Laser Engraver Focal Length by Lens: Spot Size, Kerf and Cut Depth
Focused spot diameter follows 4λf/πD — for a 10.6 µm CO2 beam of 7 mm raw diameter, a 2-inch lens focuses to about 98 µm and a 4-inch to about 196 µm. The short lens gives finer detail and a tighter kerf; the long lens gives a focus window roughly four times deeper, which is what actually lets it cut thick stock. Own a 2-inch first, then add a 4-inch only if you cut above 10 mm.
Swap a 2-inch lens for a 4-inch on the same machine and your engraving gets coarser while your cutting gets deeper. Same tube, same watts, entirely different result — because the lens sets how tightly those watts are packed and how far that packing survives below the surface.
Two numbers control it: focal length, and the diameter of the raw beam arriving at the lens. Most charts omit the second one, which is why two people with the same lens report different spot sizes and both are telling the truth.
Spot size comes from two numbers, not one
For a well-behaved beam, focused spot diameter is 4λf / πD, where λ is wavelength, f is focal length and D is the raw beam diameter hitting the lens. Plug in 10.6 µm for CO2 and it collapses to a shortcut worth memorising: spot in microns ≈ 13.5 × f ÷ D, with both lengths in millimetres.
That means a beam expander, or simply a machine whose optics deliver a fatter raw beam, gets you a smaller spot with the exact same lens. It also means a badly aligned machine clipping the beam on a mirror aperture is quietly giving you a larger spot than the chart promises.
Focal length Raw beam 5 mm Raw beam 6 mm Raw beam 7 mm Raw beam 8 mm 38.1 mm (1.5 in) 103 µm 86 µm 73 µm 64 µm 50.8 mm (2 in) 137 µm 114 µm 98 µm 86 µm 63.5 mm (2.5 in) 171 µm 143 µm 122 µm 107 µm 76.2 mm (3 in) 206 µm 171 µm 147 µm 129 µm 101.6 mm (4 in) 274 µm 229 µm 196 µm 171 µm Measure your raw beam with a tape shot just before the lens, then read your real spot size off this grid. Add 10-30% to every figure for beam quality — a typical sealed CO2 tube has an M-squared around 1.1-1.3, and these values assume a perfect beam. Worth sitting with for a moment: a 1.5-inch lens fed a 5 mm beam gives a bigger spot than a 2-inch lens fed an 8 mm beam. Focal length alone predicts nothing.
The lens table
Kerf figures below are measured at the top surface of 3 mm cast acrylic, which is the fairest common reference — wood kerf varies with species and char, and thin card barely registers a kerf at all. Cut depth is the practical per-pass figure on a 60W machine with working air assist.
Lens Spot at 7 mm raw beam Kerf in 3 mm cast acrylic Usable focus window Practical cut depth per pass What it's genuinely for 38.1 mm / 1.5 in 73 µm 0.10-0.16 mm ±0.4 mm 3 mm Fine detail engraving, small text, thin card and veneer 50.8 mm / 2 in 98 µm 0.15-0.22 mm ±0.7 mm 6-8 mm The default. Handles almost everything up to 6 mm. 63.5 mm / 2.5 in 122 µm 0.18-0.26 mm ±1.1 mm 10-12 mm The compromise lens for 6-12 mm stock 76.2 mm / 3 in 147 µm 0.22-0.30 mm ±1.6 mm 15 mm Thick acrylic and ply where edge squareness matters 101.6 mm / 4 in 196 µm 0.28-0.40 mm ±2.8 mm 20-25 mm Deep cuts, deep relief, uneven or curved workpieces Pick the shortest focal length whose focus window still spans your material thickness. That single rule resolves most lens arguments. The focus window column is calculated as the Rayleigh range either side of the waist, which is the strict optical definition. Manufacturers routinely quote roughly double these numbers because they use a looser criterion — usually the point where the spot has grown 40-50%. Neither is wrong; they answer different questions. Use the strict figure when detail matters and the loose one when you just want the cut to sever.
Depth of field is the spec that decides thick cuts
Here's the part that surprises people. The 4-inch lens doesn't cut 20 mm acrylic because it delivers more power — it delivers less power density than the 2-inch, roughly four times less. It cuts thicker because the focus window scales with the square of focal length, so the beam stays converged through the full depth instead of blooming out halfway down.
Double the focal length and the spot doubles, the power density quarters, and the focus window roughly quadruples. That trade is the whole lens decision, written in one line.
