Focal Reducer & Barlow Effects
See exactly what the glass changes.
Compare a native optical system with one reducer, field flattener, or Barlow factor. Every result updates locally as you change the factor.
Purpose and method
What this calculator does
What does this reducer or Barlow do to my telescope? Calculate how an ordered modifier train changes focal length and focal ratio.
In words: Multiply native focal length by every optical multiplier in path order.
The saved train supplies the native telescope, current optical multiplier, camera geometry and binning. Change the multiplier to compare another reducer or Barlow without altering the equipment bookmark.
Working geometry
Focal transformation
The ordered optical multiplier changes the converging cone before it reaches the sensor.
Inputs
Optical path
Enter the native telescope, sensor, and the modifier's dimensionless factor.
Below 1× reduces focal length; above 1× increases it.
Calculated now
Native versus modified
The aperture stays fixed; the modifier changes effective focal length and everything derived from it.
- Native focal length
- 1,000 mmf/5.0
- Modified focal length
- 700 mmf/3.5
- Native field
- 1.35° × 0.90°0.776″ / px
- Modified field
- 1.92° × 1.29°1.108″ / px
What changed
Focal length changed by -30.0%; image scale changed by 42.9%. A reducer widens the field and increases arcseconds per pixel; a Barlow does the opposite.
The calculator models the stated multiplier as ideal. Real reducers and Barlows can introduce aberration, vignetting, spacing sensitivity, and illumination changes.