Back Focus

It refers to the measured distance between a camera’s image sensor plane and the rearmost glass element in the attached lens. Changing the distance between the rear lens element of an optical system (like a camera lens) and the images sensor, sharpness across the entire image, while focusing refers to the act of adjusting the lens to bring a specific subject at a particular distance into focus. Back focus is the precise distance between the rear lens element and the image sensor, which needs to be set correctly for optimal image quality across the entire frame, especially when using different accessories or lenses with varying flange focal distances. 

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