A 12mm zinc selenide lens, the size the stock K40 head takes, in a long 4-inch focal length option. ZNNRKMYO sells the same lens in 12, 18, 19.05 and 20mm and in focal lengths from 38.1 to 127mm.
Here is the rare lens that fits a K40 without a new head. Factory K40 heads hold a 12mm lens, and this ZNNRKMYO option is exactly that. Unscrew the lens tube, swap the lens, and the hardware stays as it came from the factory.
That matters because many lens upgrades quietly assume you have already replaced the head. An 18mm or 20mm lens needs an upgraded head. A 12mm lens works in the stock one, and also in most upgrade heads, which usually keep a 12mm option with a spacer ring.
The catch is the focal length on this particular option. The stock lens is almost always 50.8mm, 2 inches. This one is 101.6mm, 4 inches, which puts the focal point two inches further down. For a plain stock replacement, order ZNNRKMYO's 12mm lens in the 50.8mm focal length instead.
The blank is Chinese PVD zinc selenide, coated on both faces against reflection, with 99.5% transmittance quoted. PVD is the budget grade of zinc selenide, well suited to a spare or a lens for occasional special jobs.
Start with the lens you have. Measure its diameter with calipers, and read the focal length off the old packaging or measure the focus height with a ramp test. Guessing costs a week.
| Spec | Value |
|---|---|
| Diameter (this option) | 12mm |
| Focal length (this option) | 101.6mm |
| Focal length in inches | 4 in |
| Diameters offered | 12, 18, 19.05 and 20mm |
| Focal lengths offered | 38.1 to 127mm |
| Material | PVD ZnSe |
| Material origin | China |
| Coating | Antireflective, both sides |
| Transmittance | 99.5% |
| Heat | High-temperature resistant (maker) |
| Durability | Crack and scratch resistant (maker) |
| Laser type | CO2 engraving and cutting machines |
| Package size | 1.18 × 0.79 × 0.39 in |
| Shipping weight | 1.76 oz |
Longer focus is not better focus. It is different focus, with a trade-off that suits a narrow set of jobs. Know the trade you make with the 101.6mm option.
For the same incoming beam, the focused spot grows roughly in proportion to focal length. A 4-inch lens makes a spot about twice the size of a 2-inch lens. In return, the beam stays near its narrowest over a much deeper band, so the cut or mark changes less as the surface height varies.
| 12mm option | Spot | Depth of focus | Good for |
|---|---|---|---|
| 38.1mm | Smallest | Shallowest | Fine engraving on flat work |
| 50.8mm | Stock | Stock | Everyday K40 work |
| 63.5mm | Larger | Deeper | Thicker cuts |
| 101.6mm (this) | About twice stock | Much deeper | Uneven, curved or tall work |
Measure your cabinet too. With the stock clamp removed, check the distance from the lens to the floor of the work area. If there is not roughly 4 inches plus the thickness of your work, the 101.6mm option cannot be focused.
A cheap way to judge whether you need this lens: engrave your uneven item with the stock lens first. If the mark fades at the high and low points, a deeper focus will help. If it looks even, keep the 2-inch lens.
The K40 dimensions guide covers the work area and cabinet depth.
A 12mm lens is small, thin and easy to drop. Work over a clean towel, not over the open cabinet, so a slip does not send it into the bed or the exhaust duct.
A small lens clouds fast in the stock head, so clean it often: a few puffs from a bulb, then a fold of lens tissue wet with isopropyl or acetone, pulled across in one stroke. No circles, no dry wiping.
The stock K40 head has no air-assist nozzle, so smoke rises straight onto the lens. That is why stock lenses cloud quickly. If you keep the stock head, clean the lens often. If you are ready to upgrade, an air-assist head keeps any lens cleaner for far longer.
The upgrade order explains why the head comes before most other parts.
ZNNRKMYO's range is a grid: four diameters and focal lengths from 38.1 to 127mm. The grid is useful, but it also makes it easy to order the wrong lens. Work through it in two steps.
Compared with fixed single-spec lenses, the ZNNRKMYO range wins on choice. Rivals win on material grade and on clear, single-option packaging:
Before you check out, confirm the option shows the diameter and focal length you intend. The two numbers sit side by side in the option name, and swapping them by mistake is common. A 12mm lens at 101.6mm and an 18mm lens at 50.8mm look alike in a hurried order, and neither behaves like the other in the head.
New to the machine? Start with the stock focal length and learn it well. How to use a K40 covers focus, test cuts and safety before any specialist lens.
Yes, this option is 12mm, which is the lens size the stock K40 head takes. Most upgrade heads also accept 12mm.
No. The stock lens is 50.8mm. A 4-inch lens puts the focal point about two inches lower and makes a larger spot, so choose the 50.8mm option for everyday work.
Work that is uneven, curved or textured, where a deep band of focus matters more than a fine spot.
Yes. The bulge faces mirror three and the flat side faces the bed, exactly as on larger lenses.
For a spare or a specialist lens, yes. CVD ZnSe is the higher grade and generally lasts longer on a machine that cuts heavily every day.