Parfocal lens

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A parfocal lens is a microscope objective or camera lens that stays approximately in focus when the magnification is changed. For example, if a microscope is switched from a low power objective (10×) to a higher power one (40× or 43×), the object stays in focus. Similarly, in professional cameras (as well as some compact ones, such as the EX-S770), the object remains in focus as the zoom changes.