The ability of the compound microscope to remain relatively focused at higher powers once focused at a lower magnification is called
parfocal ability.
Parfocal capability is the ability of different eyepieces, such as telescopes or microscopes, to focus their images in the same plane. These eyepieces or lenses can then be interchanged without readjusting the instrument.
Parfocal refers to the ability to change objectives with differing powers and have the focus remain sharp. The distance from the top of the objective to the object being viewed is the parfocal distance.
DIN and JIS standard objectives are both parfocal but at different distances. For DIN it’s 45 mm; for JIS, 36 mm.
If you don’t know if yours is a DIN or JIS scope, you can check to see if it’s parfocal. Select a slide or object that can be sharply focused with the most powerful objective on your scope. Then switch to the less powerful objectives in order down to the lowest power. With a parfocal microscope, a slide that is in focus with the 40x objective will still be in focus when using less powerful objectives with little or no refocusing.
You can go from low to high powers to check parfocalization as well. But it's quicker going from high to low, since high-power objectives have very narrow ranges of focus and successively switching to lower powers (wider ranges of focus) will guarantee you’re very close to focus with parfocal objectives.
A similar term is parcentered. It means that objects will stay at or very close to the center of the field of view when changing objectives.
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