There is a significant difference between parfocal objectives and objectives that are not. Having parfocal objectives means that the sample remains in focus when the objectives are changed.
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The most significant characteristic of parfocal objectives is that the image or sample under observation remains focused even when changing objectives.
When you are focussing a sample using a low-power objective and want to change to a higher one, you do not need to refocus anymore. And if you do, the adjustment is minimal. The focus keeps being the same because the distance between the image and the lens remains the same.
This characteristic turns to be convenient because
- <em>it avoids spending time focusing, </em>
- <em>it prevents contact between the lens and the coverslip. </em>
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