Astronomy
Eyepieces Many Astronomers prefer to use a Barlow Lens to double the focal length of their objective lens, thus doubling the magnification obtained from any eyepiece used with the Barlow lens.  A Medium Power eyepiece can be used to attain a High Power magnification:  For instance, on an 8", F:5 telescope you could reach maximum useful magnification by using a 5 mm eyepiece with a Barlow lens. This will give you a magnification of 2 X 203 mm = 406X.  You could get this same magnification by using a 0.5 mm X 5 = 2.5 mm. eyepiece,- without a Barlow lens.
The only available 2.5 mm eyepiece is one of an Lanthanum LV type. These have an apparent field of view between 45 and 50 degrees. The Barlow lens will cut the apparent field of view of whatever eyepiece is used with it in half. So if you want the same field of view, the eyepiece you use with the barlow lens should have at least twice the apparent field of view. There are no 5 mm focal length eyepices of any type, with an apparent field of view of 90 degrees.