In many cases, the user will turn the lens back and forth several times on the focus ring to determine approximately the correct centre position for the image sharpness. The lens may then be stopped down to achieve sufficient depth of field for a consistently sharp image.

But how do you focus precisely on a specific point? The first step is to open the aperture completely, but the image will be much too bright. Correct this by reducing the intensity of the lighting or shortening the camera's exposure time. With this setup, the depth of field is minimal and only a very small area in the z-direction appears sharp to the user.

Now focus precisely on the inspection area you think is most important (or exactly half height between the inspection features you want to capture). Then, usually, stop down again and adjust the brightness of the lighting (or the exposure time).

The depth of field will now be large enough that height variations during part feed will no longer cause blurred images.

Please note, however, that the changed working distance with eccentric standard lenses will change the scale and the measurement results will be larger or smaller!

Observe a small image area with a pattern structure. The grey value histogram on a sharp image shows larger minimum and maximum values and a larger standard deviation than on a blurred image. However, the average value of the grey tones does not change.

Attention: The maximum value of such functions depends on the image content and the general image brightness. Please also ensure that there is no overexposure in the area of the test tool that destroys grey tones.

Please use such software methods to check whether all object planes to be captured spatially are correctly focussed and that you have sufficient depth of field, especially for your measurement application.