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Understanding aberrations using double-pass techniques

The double-pass technique has been used extensively in physiological optics since first introduced by Flamant in 1955. 1 It is based on recording images of a point source projected on the retina after retinal reflection and double-pass through the ocular media. 2,3 From these double-pass images, the ocular modulation transfer function is calculated. The modulation transfer function yields the relationship between the contrast of an object and its associated image as a function of spatial frequency. However, in different applications, it is also necessary to have access to the retinal image of the point source, the point-spread function, and the wavefront aberration of the eye. An important advance in the technique was the understanding of the image formation process. When using a symmetric double-pass setup, with equivalent first and second passes, the double-pass image is the autocorrelation of the retinal image.4 This means that even in the case where the retinal image has an asymmetric shape (due to odd aberrations, such as coma), the double-pass image is always symmetric. We proposed a simple modification of the technique consisting of asymmetrizing the two passes by using a small aperture in one of them. 5 In this way, the double-pass image keeps the asymmetries present in the retinal image, and the ocular point-spread function (actual retinal image) can be obtained. 6 In addition, as the wave aberration is related directly to the point-spread function by an integral equation, it can be computed by phase retrieval techniques.7 This is a useful procedure to estimate ocular

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