Lowering the focus per pass: cutting thick sheets cleanly
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When you cut a thick sheet in several passes, the laser eats deeper into the material with every pass. The focal point, however, stays where you set it at the start. With a motorised Z axis you can move the focus down as you go: in xTool Studio this is called focus lowering (Fokusabstieg), in LightBurn it is Z Offset and Z Step per Pass. This guide shows what the functions do according to the documentation, which machines support them and how to get started safely.
Why the focus is off after the first pass
The laser beam is narrowest at the focal point and widens again in front of and behind it, like an hourglass. The basics are covered in the article Laser focus.
After the first pass, the point where cutting continues is no longer at the surface but at the bottom of the kerf. The beam is already wider there, the energy spreads over a larger area, and every further pass gains less depth. The LightBurn documentation describes exactly this purpose: with Z Step per Pass you move the focal point deeper on every pass so that cutting stays efficient.
Important: lowering does not replace suitable basic settings. xTool states explicitly that lowering is not a fix for failed cuts. Power, speed and number of passes have to be right first, see Laser parameters and Material test.
Requirement: a motorised Z axis
The focus can only be lowered by software if the machine can move the laser head or the work surface in height by itself. This is usually the case for machines with autofocus, but not for open diode lasers with a fixed, screwed-on module.
| Machine (example) | Cutting claim according to the manufacturer | Lowering the focus by software |
|---|---|---|
| xTool S1 40W, diode 40 W | up to 15 mm basswood plywood in one pass | yes, focus lowering 1 to 6 mm according to the xTool help |
| xTool P2S, CO₂ 55 W | up to 18 mm solid wood, plywood up to 12 mm recommended | according to the xTool help for all models except the D series, F1 and F1 Ultra; 1 to 2 mm on the predecessor P2 |
| xTool F2 Ultra, diode 40 W | up to 23 mm wood in several passes | xTool recommends focus lowering for cutting problems |
| Acmer P2 33W, open | up to 30 mm plywood, passes not stated | no, focus by hand |
Without a Z axis, the only option is to do it by hand: set the focus on the surface, lower the module by a fixed value and cut all passes with that value. Adjusting between passes would mean pausing the job and turning screws on the module. Things easily shift in the process, so this is not a good idea.
Focus lowering in xTool Studio
In xTool Studio (formerly XCS) you find the function in the advanced settings of an element. It only applies to vectors in Cut mode, not to engravings. You switch focus lowering on and enter by how many millimetres the focus is lowered.
What the official xTool help documents:
- Setting range: on the xTool S1 the default is 3 mm, range 1 to 6 mm. On the P2, Laserbox Rotary and M1 the default is 1 mm in each case, range 1 to 2 mm.
- Starting value: according to the xTool lab, lowering by 3 mm on the S1 improves the cut in most cases. Example from the help for the P2: 12 mm basswood plywood with 2 mm lowering.
- Limit: the lowering amount must not be larger than the material thickness, otherwise the laser head collides with the material.
- What the best value depends on: material thickness and type, power of the laser module, focal length of the lens and size of the laser spot.
Single-step lowering and stepwise lowering
Newer versions of xTool Studio offer two modes: single-step lowering (Einzelstufenabstieg) and stepwise lowering (Schrittweises Absenken). The release notes for xTool Studio Desktop 1.3.6 only say that focus lowering now supports stepwise lowering and that this can significantly improve cutting performance. xTool does not explain the two modes in detail (as of 10/2026).
Judging by the names and by a project example from the xTool community on Atomm, they work like this:
- Single-step lowering: the focus is lowered once by the set value and stays there for all passes.
- Stepwise lowering: there is a start value, a step amount and an interval in passes. In the community example: start 2 mm, 1 mm deeper every 2 passes.
This is not manufacturer documentation. Check the behaviour on your machine with a test cut on a scrap piece before you rely on it. The maximum value also depends on the machine.
Z Offset and Z Step per Pass in LightBurn
In LightBurn, both values are in the layer’s Cut Settings Editor:
- Z Offset moves the head once for the layer. According to the documentation, positive values move the Z axis inwards, that is closer to the material, and negative values outwards. A deeper focus can help when cutting thick materials.
- Z Step per Pass raises or lowers the laser by the same amount on every pass. This only has an effect if the layer has more than one pass.
