Tune sensor placement tolerance with depth of focus. Use f-number, magnification, or wavelength criteria rigorous. Export tables, verify units, and document your calculations easily.
Depth of focus describes how far the image sensor (or film) can shift along the optical axis while maintaining acceptable sharpness. This is an image-space tolerance and differs from depth of field.
A widely used guideline is t ≈ 2 N c, where t is total depth of focus, N is f-number, and c is the acceptable circle of confusion on the sensor.
When working closer (macro), an improved estimate is t = 2 N c (1 + m), where m is magnification. If image distance v and focal length f are known, you may use (1+m) = v/f.
For diffraction-limited systems, a common defocus tolerance uses Δf = ± 2 λ N², where λ is wavelength. The total span is 4 λ N².
These sample cases illustrate typical outcomes. Values are approximate.
| Method | F-number | CoC / Wavelength | Magnification | Total Depth of Focus |
|---|---|---|---|---|
| Guideline | 8 | 15 µm | — | 0.24 mm |
| Magnification-aware | 8 | 15 µm | 0.5 | 0.36 mm |
| Guideline | 2.8 | 10 µm | — | 0.056 mm |
| Diffraction-limited | 10 | 550 nm | — | 0.22 mm |
| Diffraction-limited | 4 | 550 nm | — | 0.0352 mm |
Depth of focus is the allowable axial shift of the sensor or film while the image remains acceptably sharp. It is a tolerance problem: if the sensor plane moves beyond this limit, blur grows beyond your chosen criterion. This matters in cameras, microscopes, relay optics, and machine-vision modules where assembly variation is real.
Depth of field describes object-space range that appears sharp in front of the lens, while depth of focus is image-space tolerance behind the lens. A system can have large depth of field but still require precise sensor positioning. Treat depth of focus as a mechanical and alignment specification, not a scene-property.
For geometric blur, the calculator uses total depth of focus t ≈ 2Nc, where N is f-number and c is the acceptable circle of confusion. If N = 8 and c = 15 µm, then t ≈ 2×8×0.015 mm = 0.24 mm. Tightening c from 15 µm to 8 µm reduces tolerance by about 47%.
When you do not have a project-specific blur limit, a common engineering approach is to set c from sensor diagonal. Using diagonal/divisor, a 43.3 mm diagonal with a divisor of 1500 yields c ≈ 0.0289 mm (28.9 µm). Increasing the divisor to 2000 gives a stricter 21.7 µm value.
With small pixels, you may prefer a pixel-based tolerance. If pixel pitch is 4.3 µm and you choose a 2× factor, then c ≈ 8.6 µm (0.0086 mm). This can better match digital sampling expectations and helps avoid overly optimistic tolerances when sensors have very fine resolution.
In close-up and macro work, magnification affects defocus behavior. The magnification-aware estimate uses t = 2Nc(1+m). At m = 0.5, the tolerance increases by 50% compared with the small-magnification guideline. This is useful for inspection systems and macro rigs where m is known from design.
If diffraction dominates, defocus tolerance scales as N². The calculator uses Δf = ±2λN² (total span 4λN²). For λ = 550 nm and N = 10, total span is 4×0.55 µm×100 = 220 µm (0.22 mm). Shorter wavelengths tighten tolerance; longer wavelengths loosen it.
Real assemblies face tilt, spacing stack-ups, filter thickness variation, and temperature drift. If you allocate only half the computed depth of focus to sensor placement, the remainder can cover tilt and mechanical aging. Use the export buttons to document assumptions, then iterate CoC and method to match your quality target.
Many designs start with 10–30 µm on the sensor, then adjust using pixel pitch or image-quality requirements. Smaller CoC means stricter sharpness and a smaller depth of focus.
Use the geometric guideline for general estimates. If you know magnification or you work near macro distances, choose the magnification-aware method for a more realistic tolerance.
Use it when the system is near diffraction-limited, such as microscopes, well-corrected optics, or small apertures. It is especially useful when CoC is not the primary limitation.
A smaller aperture reduces the cone angle of rays, so defocus creates a smaller blur circle at the image plane. That increases allowable sensor shift, but reduces light and can increase diffraction blur.
In these formulas, f-number and tolerance dominate. Focal length matters indirectly through magnification and working distance. If you provide image distance and focal length, the calculator uses v/f to estimate (1+m).
The calculator reports a symmetric ± tolerance for simplicity. In real optics, asymmetry can occur from aberrations, sensor stack effects, or non-ideal focusing mechanisms.
Pick a method, record your CoC or wavelength choice, and export the results. Then state the allowed sensor spacing tolerance in your drawing notes and reserve margin for tilt and thermal drift.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.