Doublets are suitable for many applications when the aperture is fairly small (up to about f/4) and when the field angle is limited to a few degrees. The two components of a doublet may either be cemented together or separated by an air space. In most cases the cemented solution is preferred because its tolerances are easier to meet and it is obviously more robust.
In addition to being used alone, doublets are used in collimators and as objectives of telescopes. Many lens systems contain one or more doublets, and most of the comments made below concerning "stand-alone" doublets will also apply to doublets that are used in these complex systems.
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