The working distance of an objective is the distance between the objective and the object. That quantity is relevant in various situations; some examples:
- When using a photo camera for imaging insects, for example, it is beneficial to have some working distance in order not to irritate the insects. That can be a problem with some macro objectives.
- A microscope objective may have a very small working distance of less than 1 mm in cases with large magnification. That can be inconvenient with some kind of objects. It also makes it difficult to illuminate the sample from the top, if this cannot be done through the objective.
- In laser material processing, a large working distance can be important for different reasons. For example, one may avoid debris to be deposited on the optics. Also, one can more easily move the beam focus over large distances if one only needs to slightly turn a laser head which is placed in a large distance. In such situations, a large working distance is only possible if one has a high beam quality.
In some cases, it may not be clear how exactly the mentioned distance is measured – for example, if some mechanical parts protude beyond the final lens. It should then be made clear what exactly is meant with the working distance.
Note that the working distance is not necessarily the same as the front focal distance. Even if the object is placed in a beam focus, that focus does not need to lie in the focal plane if the input light of the objective is not collimated.