Focus Lens Playbook (50 / 63.5 / 100 mm): When & Why

Choosing the right CO₂ focusing lens is one of the highest-leverage decisions you can make for cut quality. Whether you’re engraving leather or cutting 20 mm knife templates, the correct focal length determines energy density, Rayleigh range (focal depth), kerf straightness, and process stability.

This guide explains: how 50 / 63.5 / 100 mm lenses behave, which materials & thicknesses they suit, how to map thickness → lens, speed windows from real application data, and how to select with a 1-minute decision tree.
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Why focal length matters

Focal length determines the spot size and focal depth (Rayleigh range).

Rule of thumb:
Thin → short focus (speed / detail) Thick → long focus (straightness / penetration)

Rayleigh range (focal depth)

The longer the focal length, the more uniform the beam remains as it goes deeper. That’s why long lenses handle 15–25 mm materials with vertical kerf. 

Short focus → shallow focal depth → only ideal near surface  
Long focus  → deep focal depth   → ideal for 10–25 mm

So, the real question is not “Can the power cut?” but “Can the focal depth deliver stable energy through the full thickness?”

Many users upgrade power first. But for thickness, the lens is often more important than watts.

50 / 63.5 / 100 mm — The 1-minute summary

Lens Spot Focal depth Strength Weakness
50 mm Small Shallow Fast / Fine Weak for ≥10 mm
63.5 mm Balanced Medium Most versatile Not ideal for ≥15 mm
100+ mm Larger Long Thick / Straight kerf Slower on thin

The “best” lens = best match for thickness + material + kerf requirement.

Material–thickness → lens mapping

Based on GWEIKE application data (90% power reference).

Material Thickness Best Lens Why
Acrylic 3–5 mm 50–63.5 mm High detail + decent focal depth
Acrylic 8–15 mm 63.5 mm Better kerf verticality
Acrylic 20–25 mm ≥100 mm Deep focal depth → straight kerf
Knife template 15–20 mm ≥100 mm Vertical slot for steel-rule dies
MDF 3–10 mm 63.5 mm Balanced depth + clearing
MDF 12–18 mm ≥100 mm Resin → deeper heat → need depth
Leather <3 mm 50 mm Fast + fine
Cloth Single 50 mm Fast light cut
PVC 2–4 mm 63.5 mm Stable edge
Mild steel 2 mm 50 mm Higher energy density + O₂ assist

These mappings reflect focal depth + application data.

Real speed windows — What the data shows

These values show why focal length matters for thickness.

Material Thickness Power Lens Best Speed
Acrylic 3 mm 60–150 W 50–63.5 mm 10–35 mm/s
Acrylic 20–25 mm 150 W ≥100 mm ~0.8–2.0 mm/s
Knife template 15–20 mm 150 W ≥100 mm ~1.8–4.5 mm/s
MDF 12–18 mm 150 W ≥100 mm ~3–9 mm/s

The vertical improvement on ≥100 mm lenses is what enables clean and repeatable rule-slot cuts for die boards. This is where 50–63.5 mm simply cannot maintain beam stability.

Lens decision tree

Thickness ≤ 5 mm    → 50–63.5 mm
Thickness 5–12 mm   → 63.5 mm
Thickness ≥ 15 mm   → 100–125 mm

Knife template       → ≥100 mm
Leather / cloth      → 50 mm
Mild steel 2 mm O₂   → 50 mm
Acrylic 20+ mm       → ≥100 mm
If unsure: Start with 63.5 mm → Upgrade up/down based on speed vs verticality.

Kerf straightness — Why long focus wins

Thick materials challenge the beam to stay collimated. Short lenses lose intensity deep into the work → widening bottom kerf.

For die-boards & acrylic ≥20 mm → ≥100 mm is not optional, it is necessary.

SOP — Choosing & validating a lens

  1. Identify material & target thickness
  2. Select lens by mapping table
  3. Set focus:
    • Thin → surface
    • Thick → 25–40% into thickness
  4. Run best-speed baseline
  5. Inspect kerf straightness
  6. Record speed / focus / air / lens

Troubleshooting

Symptom Cause Fix
V-shape kerf Focal depth too shallow Switch to ≥100 mm
Edge burns at bottom Poor clearing Adjust air / deeper focus
Slow & unstable 50 mm too short Upgrade to 63.5/100 mm
Kerf wide Overheated / focus shallow Increase speed / deeper focus

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More lens information 50 mm lenses 63.5 mm lenses

FAQ

Is 50 mm always best for fine work?

Yes — it gives the smallest spot for detail and engraving.

Why is 100 mm necessary for 20 mm acrylic?

Because the deep focal range keeps kerf vertical over large depth.

Can 63.5 mm cut 18 mm MDF?

It can, but 100 mm gives straighter walls and fewer burn marks.

What if my kerf is V-shaped?

Your focal depth is insufficient — upgrade to 100 mm.

Does lens affect speed?

Yes — longer lenses tend to be slower because of larger spot.

Which lens for steel?

50 mm with O₂ assist for 2 mm mild steel.