The lowest measuring uncertainty can be achieved using telecentric lens systems. The advantage of telecentrics is that the lateral magnification remains constant when the working distance is altered within the telecentric range, thus preventing errors of dimension. This is especially important when working with lower magnifications. The best quality can be attained using telecentric lenses with a fixed magnification.

From an applications standpoint, it makes sense to combine high and low magnifications. This is especially true in cases where features with less precise tolerances should be measured in one image quickly and in addition, high-precision measurement of closely toleranced features in small image fields should also be possible or the positions of the elements to be measured must still be better located after coarse positioning (of the workpiece) on the measuring machine. Using telecentric lenses with a fixed lateral magnification, this can be achieved in two different ways. The first way is simply to change the lens. This can be done automatically (for example, with a revolving lens changer). The main problem here is the high repeatability required for a lens change. This approach thus has a negative effect on the measuring uncertainty. Since only two different magnifications are required in most cases, the simplest approach is to switch back and forth between two different image processing sensors of varying magnification. The greatest flexibility can be achieved using a zoom lens system (Fig. 9). Due to the positioning movement of the optical components in the lens, slight losses of accuracy can be expected. However, such losses should be negligible in modern systems. Linear guides are used in the Werth Zoom to ensure high positioning repeatability. The movements of the lens packages required for the zooming processes are motorized. This design enables a 1 × to 10 × magnification and working distances ranging from 30 mm to a maximum of 250 mm. Optimization between the measuring range of the sensor and the measuring uncertainty can thus be attained (via magnification). Regardless of this, the working distance can be adapted to the specific requirements of the workpiece (to prevent collision problems).