The documentation does not explicitly state a sign for Z Step per Pass. So always check the direction with a dry run with the laser switched off or at very low power.
For any of this to work, Enable Z Axis has to be active in the device settings, and your controller has to support Z movements. The documentation warns: with the Z axis enabled you need either Relative Z moves only or a correctly entered material height. Otherwise the default height of 0 can drive the head into the workpiece. If the axis moves the wrong way round, there is the Reverse Z direction switch.
There is a limitation with autofocus machines: many systems treat the autofocus height as the lowest permitted position. In that case you may not be able to set the focus deeper in LightBurn at all. On the xTool S1, LightBurn supports only basic functions according to the data sheet. Only a test shows whether Z commands reach your machine.
How much to lower? Starting values to test
Only the xTool values above are documented. Everything else in this table is a starting value for your own test, not a manufacturer claim.
| Situation | Starting value to test |
|---|---|
| one pass, material over 3 mm | lower once, roughly to the middle of the material, at most the machine maximum |
| several passes, stepwise | step per pass = half the material thickness divided by the number of passes |
| example: 6 mm plywood, 3 passes | 1 mm per pass, ending 3 mm below the surface |
| machine with a small range (1 to 2 mm) | maximum value once, the rest through passes and speed |
The rule of thumb of the middle of the material comes from the article Laser focus. For lowering, xTool names use from a material thickness of about 3 mm.
How to test properly:
- Place a small square, for example 20 x 20 mm, on a scrap piece.
- Keep power, speed and passes the same, and change only the lowering value.
- Cut three variants: without lowering, with a small value, with a larger value.
- Compare the underside: is the part cut through? How wide and how slanted is the edge?
- Note down the best value with material, thickness and machine.
Interplay with passes, air assist and lens
- Passes: lowering only helps if the cut actually gets deeper after every pass. If the first pass hardly gains any depth, a slower speed helps first. Never move the workpiece between passes.
- Air assist: in the deep kerf, smoke would otherwise linger and absorb energy. A good airflow is almost mandatory for thick sheets, see Air assist.
- Lens choice: with CO₂ lasers, a lens with a longer focal length has a greater depth of field. The beam stays narrow over more millimetres, so lowering is less critical. Short focal lengths with a small spot benefit more from tracking.
- Material: with wood, glue layers and density vary. With acrylic, the result depends heavily on the type and the kind of laser. Always test values per material type.
Risks and typical mistakes
- Head too low: if the lowering amount is larger than the material thickness, the head drives into the material. xTool and LightBurn both warn of collisions and damage to the machine.
- Wrong sign: in LightBurn the head rises instead of lowering, or the other way round. Always do a dry run first.
- Focus below the material: if you set a large step with many passes, the focal point eventually ends up below the sheet. Do the sum: step times passes must not exceed the material thickness.
- Uneven material: a warped sheet sits higher in some places. The distance to the head is smaller there and the risk of collision rises. Fix the sheet flat.
- Autofocus measured wrongly: if the sensor measures on an edge or a reflective spot, the starting point is wrong. Lowering then builds on a wrong value.
- Lowering as a cure-all: it does not help with a dirty lens or a laser that is too weak. Further causes are listed under Won’t cut through.
In short
- With several passes, the bottom of the cut lies deeper than the focal point. Tracking keeps the spot small.
- Only possible with a motorised Z axis or an autofocus module. On open machines without a Z axis you focus once by hand.
- xTool S1: 1 to 6 mm, default 3 mm. P2 and M1: 1 to 2 mm. Never more than the material thickness.
- LightBurn: Enable Z Axis, Relative Z moves only or material height, check the direction in a dry run.
- Start the step size test with half the material thickness divided by the passes, then adjust on a scrap piece.
- Air assist on, workpiece fixed, note the value.
Sources
- xTool Support: How to Set Lower the Focus
- xTool Support: How to Lower the Focus for xTool S1
- xTool Support: xTool S1 Focus Lowering
- xTool Support: xTool Studio Desktop V1.3.6 Release Notes
- xTool Support: Materials not cutting through (F2 Ultra)
- Atomm Community (unofficial): Cutting 20 mm Wood with the M1 Ultra
- LightBurn documentation: Cut Settings Editor, Line Mode
- LightBurn documentation: Device Settings, Dimensions and Units