This is a device that enables you to check the autofocus on a DSLR. The unit consists of a flat panel high contrast B/W target plus a ruler with clear markings. The base plate has a built in spirit level, align the camera so it is perpendicular to the target and then let your camera perform its autofocus. The target is in line with the zero position on the ruler, now if the autofocus is not hitting the correct focus you will be able to see by the ruler markings how far it is out. The SpyderLenscal is useful if your DSLR camera has a Micro focus adjustment setting, but most DSLR cameras do not have this feature or ability to make any adjustment, unless you are using a top of the range model from Canon or Nikon.