A photographic lens is evaluated by calculating the optical transfer function (OTF) from its design data and by measuring experimentally its OTF. Through the finite ray tracing, the wavefront aberration of the lens under test is obtained numerically to the second order for the extra axial object point and to the sixth order for the axial object point. The optical transfer function is then obtained as the auto-correlation of the pupil function. The computer calculation is carried out by using the mini computer NOVA 840 or FACOM 230-25 at the field angles 0˚, 7˚, 14˚, 21˚, and 28˚.
In the experimental measurement of its OTF, the single slit object (4 ㎛ ×1cm) is placed on the optical axis and illuminated by mercury monochromatic light source. The image formed by the lens under test is scanned by the motorized knife edge and the data processed by a digital computer to obtain OTF. The MTF curves are obtained at F/2.8, F/4, and F/8 for the accurately focussed image and several defocussed ones, and then are compared with the theoretical curves.
The OTF's of aperture modulated optical system, free from aberration, are investigated to examine their characteristic spatial frequency responses. In this work, the aperture modulation is carried out by providing an opaque annulus and/or an opaque and an opaque central disc. Those are all double annular apertures. Including the transmission band at s=O, we may say the double annular apertures provide three, four, or five frequency transmission bands throughout the whole frequency region within the cut-off frequency. The location and width of the broad frequency response peaks change as the aperture modulating conditions vary.
The OTF's of the Tessar type photographic lens produced in Korea are extensively investigated. As the lens design data is known, both axial and off axial aberration characteristics are derived through computation. And the OTF is calculated for modulated aperture which are loaded with aberrations. The spatial frequency filtering properties differ greatly from those of aberration free modulated aperture.
The OTF's of the lens are improved by properly positioning an opaque annulus. The performance of any optical system can be improved and specialized in optical uses by proper aperture modulation. The spatial frequency filtering property in aperture modulated lenses in the new special physical nature in addition to the coherent optical frequency analyzing feature of lenses.