It is an essential tool that is particularly useful when shooting with a shallow depth of field, where focusing should ideally be "spot on". Inspect the photo to see if focusing is accurate. If focusing is accurate, the '0' on the ruler should be the sharpest point on the image and the other numbers get blurrier as you move away from the 0.
This product would be ideal for photographers, camera enthusiasts, and hardware engineers who require a lens focus calibration tool, alignment ruler, and folding card for accurate lens adjustments and maintenance.
