F Theta Lens vs Co2 Lens: Which One Should You Buy?

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F Theta Lens vs Co2 Lens: What Actually Differs

The difference starts with how the beam moves. On a galvo system, mirrors steer the beam across the work area while the lens stays fixed, and the F-theta lens keeps the focus flat across that field so the mark stays sharp from the center to the edge. On a CO2 gantry machine, the head itself travels over the work and carries a fixed-focus lens, so the focus point moves with the head. That is why the two lens types are not substitutes: each one is built for a different beam path, a different mount and a different way of moving the beam.

Focal length is the other half of the choice. A fixed-focus CO2 lens focuses at one working distance set by its focal length, so changing the focal length changes where the work must sit under the head. A flat-field F-theta lens is chosen for the scan field of its galvo system instead. In both cases, the lens must match the machine that carries it: diameter, focal length, material and wattage range all have to fit the head and the laser, or the lens cannot do its job.

Safety stays the same whichever lens type you run. Vent fumes out of the room, never leave a running job unattended, and use eyewear only if its listing states an optical density and a wavelength range that matches your laser. Never laser PVC, vinyl, PVC-based faux leather, polycarbonate or ABS, since those materials can release harmful fumes and damage the machine.

The CO2 Gear in This Guide

No F-theta lens appears in the listings for this guide, so the gear below covers the CO2 side: three focus lenses and one mirror, ranked with the focus lenses first. Read them as examples of what a CO2 listing states, and match any purchase to your own machine head.

MCWlaser CVD ZnSe Focus Lens. A 20mm diameter focus lens with a 50.8mm focal length, made from CVD ZnSe, with transmittance of at least 99.5 percent and a thickness of 2.2mm. The listing names wattages from 40W to 200W and a wavelength of 10600nm, so it matches the widest stated wattage range among the focus lenses here, shared with the Cloudray. It ranks first here.

Cloudray CO2 Focus Lens. A 20mm diameter meniscus focus lens with a 50.8mm focal length, also in CVD ZnSe, with transmittance of at least 99.5 percent and a thickness of 2mm, for CO2 machines from 40W to 200W. It matches the MCWlaser on diameter, focal length and transmittance, and differs mainly on thickness, so it ranks second. The listing also notes a double-sided solid coating and wiping resistance.

OMTech ZnSe Focus Lens. An 18mm diameter focus lens with a 2 inch focal length, in CVD ZnSe, with transmittance better than 99.5 percent at a wavelength of 10.6 microns, for 50W to 80W CO2 machines. Its plano-convex shape is listed as easy to install in the correct orientation, and the listing mentions detailed cleaning instructions. Note that its spec table also carries camera fields such as zoom ratio and diaphragm blades, which do not belong to a laser lens; the title and bullets above are the trustworthy part. It ranks third because its stated wattage range is narrower than the two 20mm lenses.

CNCOLETECH Si Mirror Kit. This one is not a lens at all: it is a 3-piece kit of 25mm gold-plated silicon reflective mirrors, 3mm thick, with reflectivity up to 99.8 percent at 10600nm, for 40W to 400W CO2 machines. A mirror reflects the beam along its path instead of focusing it, so it belongs in the beam path rather than the focus position, and it ranks last here for that reason. Note that its spec table lists ZnSe material, a maximum of 150W, and diameters and focal lengths that do not match the mirror described in the title and bullets; trust the title and bullets, which describe a 25mm silicon mirror kit.

Choosing Between Them

If you own a galvo system, buy an F-theta lens made for that system, and check the mount and scan field with the system maker before ordering, since no F-theta lens is featured in the gear above. If you own a CO2 gantry machine, match the focus lens to the head: diameter first, since the lenses here are 20mm or 18mm, then focal length, since all three focus lenses here list 2 inches, then wattage range and ZnSe material for the 10.6 micron CO2 wavelength. A lens that fits on paper but not in the head is no use at all.

For upkeep, follow the instructions that come with the lens, and see our how to clean a laser engraver lens guide before the first cleaning. A scratched coating costs transmittance, and a dirty lens costs focus, so treat the optic as the consumable it is.

FAQ: lasers and CNC

Can I use a CO2 lens on a fiber galvo machine?

No. A CO2 lens is a fixed-focus lens for a moving CO2 head, while a galvo machine needs a flat-field F-theta lens for its mirror-steered beam. The mounts and beam paths differ, so match the lens to the machine type.

What focal length should I choose?

Choose the focal length your machine head takes. All three focus lenses featured here list a 2 inch focal length, which sets one working distance under the head. A different focal length means a different working distance, so confirm the fit before buying.

What is the difference between a lens and a mirror?

A focus lens transmits and focuses the beam: the lenses here state transmittance of at least 99.5 percent. A mirror reflects the beam along its path: the CNCOLETECH kit states reflectivity up to 99.8 percent at 10600nm. They do different jobs and are not interchangeable.

How do I know a CO2 lens fits my machine?

Match four stated figures to your machine: diameter, focal length, wattage range and material. The lenses here are 18mm or 20mm in diameter, list a 2 inch focal length, cover 50W to 80W or 40W to 200W, and are CVD ZnSe for the 10.6 micron CO2 wavelength. If any figure does not match your head, keep looking.

How do I clean a laser lens?

Follow the instructions that come with the lens, since coatings differ and the wrong cloth or fluid can scratch them. The OMTech listing mentions detailed cleaning instructions, and our how to clean a laser engraver lens guide walks through the steps.

Related: the laser engraver hub, best CO2 laser lens and best CO2 laser lens under 30.