A laser isn't a straight cylinder of light; it's an hourglass. Understanding the "Rayleigh Range" determines whether you engrave with surgical precision or vibrant color.
The Hourglass Concept
Every laser beam converges to a point (the waist) and then diverges. The region where the beam is at its tightest and most powerful is called the Depth of Field (or technically, twice the Rayleigh Range).
IR vs. Blue Diode
The F2 Infrared laser (1064nm) behaves very differently from the Blue Diode (455nm) due to how light waves interact with the focusing optics.
Short focal waist. Highly sensitive to height. Moving just 1mm out of focus can drop power density by 50%+. This sensitivity is exactly why we use it for color etching—defocusing "spreads" the heat gently.
Typically a longer Rayleigh range. More "forgiving" of uneven materials like curved wood. The spot is larger, so while it doesn't engrave as deeply into metal, it is more stable over a wider height range.
Practical Guide
01The Dual-Dot Method
The xTool F2 uses two red pilot lasers. When you adjust the Z-height so the two dots merge into one, you are exactly at the focal waist (Z=0).
02Deliberate Defocusing (The "Color" Trick)
For vibrant blues on stainless steel, try moving 2mm higher than focus. This widens the spot, preventing the metal from "boiling" and instead allowing a smooth oxide film to grow.
03The Focal "Wall"
If you are cutting material, focus 1/3 into the thickness. This puts the narrowest part of the hourglass inside the material rather than on the surface, ensuring a straighter cut.