Back-focus Spacing
Build the imaging train to the right depth.
Add the optical thicknesses between the reference shoulder and sensor, then account for the first-order focus shift introduced by a filter.
Purpose and method
What this calculator does
Which spacer thickness do I need? Balance adapters, filters and camera depth against required back-focus.
In words: Required spacer equals specified back-focus minus installed component depth.
Mechanical sensor depth, accessory thicknesses and the reducer or flattener's specified back-focus are not part of the optical equipment profile, so they must be measured or read from drawings.
Working geometry
Mechanical back-focus stack
Adapters, filter hardware and camera depth fill the distance from reference shoulder to sensor.
Mechanical stack
Spacing inputs
Measure each item along the optical axis from shoulder to shoulder.
1.5 is a useful estimate when the glass specification is unknown.
Calculated now
Required spacing
Add 0.17 mm of spacing.
- Corrected target
- 55.67 mm
- Installed train
- 55.50 mm
- Required spacer
- 13.17 mm
- Spacing error
- -0.17 mm
- Filter allowance
- 0.67 mm
Equations
Manufacturer back-focus values and tolerances take precedence. Filter shift is a first-order paraxial estimate; tilt, thread engagement, shoulder placement, and wavelength can change the practical setting. Negative required spacer means the fixed train is already too long.