Eyepiece Projection Back | Up | Next
Formula 20    Calculating the Magnification for Eyepiece Projection

M = (ED - EF) / EF

Where:

  • M = Magnification
  • ED = Eyepiece distance to camera sensor
  • EF = Eyepiece focal length

Example: What is the magnification with an 18mm eyepiece that is 90mm from the camera sensor?

M = (ED - EF) / EF
M = (90 - 18) / 18
M = (72) / 18
M = 4

The magnification is 4x.

Formula 21    Calculating the Effective Focal Length

EFL = FL x M

Where:

  • EFL = Effective Focal Length
  • FL = Scope Focal Length
  • M = Magnification

Example: what is the effective focal length of a telescope with a 125mm aperture at f/8 with 1,000mm of focal length with the 18mm eyepiece that is 90mm from the camera sensor.

The magnification is 4x, so:

EFL = FL x M
EFL = 1,000 x 4
EFL = 4,000mm

The effective focal length is 4,000mm.

Formula 22    Calculating the Effective Focal Ratio

F/# = EFL / A

Where:

  • F/# = F/number or Focal Ratio
  • EFL = Effective Focal Length
  • A = Aperture

Example: With a 125mm aperture f/8 telescope with 1,000mm of focal length and an 18mm eyepiece that is 90mm from the camera sensor, the magnification is 4x and the effective focal length is 4,000 millimeters. What is the effective focal ratio?

F/# = EFL / A
F/# = 4,000 / 125
F/# = 32

The effective focal ratio is f/32.


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