A microscope can be said to be “parfocal” when an object can be observed from the lowest magnification to the highest without having to re-focus
In most modern microscopes, this distance (45 millimetres) is the chosen distance from the opening of the nosepiece to the focused specimen. By having a standard distance for all objectives, it is easy to rotate each objective, in turn, into the light path with a minimum of refocusing. Such a battery of objectives, despite various lengths of the objectives themselves, is described as being parfocal. If, in addition, a focused feature which has been centered in the field of view remains centered as objectives are changed, the objectives are described as “parcentric”.

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