Backfocus Calculator

Calculate and verify your backfocus spacing to ensure flat field and round stars across the frame.

Required Backfocus

Reducer/Flattener Preset

Required Distance (mm)

Optical Train Components

Add Common Component

Add Custom Component

This is a filter (glass)

Backfocus Analysis

0.0

Stack (mm)

0.0

Effective (mm)

55.0

Required (mm)

Difference-55.0mm

Under required backfocus

PROBLEMATIC

Spacing issues likely - stars may be elongated at edges

Insufficient backfocus. Add 55.0mm

Tips

Filters: Glass adds about 1/3 of its thickness to effective backfocus due to refractive index.

Tolerance: Most reducers/flatteners work well within ±0.5mm. Beyond ±1mm, expect edge distortion.

Verify: Always take a test image and check corner stars after assembly.