What is depth of focus or focus 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 its basic physical assumptions are significant in laser technology, as it is used to select the right lens for the respective application.

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 formed. This has led to depth of focus being referred to as depth of sharpness as an alternative. This means that the imaging system, in this example the laser beam, can be moved in the range of image depth without blurring the result. 

The topic of focus/focusing and focus tolerance is experienced directly when laser cutting or laser engraving, as choosing the correct lens for your application is just as important as optimising the focus. 
Finer and more detailed graphics that are to be laser engraved require a smaller focal length of the lens. However, if thick materials are to be cut, lenses with a large focal length are recommended. As a practical example on the topic of focus/focusing, we use the 1.5 inch lens for a CO2 laser. This lens with a low focal length allows the engraving of graphics with fine details thanks to a resolution of more than 500 dpi.

The conflict of objectives lies in the fact that with decreasing focus diameter, the depth of focus also decreases, which requires it to be focused more precisely.
Unlike the 7.5 inch lens. The large focal length allows a high laser power, for example, to cut acrylic sheets that are thicker than 25 mm.