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Eyepiece apparent vs real field of view
Eyepiece Projection

Eyepiece Apparent vs Real Field of View

Two methods, the first is an approximation but is often the easier to work out due to lack of information about field stop sizes.

Method 1

Magnification = Focal Length Scope / Focal Length Eyepiece

Real FoV = Apparent FoV / Magnification

Focal length of scope
Focal length of eyepiece
Eyepiece apparent FOV
Calculated Real Field of View

Method 2

RealFoV = (Eyepiece Field Stop / Focal Length Scope) * 57.3

Focal length of scope
Eyepiece field stop diameter
Calculated Real Field of View

Eyepiece Projection

Use this formula to calculate the resulting focal length and focal ratio when using eyepiece projection:

magnification = (epid - epfl) / epfl

Resulting FL = scope FL * Magnification

Resulting fRatio = scope fRatio * magnification

CCD-EP distance (mm)
Scope focal length (mm)
Scope focal ratio
Eyepiece focal length (mm)
Calculated focal ratio
Calculated focal length (mm)
Magnification factor


epid = Distance of CCD from Eyepiece
epfl = Eyepiece focal length

Note: This only gives an approximate value for the resulting focal length and focal ratio. The distance from the eyepiece is hard to measure, and the nodal point of the eyepiece (the point of its effect - which is usually internal to the eyepiece) is normally unknown.