A cordless, balanced venetian blind or shade with a constant or a consistent, variable force spring motor includes conventional window covering components without the outside hanging lifting cords or cord locking mechanisms. The method comprises determining mathematical relationships between Fourier Transforms of logical slices through the object and Fourier Transforms of projection images of the object. In the process, a first value is loaded from a first location, and a second value is loaded from a second location. The insert has a top surface with a curved portion, a bottom surface with a curved portion, and a stop surface. The focus adjusting system operates within a limited searching range including a current position of the focusing lens group when predetermined conditions are satisfied, including a condition upon which it is determined that the first image and the second image match each other completely or by a predetermined degree. The sensor may be a photometric or colorimetric sensor or an electrochemical sensor.