Abstract
Several methods for measurement of focal length of a positive lens involve the measurement of distances of the object and image planes from principal planes, which are mathematical planes; the positions of which are difficult to figure out accurately. The Talbot method for focal length measurement is a method, which does not require the distances from the principal planes. Further Talbot interferometry can be used to measure focal length variation over the surface of a progressive power lens. A technique for the measurement of focal length using Talbot interferometry and moire effect has been described. The Talbot image of a Ronchi grating placed just after a test lens is superimposed on the second grating to give us the moire fringes. The angle of tilt of the moire fringes can be used to find out the focal length. The effect of unequal pitches of the gratings on moire formation is also investigated. The investigation is also carried out using binary gratings having equidistant circular rings (circular gratings). The theory and experimental results obtained with both types of gratings have been discussed.
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