 # Ken's Eyepiece Corner - Formulas

In 1996-1997 as an experiment in using Javascript, I wrote this calculator with a suite of additional pages focusing on eyepieces. At that time, these pages won an award from Griffith Observatory. You can find my contemporary website at: https://klhess.com

Enter two of the three values for the objective diameter, f/number, and focal length, then press Compute to calculate the remaining value.

### f/Number = Objective focal length / Objective diameter

Objective focal length
Objective diameter
f/Number of objective

Enter two of the three values for the effective focal length, objective focal length, and Barlow X factor, then press Compute to calculate the remaining value.

### Effective focal length = Objective focal length * Barlow X factor

Objective focal length
Barlow X factor
Effective focal length

Enter two of the three values for the eyepiece field stop, objective focal length, and field angle, then press Compute to calculate the remaining value.

### Field angle (degrees) = (Eyepiece field stop / Objective focal length) * 57.3 degrees/radian

Eyepiece field stop
Objective focal length
Field angle (degrees) 