Setting the laser focus: focal length, focal point and cutting
are explained briefly when you hover over or tap them.
A laser concentrates its light onto a small spot through a lens. Only there is the energy high enough to engrave or cut cleanly. If the material sits too high or too low, the spot gets larger, the engraving paler and the cut wider. Incorrect focus is therefore often the reason why values from a table do not work for you.
Focal point and focal length
The focal length is the distance from the lens to the point where the beam is narrowest. This point is called the focal point. In front of and behind it, the beam widens again, like an hourglass.
The range around the focal point in which the beam is still almost as narrow is called the depth of field. The shorter the focal length, the smaller the spot, but also the smaller the depth of field. With CO₂ lasers you can see this clearly in the usual lenses:
| Lens (CO₂) | Laser spot | Depth of field | Typical use |
|---|---|---|---|
| 1.5 inch | very small | low | fine engraving, photos, small text |
| 2 inch | small | medium | all-rounder for engraving and cutting |
| 2.5 inch | somewhat larger | larger | cutting medium thicknesses |
| 4 inch | large | large | thick or uneven materials over 10 mm |
The classification follows the lens guide of the CO₂ manufacturer Thunder Laser. Diode lasers have fixed optics. Here the manufacturer specifies the correct working distance, usually via a spacer or a focus gauge.
Focusing manually
With many open diode lasers you set the focus by hand:
- Place the material and align it flat. Fix warped wood at the edges with weights or magnets.
- Hold the supplied spacer or focus gauge between the laser module and the material surface.
- Loosen the module, lower it until it rests on the gauge, and tighten it again.
- Remove the gauge. Do not press on the module afterwards.
CO₂ machines often have a focus rod or a small acrylic block that you use to set the height of the table or head.
If you are unsure whether the focus is right, a ramp test helps: place an offcut at an angle so that one end sits a few millimetres higher. Laser a straight line along the whole length. The focus is at the thinnest point of the line. Measure the distance between nozzle or module and material there and use it as a reference in future.
Automatic focus
Many newer machines measure the distance themselves, for example with a probe, a camera or a distance sensor. According to the manufacturer, the Creality Falcon A1 Pro uses a camera and laser distance measurement, and the xTool P2S a LiDAR sensor. This saves time and prevents mistakes with changing material thicknesses.
However, an autofocus only measures one spot or a few points. With curved workpieces, edges or very dark and reflective surfaces it can be off. With new materials, check the result with a short test run. Open machines without a Z axis, such as the Acmer P2, are focused manually.
Focus when cutting
When engraving, the focus is on the surface. When cutting, this is usually also right for thin materials up to about 3 mm. With thicker sheets it helps to place the focus inside the material. A common rule of thumb is the middle of the material. Then the beam is similarly wide at the top and bottom, and the cut edge becomes straighter.
This is how you put it into practice:
- With a Z axis: In LightBurn, the layer settings contain the Z offset for this, which moves the head closer to the material. The LightBurn documentation explicitly states that a deeper focus can help when cutting thick materials.
- Without a Z axis: Focus on the surface with the gauge and then lower the module by half the material thickness. With 6 mm plywood, that is about 3 mm.
- Several passes: With “Z Step per Pass” in LightBurn, the focus moves deeper by a fixed value on each pass. This is practical for thick sheets that need several passes.
Be careful with Z movements: the LightBurn documentation warns that the head can drive into the material and damage the machine. First check values and direction with an empty run. If cuts still do not go through, you will find further causes under Does not cut through.
Defocusing on purpose
Sometimes a blurred beam is wanted. If you place the material slightly below or above the focus, the laser spot becomes larger and the energy is spread out.
- Wider lines: For thick outline lines or markings, a single pass is then enough instead of several side by side.
- Areas without stripes: A slightly enlarged spot can hide line stripes in area engravings. You then usually also have to adjust the line interval, see Laser parameters.
- Sensitive surfaces: On glass, stone or paper, lower energy density can help to avoid chipping or burning through.
Defocusing is always an experiment. A distance of one to three millimetres is a sensible range to test. Note down the value so that you can repeat the result.
Quickly checking whether the focus is right
| Observation | Probable cause |
|---|---|
| Line wide and pale | focus clearly off |
| Engraving strong on the left, pale on the right | material lies at an angle or the table is not level |
| Cut clean at the top, not through at the bottom | focus too high for thick material |
| Result varies from day to day | module has come loose or lens is dirty |
A dirty lens acts like poor focus. Clean it regularly according to the manufacturer’s instructions. And when focusing, too, remember: operate open machines only with suitable safety glasses and never leave them running unattended.
Continue with: Material test and Air assist.
Several passes through thick material? Then it is worth lowering the focus with each pass: Lowering the focus per pass.