Laser Focusing Lenses 101: How Focal Length Works

OMTech Laser Updated On September 08, 2023

Focusing Your Laser Engraver Machine

Trying to focus a laser engraver can be a tough task. It’s not always easy to learn how to focus your laser engraver machine. As a business owner, learning the fundamentals of your laser focusing lens will help you maximize profit and deliver quality work. One of the most important factors to look out for when using a laser engraver is the focal length.

Focal length is embedded in the concept of laser focusing. It determines the quality of your laser markings for a perfect engraving. You can liken this concept to how you have to adjust a camera's focus to get a clear image.

What is Laser Focal Length?

Focal length is the distance from the focal lens to the laser beam’s convergence point. When a laser beam is emitted from a CO2 laser tube, the laser beam has inconsistent intensity and is too wide to engrave with any precision. That’s where the focal lens comes in — laser focusing lenses have a convex contour that makes the laser beam converge to an extremely narrow point, bending the laser beam into an X shape. This point of convergence, or center of the “X,” is precisely where the laser should contact the target material.

What is Laser Focus for Laser Engraving?

Focal length refers to the distance from the laser focusing lens to the top surface of the material being processed, which is often measured in inches. On the other hand, focus refers to the smallest possible beam of a laser. And this beam has the maximum density of laser packed in it. Focus is an inherent property of focal length; it’s basically the focal length at which the best engraving quality is realized. For example, when an engraver is designated to have a focal length of 1.5 inches, this implies that when you place a material 1.5 inches from the focal lens to the top surface of your material, only at that distance to the material is the engraving sharpest. Therefore, you must note the lens's designated focal length when purchasing your laser engraver.

How to Select the Right Focus Lens for Your Laser Engraver

Knowing the right focus lens for you is not rocket science. The truth is these factors are interwoven and intertwined in some ways. When determining laser focal length, think about the maximum cutting thickness you want to achieve. These are some factors to consider when trying to choose the best focal lens for your laser engraving machine.

The Thickness of Engraving Material

The rule of thumb guiding laser focusing states that the larger or thicker material, the larger the lens, spot size, and depth of focus. Consequently, it is evident that a thin material will require a small spot size and a small lens. Also, the lens will only be able to engrave small and fine details on the material. For fine detail and efficient cutting, the proper focused laser spot will be between .1 - .3 mm. This analysis is the opposite if you have thick materials.

There is an “unfocused” method of engraving applied to create larger lettering on bulky materials, for example, 3" x 6" letters on a 3' x 4' piece of plywood. You can "unfocus" the laser spot and enlarge it to around 1mm-1.5mm. Always expect some trial and error when trying out a new engraving technique.

The takeaway is that you must consider the thickness of the material you will be working with when purchasing an engraver. Knowing fully well that your laser engraver must not underperform, neither should it overperform to save cost. Failure to do this could limit your profit and capacity as a business entity in the laser engraving space.

Degree of Engraving Detail and Resolution

This factor determines how sharp the engraving comes out when it's done. Here is where the thickness of material and resolution intersects: Larger laser focusing lenses have lower resolutions, and remember that for thick materials, you need a large lens. This implies that you must be able to strike a balance between these two factors. In the end, you want a lens that can work on thick materials and still give good details and a fine resolution. Generally, the 2-inch lens (focus) is considered as the most universal among the classes of lenses available - 1.5, 2, 2.5, and 3-inch, respectively.

Laser Power

The focal lens can affect the machine's power by changing the laser beam's dot size diameter. A 60W machine can only provide 60W of maximum power, no matter which focus lens is used. But, larger lenses will typically increase the beam dot size, which lowers the energy density of laser beam. That means that using a typical 4-inch focal lens with a 50W laser tube will actually decrease your cutting ability because the laser beam is wider and less dense. Keep this in mind when choosing your focal lens for laser cutting. While a 3 or 4-inch lens is great for cutting on a High-Power laser, it's not ideal for a Mid-Range laser.

However, you must have the proper focal distance designed for your specific application, and it must be set up in perfect focus to maximize the laser tube’s full wattage power. While you may think that the thicker the material, the larger the lens needed, the more likely the resolution and detailing is lower, and the more power is required. But this relationship isn’t always true. The main factor of power depends on the focal length of the lens. You can have an 18mm diameter focal lens with a focusing distance of 3". You can also have a 25mm diameter focal lens with a 1.5" focusing distance. The trick is in manipulating your focal length to achieve your desired laser power relative to the size and thickness of your project materials.

Type of Engraving Material

Typically, all materials vary in their composition, and this should inform the right lens choice for you. Different materials have varying absorptivity for laser engraving via the focal length of the lens. For example, the appearance of engraving on glass and wood differs even at the same lens focus. This means that you have to pay keen attention to how each material responds to the engraver's focus.

Benefits of Common Types of Laser Focal Lenses

1.5-inch laser focusing lens:

Finest detail and best resolution (greater than 500dpi).
Suitable for rubber
Easy to fall out of focus
Most suitable for thin materials and small engravings.

2-inch laser focusing lens:

The overall best lens with the right balance of detail, resolution, and depth of focus
Medium resolution between 100 and 500 dpi.
Average laser power input requirements

2.5-inch laser focusing lens:

Low graphics detail and resolution less than 500 dpi
Appropriate for heat-sensitive material (e.g., laminate).

3-inch laser focusing lens:

Not suitable for engraving, best for laser cutting wood and acrylic.

Your Guide to Laser Focusing — Conclusion

We have done our best to highlight laser focusing and its relation to some factors considered above. More so, these lenses and their focal length always determine the quality of the laser beam available for engraving. In general, ensure you do your homework and define your expectations before purchasing a laser engraver. OMTech has the best CO2 and fiber laser machines with the appropriate accessories for optimal result and marking quality. Our doors are wide open to receive you for demonstration and consultancy at our Southern California showroom.