Autofocus planning
Sample focus finely enough to find the curve.
Relate focal ratio, wavelength and focuser mechanics to the critical focus zone and autofocus sweep.
Purpose and method
What this calculator does
What autofocus step size and range should I use? Relate optical depth of focus to focuser steps and sweep size.
In words: Critical focus zone grows with wavelength and the square of focal ratio.
Change a value and the result updates immediately. The diffraction expression is a starting point. Seeing, filter offsets, backlash, mirror motion and the focuser algorithm may require a wider sweep or different step spacing.
Working geometry
Focus-curve sampling
A sequence of focuser positions samples both sides of best focus across the critical focus zone.
Inputs
Optics and focuser
Calculated now
Focus run
- Critical focus zone
- 30.25 µm
- Critical focus zone steps
- 30.25 steps
- Suggested sample spacing
- 15 steps
- Total sweep
- 120 steps
- Temperature compensation
- 20.00 steps
Governing equation
In words: The approximate critical focus zone equals 2.2 times wavelength times the square of focal ratio.
Show the working
The calculator applies the equation above to the values currently entered, without rounding until this display.
- Critical focus zone
- 30.25 µm
- Critical focus zone steps
- 30.25 steps
- Suggested sample spacing
- 15 steps
- Total sweep
- 120 steps
- Temperature compensation
- 20.00 steps
The diffraction expression is a starting point. Seeing, filter offsets, backlash, mirror motion and the focuser algorithm may require a wider sweep or different step spacing.