The effect on confocal imaging of spherical aberration caused by a weak refractive-index mismatch is discussed. Aberration balancing that uses a change in the objective’s tube length is studied. It is found that the range of depths that can be imaged satisfactorily by a high-numerical-aperture objective with compensation is an order of magnitude greater than that without compensation. The aberration balancing tends to break down for extremely high numerical apertures.
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