2.5 inch lens (63.5 mm) for cutting 6 mm and thicker material
Quick answer
A 2.5 inch (63.5 mm) lens focuses a CO2 beam to roughly 0.107 mm with about 1.7 mm of usable depth. That makes it a good match for cutting 6 mm and thicker material.
The Gaussian beam relationship is standard optics. Real machines use compound lenses on imperfect beams, so the figures are indicative and the comparison between lenses is the useful part.
| Lens | Focal length | Spot diameter | Depth of field | Best for |
|---|---|---|---|---|
| 1.5 inch | 38.1 mm | 0.064 mm | 0.61 mm | Fine engraving and detail on flat stock |
| 2.0 inch | 50.8 mm | 0.086 mm | 1.09 mm | The general purpose default, engraving and thin cutting |
| 2.5 inch | 63.5 mm | 0.107 mm | 1.7 mm | Cutting 6 mm and thicker, and uneven surfaces |
| 4.0 inch | 101.6 mm | 0.171 mm | 4.35 mm | Thick stock and deep vessels, at the cost of fine detail |
The trade you are actually making
Focal length sets two things at once, and they move in opposite directions. A shorter lens concentrates the beam into a smaller spot, which means higher power density and finer detail, but the distance over which the beam stays that small shrinks quickly. A longer lens gives a larger spot with a much deeper usable range.
At 63.5 mm the spot is about 0.107 mm across with roughly 1.7 mm of depth. That generous depth is what lets it cut thick material cleanly, at the cost of the fine detail a shorter lens would give on a flat surface.
Why power density is the real currency
The same wattage concentrated into half the spot diameter delivers four times the power per unit area, and power density is what actually cuts. This is why a well focused beam cuts material that a slightly defocused one only scorches, and why focus is the first thing to check when a machine that used to cut suddenly stops.
It also explains a common frustration: engraving that looks crisp in the middle of the bed and blurry at the edges. If the gantry rails are not parallel to the bed surface, the lens to work distance changes across the travel, and with only 1.7 mm of depth to play with, a fraction of a millimetre is visible.
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Common questions
Is a 2.5 inch lens right for cutting 6 mm and thicker material?
Yes. At 0.107 mm spot and 1.7 mm depth it is well matched to cutting 6 mm and thicker material, giving the depth of field that thick material needs. It is the lens most people reach for on this kind of job.
How do I set focus accurately?
Most machines ship with a focus gauge, a small block or pin that sets the exact lens to work distance, and using it is more reliable than judging by eye. For a machine without one, the ramp test works well: cut a line across a board tilted at a shallow angle and look for the narrowest, cleanest point, then measure that height.
Does a longer lens mean less power reaches the work?
The same watts arrive either way, spread over a larger spot. What falls is power density, which is what does the cutting, so a longer lens cuts more slowly through thin material than a short one. The compensation is that it holds its focus through depth, which is exactly what thick stock needs.
Can I use one lens for everything?
A 2 inch lens is the usual single choice, because it sits between the extremes and handles both engraving and moderate cutting acceptably. If you mostly do one kind of work, matching the lens to it is a genuine improvement, but it is not the first upgrade to make on a machine that is otherwise working well.