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Light Microscopy: Comparison of Optics

Author(s): David R. Caprette, PhD

Types of Optics Compared

The best choice of optics depends on the specimen to be viewed. Specialized optics can give wonderfully detailed views of objects that are disappointing in bright field, but bright field often is a better choice. Bright field optics are usually-but not always-the best choice for viewing stained tissue.
A naturally pigmented specimen, such as Spirogyra may appear more dramatic with dark field or D.I.C. optics, but the cell divisions and chloroplasts are distinct in bright field, and the colors are true to nature. Phase contrast does not contribute additional information, and the halo that typically surrounds a specimen actually detracts from ideal contrast. On the other hand, phase contrast optics give the best view of spore-forming bacteria such as Bacillus thuringiensis. Dark field optics show the cell walls and spores with excellent resolution. To see such features in bright field, one must stop down the condenser aperture, causing distortion of the details.
Differential interference contrast does not do much for our views of a specimen such as Spirogyra or Bacillus. On the other hand, D.I.C. optics increase the depth of focus, making features of an object such as an amoeba very distinct even if they do overlap each other.