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. Which is why “depth of focus” is also called depth of sharpness. This means that an imaging system (in our case, the laser beam) can be moved in the range of image depth without blurring the result. 

The topic of focus/focusing and focus tolerance can be experienced directly if you want to cut or engrave using a laser. Because choosing the right lens is just as important as the optimizing the focus.
The finer and more detailed a graphic that is to be applied to an object by means of 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 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 the finest 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, i.e. it needs 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.