It also explains a common frustration: someone buys a 4-inch lens hoping for more cutting power on 3 mm ply and gets a slower, uglier cut than the 2-inch gave them. For thin stock, the long lens is the wrong tool.
Diode modules work on a different principle
You don't swap focal lengths on a diode module, you swap modules. The optics are a fixed short-focus assembly built around a rectangular semiconductor emitter, so the focused spot is a rectangle, not a circle, and its long axis runs in a fixed direction relative to the gantry.
That rectangularity has a practical consequence most people meet by accident: a fine vertical line and a fine horizontal line of the same width in your design come out at slightly different weights. Rotate the artwork 90 degrees and the effect swaps sides.
Module class Typical quoted spot Spot area vs a 5W single emitter Finest reliable engraved line Where the extra emitters actually help 5W, one emitter 0.06 x 0.06 mm 1.0x 0.10 mm Nothing — this is the detail benchmark 10W, two emitters combined 0.06 x 0.08 mm 1.3x 0.12 mm Cut speed roughly doubles, detail barely suffers 20W, four emitters combined 0.08 x 0.10 mm 2.2x 0.15 mm Thicker single-pass cuts; fine text starts to fill in 40W, eight emitters combined 0.10 x 0.15 mm 4.2x 0.25 mm Depth only. Small serif text is off the table. 2W 1064 nm infrared module 0.02-0.03 mm round 0.15x 0.04 mm Bare metal marking and very fine detail — it will not cut wood Read the last two columns before assuming a bigger module is an upgrade. If your work is jewellery tags and small text, a 5W module out-resolves a 40W one and always will. Beam combining is the reason the trade-off exists. Four emitters get folded into one path with mirrors and polarisers, and each combination step costs you a little spot quality. There's no free lunch where power quadruples and the spot stays put.
Which lens to own first
Buy a 2-inch and stop. It's the right answer for the overwhelming majority of CO2 owners, because it covers 0-6 mm cleanly, engraves at a resolution nobody complains about, and forgives a bit of Z error.
Add a 4-inch only when you have a repeating job in stock thicker than 10 mm. Skip the 2.5-inch and 3-inch unless you've already got both ends covered and you're chasing a specific edge quality on 8-12 mm acrylic. They're perfectly good lenses; they're just a third purchase pretending to be a second one.
One more thing worth the money: a spare of whatever lens you use daily. Lenses die from spatter, not from wear, and they die in the middle of a job.
Refocusing after a lens change
- Fit the lens with the convex face toward the incoming beam on a plano-convex CO2 focus lens. Upside down still cuts — badly — which is why the mistake survives so long.
- Set an approximate Z using the manufacturer's focus gauge or a stack of feeler gauges, then confirm by test rather than trusting it.
- Run a ramp test: tape a strip of card or thin ply at a shallow angle across the bed, cut a single line along it at low power and moderate speed, then find the narrowest, darkest point of the burn.
- Measure the height difference between that point and your bed. That's your correction. Apply it and re-run once to confirm.
- Record the number, the lens and the nozzle in a note taped inside the lid. You'll change lenses more often than you think and you will not remember.
Symptoms of the wrong lens
- Cut opens up wide at the top and pinches at the bottom, on stock well within the machine's power range — focal length too short for the thickness.
- Engraved text under about 4 mm cap height fills in and loses its counters — spot too large, so either a longer lens than you need or a big multi-emitter diode.
- Cut severs at the front of the bed and fails at the back — that's alignment or bed level, not the lens, and swapping lenses will hide it rather than fix it.
- Acrylic edge comes out frosted and rippled instead of clear — usually air assist or speed, occasionally a lens sitting too far from the surface for its focus window.
- Every cut needs one more pass than it did last month — that's contamination or tube age, not a lens spec.
Kerf compensation, once you know your spot
Finger joints and press-fit parts live or die on kerf. The beam removes material centred on the vector path, so a slot cut to nominal size ends up one full kerf too wide and a tab one full kerf too narrow.
Measure it rather than calculating it. Cut a 50 mm square from scrap, measure the square and the hole it came out of, and the difference between them is one kerf. Do it for each combination of lens, material and thickness you use often, and write the numbers on the inside of the lid next to your focus offsets.
As a sanity check, measured kerf usually lands between one and two times the focused spot diameter. If yours is coming in at four times the spot, your focus is off or your lens needs cleaning — the arithmetic is telling you something before the parts do.