A good, inexpensive eyepiece design that uses one simple (single element) lens, and one two-element achromatic lens.
Inexpensive three-element eyepiece.
a good general-purpose eyepiece and a Barlow lens placed between a focuser and eyepiece can triple magnification
An economy eyepiece employing a cemented doublet lens and singlet lens.
A 3-element eyepiece design that can perform reasonably well at low-power (wide-view) focal lengths. Although this is a very old eyepiece design it is often sold at reasonable prices, under the â€œmodernizedâ€ name Super Modified Achromat (SMA).