Home › Guides › How to focus a laser properly (and the ramp test that proves it)
How to focus a laser properly (and the ramp test that proves it)
Soft edges, weak engraving and cuts that taper are focus problems far more often than power problems. Here is how to set it, how to verify it, and the one rule that changes for thick cuts.
Check your job time →1. Focus on the material, not the bed
The focus gauge sets the distance between the lens and the top surface of the workpiece - not the bed. Swap 3 mm ply for 6 mm and your focus is now 3 mm out, which is enough to visibly soften an engraving. Re-focus every time the stock thickness changes. On machines with autofocus, make sure the probe is landing on the material and not through a cut-out.
2. The ramp test: find true focus in one burn
Tape a strip of scrap at an angle so one end sits low and the other high, then engrave a single thin line along it. The line is narrowest and darkest at the exact point the beam waist crosses the surface. Measure from that point back to your lens and you have your true focal distance - which is often a millimetre or two off the number in the manual. Do this once per lens and write it on the machine.
3. For thick cuts, drop focus into the material
This is the part most people miss. A focused beam is an hourglass: narrow at the waist, wider above and below. If you focus on the top surface of 6 mm stock, the beam is already spreading by the time it reaches the bottom, so the kerf tapers and the last millimetre never clears. Focusing roughly one third into the thickness puts the waist inside the material and gives a straighter, cleaner cut. For engraving, keep focus on the surface.
4. Lens focal length changes what you can do
A short lens (1.5") makes a tiny spot - excellent for fine engraving - but its depth of focus is shallow, so it struggles through thick material. A longer lens (2.5"-4") has a larger spot but holds usable focus over more depth, which is what you want for cutting. If you cut thick stock and engrave fine detail on the same machine, that is two lenses, not one compromise.
5. Things that quietly ruin focus
- Warped material. A 2 mm bow moves the surface out of focus across the sheet. Clamp, pin or magnet it flat.
- A bent focus gauge. Cheap acrylic gauges get knocked and shaved by the beam. Check it against a caliper.
- Play in the head. If the carriage rocks, focus changes as it moves. Check the wheels and eccentric nuts.
- Bed not parallel. Focus correct at the front, wrong at the back. Measure head-to-bed at all four corners.
Rule of thumb: Change one thing at a time. Re-focus, run a short test cut, look at it. Adjusting focus, speed and power together tells you nothing about which one mattered.
Open the calculator →FAQ
Should I focus on the surface or inside the material?
Focus on the top surface for engraving. For cutting thicker material, focus roughly one third into the thickness so the narrow waist of the beam sits inside the material - this gives a straighter kerf and helps the cut clear the bottom.
What is a ramp test?
Engrave a single line along a piece of scrap propped at an angle. The point where the line is thinnest and darkest is your true focal distance. It is the fastest way to verify focus without trusting the supplied gauge.
Why is my laser engraving blurry?
Almost always focus. Check that focus was set for the actual material thickness, that the material is flat, and that the focus gauge has not been bent or burnt. A dirty lens can also soften the spot.
Do I need to refocus for every job?
Whenever the material thickness changes, yes. Same material, same thickness, same lens - no. Machines with a motorised Z or autofocus still need the correct thickness entered.