Focal Lenses: Types and Applications

Applies to: MIRA, NOVA, REDLINE MIRA and REDLINE NOVA

This guide introduces the lenses you can use in your laser: the range generally used, and what each is best for. It is written for beginners who are receiving or working with a different lens type for the first time. Read it through before you change lenses.

Both the type and the focal length of a lens affect how your CO2 laser performs, and each suits particular work.

Lens physics

Inverse square law

The intensity of the laser beam is inversely proportional to the square of the distance from the source. Double the distance between a light source and a surface, and the intensity at the surface falls to one quarter. This is why the distance between the lens and the material affects engraving quality.

In a laser engraver, the effect shows after the beam has passed through the lens. The further out of focus the material is, the more energy the beam loses.

Divergence and depth of field

Divergence is how much the beam spreads as it travels. A beam with low divergence stays focused over a longer distance. Shorter focal lengths give higher divergence, and longer focal lengths give lower divergence.

Depth of field is the range over which the beam stays adequately focused on the material. If the surface moves within this range, engraving quality remains acceptable. Outside it, the beam diverges too much, and precision and quality are lost.

Focal length and focal point

The focal length of a lens is the distance over which the beam converges to a point. That point is the focal point, where the beam is most intense.

Lens diameter

The diameter of a lens affects how much laser light can pass through it and the overall quality of the beam.

Larger diameter lenses handle more power and distribute the energy more evenly, but generally cost more.
Smaller diameter lenses are more cost effective, but may not perform as well, especially at higher power.

Focal lengths at a glance

	Shorter focal lengths (1.5” to 2”)	Longer focal lengths (2.5” to 4”)
Beam	Tighter, more concentrated beam and a smaller spot	Larger focal spot, with lower divergence over distance
Strength	Higher resolution and finer detail	Greater depth of focus
Best for	Shallow, high-detail engraving: small text, intricate designs and detailed patterns	Deeper engraving, cutting and thicker materials

Specifications shared by all lenses

Diameter tolerance: +0 / -0.1 mm
Edge thickness: 2 mm
Focal length tolerance: ±2%
Wavelength: 10.6 µm
Power: 0 to 200 W
Transmittance: 99.8%
Clear aperture (polished): more than 90%
AR coating reflectivity: less than 0.2%
Scratch-dig: 20-10

Legacy MIRA lenses (18 mm diameter)

These lenses fit all legacy MIRA models: MIRA 5, MIRA 7 and MIRA 9.

Lens	Focal length	Best for
18 x 38.1 mm (1.5”)	38.1 mm	High-detail engraving at shorter distances, with a more concentrated beam
18 x 50.8 mm (2”)	50.8 mm	A balance of detail and depth, for general engraving or cutting
18 x 63.5 mm (2.5”)	63.5 mm	Extra clearance for full wraps on 40 oz Stanley tumblers and similar tumblers
18 x 101.6 mm (4”)	101.6 mm	Cutting thicker materials, or engraving items that need focus over a longer distance

18 x 38.1 mm focus lens — 18 x 38.1 mm (1.5”)

18 x 50.8 mm focus lens — 18 x 50.8 mm (2”)

18 x 63.5 mm focus lens — 18 x 63.5 mm (2.5”)

18 x 101.6 mm focus lens — 18 x 101.6 mm (4”)

REDLINE MIRA, REDLINE NOVA and legacy NOVA lenses (20 mm diameter)

Lens	Fits	Does not fit	Best for
20 x 38.1 mm (1.5”)	All REDLINE MIRA models	Any NOVA model, legacy MIRA models
20 x 50.8 mm (2”)	All REDLINE MIRA and REDLINE NOVA models	Legacy MIRA and legacy NOVA models
20 x 63.5 mm (2.5”)	All REDLINE MIRA and REDLINE NOVA models, legacy AEON NOVA and Super NOVA models	Legacy MIRA models	A compromise between detail and depth, for medium-range engraving
20 x 101.6 mm (4”)	All REDLINE MIRA and REDLINE NOVA models, legacy AEON NOVA and Super NOVA models	Legacy MIRA models	Cutting thicker materials, or engraving items that need focus over a longer distance