Microscopy beginners may be perplexed when they notice that parts of a sample that appear only marginally out of focus often look much more blurred in an image. The depth of field as seen by the human eye seems to be considerably larger than the depth of field seen by a camera. This bewildering effect occurs because the eye accommodates: While looking through a microscope, the user constantly - and often unconsciously - shifts the plane of focus without touching the focus knob, simply by tuning the eye lenses to a different focal length. Tunable lenses have thus helped researchers gain a more intuitive feel for the three-dimensional shape and texture of microscopic objects since the invention of microscopy.
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