True vs Apparent Field Of View

Many beginning astronomers are often confused about whether a lens with a wider apparent field of view (fov) will actually show more of the sky or just make the same true fov appear larger. Heres the answer.

Apparent fov is the edge-to-edge angular diameter of the light emerging from the eyepiece as seen by the eye. It is an inherent specification for a given eyepiece type or design. The apparent fov of an eyepiece is directly related to the true fov seen through the telescope. The true fov is the actual field of view of the eyepiece when inserted into a telescope. The true fov of an eyepiece may be determined by dividing the eyepiece’s apparent fov (provided by the manufacturer) by the magnification.

True Field = Apparent Field / Magnification

For example, if you're useing a telescope with a 1000mm focal length, and a 40mm eyepiece with a 50 degree apparent field, the magnification will be 25x (1000mm/40mm). The true fov will be 50/25 or 2 degrees! A Different 40mm lens with 75 degrees apparent fov will have a true fov of 75/25 or 3 degrees.

So if you have two of the same mm lenses, the one with the larger apparent fov will show a greater section of the sky.

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