting hole to the point of focus on the specimen. When a set of matched objectives (for example, achromatic objectives of various magnifications) are used they project an image to approximately the same plane in the body tube. Changing objectives by rotating the nosepiece, therefore, usually requires only minimal use of the fine adjustment control to re-establish sharp focus. The parfocal distance differs between manufacturers. Most manufacturers use a parfocal distance of 45 mm, while Nikon's CFI optical system uses a longer parfocal distance of 60mm. This longer parfocal distance results in objectives with longer working distances while maintaining relatively high N.A.s.
APPLICATIONS:
Parfocality is not only convenient across all microscopy applications in reducing focusing time, but is also a safety feature in cases where glass slides are used as minimal focus adjustment helps to prevent contact between the objective and the coverslip. The long working distances provided by Nikon's 60mm parfocal distance are highly desirable when examining specimens in vitro through culture vessels, and for chemical and metallurgical microscopy where the objective front lens must be protected against heat and damaging chemicals by a thick coverslip.
MICROSCOPE CONFIGURATION:
Matched sets of objectives should be mounted in the microscope nosepiece to ensure parfocality between the objectives.
RECOMMENDED SYSTEM:
Nikon's CFI60 optical system, with a parfocal distance of 60mm, provides longer working distances while maintaining relatively high numerical apertures.
LINKS:
Properties of microscope objectives:
