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
For a typical small CO2 lens (50.8 mm focal length), depth of field is a few mm. For a longer focal length (100+ mm) it is wider but the spot is also larger overall. For a diode laser at typical wavelengths, depth of field is shallower than CO2 because the focusable spot is smaller.
In practice, getting within ±0.5 mm of the focal point is usually fine for cutting. For engraving photos and fine detail, ±0.2 mm matters.
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 calibration pattern at varying Z heights. The sharpest line marks the focal point. Use the Focus Test dialog.
The Focus Test method is more accurate and accounts for material thickness, the focal point is at the top of the material surface, not the bed.
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 cutting through, focus to the middle of the thickness gives a more uniform kerf top to bottom. For surface engraving, focus on the surface.
How Beam Bench handles it
The Focus Test dialog generates the calibration pattern. Most machines have Z controlled either manually or via the controller; if your machine has Z control, the Focus Test sweeps Z automatically.
The Device Settings → Machine has fields that can store the focal offset per profile.
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.