DSLRKIT Lens Focus Calibration Tool Alignment Ruler Folding Card(Pack of 2) This compact ruler allows you to determine if your lens is auto-focusing accurately as it should. It is an essential tool that is particularly useful when shooting with a shallow depth of field, where focusing should ideally be "spot on". You can use this ruler to determine how "off" the focusing is for a particular camera / lens combination and, on supported cameras, adjust the camera's auto-focus settings to resolve this On Nikon cameras, this feature is known as "AF Fine Tune". On Canon and Sony cameras, it's known as "AF Micro Adjustment". Please refer to your camera's instructions manual to see if this feature is present. Size:19*12.3cm Package Included: Lens Focus Calibration Card X 2