Focus and depth of field
Where the beam is sharpest. Why focal height matters. How much error you can get away with.
A laser beam is narrowest at one specific distance from the lens, the focal point. Closer or farther from that point, the beam diverges and the spot grows. A larger spot means less energy per unit area, which means less cutting power and less engraving sharpness.
The vertical zone where the spot is small enough to do useful work is the depth of field. Outside it, results degrade rapidly.
The basics
Depth of field varies with the laser source, lens, beam quality, material, and process. Use the machine and lens manufacturer's focal-distance guidance, then verify the result on scrap. Do not treat a tolerance copied from another laser as a safe or equivalent setting.
How to find focus
Two common methods:
- Mechanical jig: most diode and small CO2 lasers ship with a small block of plastic or wood the height of the focal distance. Slide it under the head; lower the head until it just touches; remove the jig.
- Focus test: engrave a labeled calibration pattern across a narrow, known-safe Z range. Compare the lines and record the best result for the intended operation. Use the Focus Test dialog only with a compatible GRBL profile that supports Z moves.
The material surface, not the machine bed, is the normal reference for surface focus. The preferred focal depth for cutting depends on the lens, material, thickness, and manufacturer guidance.
Material thickness affects focus
If your work moves from 3 mm plywood to 6 mm plywood, the top of the material is now 3 mm higher. The focal point should rise with it.
For surface engraving, the surface is commonly the intended focus. For cutting, follow the machine and lens guidance and verify the chosen depth on scrap rather than assuming the middle of every material is correct.
How Beam Bench handles it
The Focus Test dialog generates the calibration pattern. Live Frame and Start require an active GRBL profile with Z moves enabled; manual-only Z machines and DSP or galvo connections cannot run the automatic sweep.
Beam Bench does not automatically store or apply the winning focus offset. Apply focus through the machine manufacturer's normal procedure and, if useful, record the value and setup in the machine profile's Notes.
When it matters
- Engraving photos: small spot is everything.
- Cutting thick material: focus position trades top-edge quality for bottom-edge quality.
- Fast engraving: out-of-focus burns lighter; you compensate by slowing down, which negates the speed.
When it does not matter much
- Quick scoring on flat thin material with default focus: life is short.