The importance of an element's axial thickness depends greatly on its role in a system and can vary dramatically. Thickness refers specifically to the center thickness of a lens or spacing between elements. For curved surfaces, a reasonable operating tolerance runs ±0.1mm. For flat surfaces, however, the production of large sheets of non-polished glass yields larger variances in thickness. Thickness will vary greatly depending on sheet size and where on the sheet the measurement is made. In order to accommodate this fact a nominal tolerance value is used meaning that no specific thickness tolerance is defined. Over time, nominal thickness tolerance has generally been accepted to be ±0.015" to 0.020". Again, this refers to glass that is not polished after fabrication.

If a specific thickness or precision surface accuracy is needed then polishing is clearly required and higher orders of tolerancing can be maintained. Typically, a 6:1 diameter to thickness ratio is used as a rule of thumb for high accuracy plano surfaces in order to prevent warping in fabrication or in the final mounting. Higher ratios may be used for lenses depending on radii and diameter values.

Edge thickness is used as a "reference" for lenses meaning that it is not a manufacturing limit. Edge thickness is actually a calculated value which depends on radii, diameter, and center thickness. It is thus used as a reference to indicate physical limitations for mounting considerations.