What is depth of focus or focal tolerance?

The depth of focus, otherwise known as double Rayleigh length, refers to the distance along an optical axis required by a laser beam to double its focus area - starting from the focus in the focal plane. The depth of focus and understanding its basic physical properties are critically important in laser technology, as it is these properties that are used to select the correct lens for any specific application.

Terminology: Depth of focus

The depth of focus represents the area in the image space of an imaging optical system, in which a sufficiently sharp image of a focused object is adequately formed. This is why “depth of focus” is also known as depth of sharpness. An imaging system (in our case, the laser beam) can only be moved within the range of image depth before blurring is the result.

Effects of focal tolerance on the laser beam

The importance of focusing and focal tolerance becomes critically obvious when you want to cut or engrave using a laser. This is because choosing the right lens is just as important as optimising the focal length of that lens.
The greater the image definition required, and the more intricate detail contained in the graphic that is to be applied to an object by means of a laser, the smaller the focal length of the lens must be. However, if thick materials are to be cut, lenses with a large focal length are recommended. As a practical example, if we were to use the 1.5 inch lens with a CO2 laser, this lens, which has a small focal length, enables the engraving of graphics with the finest detail thanks to a resolution of more than 500 dpi.

On the other hand, a 7.5 inch lens which has a much larger focal length, will allow a high laser power to cut acrylic sheets that are thicker than 25mm.
There is a conflict of objectives when choosing the correct lens due to the fact that if the focus diameter is decreased, then the depth of focus also decreases, i.e. it needs to be focused more precisely.