Computer-Simulation Analysis Of Ghost Images In Photographic Objectives

Abstract

AbstractThe results of investigations about ghost images caused by internal reflections inphotographic objectives will be described as follows. (1) Ghost images in photographicobjectives are classified according to their generation processes. (2) The analysismethod of ghost images based on computer simulation is briefly explained. (3) Thecomparisons between real ghost images recorded on a photographic film and the results ofcomputer simulation are examined. (4) Relations between objective types and the generationprocesses of ghost images are discussed about some typical objective types. (5) Themethods to reduce ghost images are investigated. Finally, some ideas to reduce ghostimages in the designing stage for photographic objectives referring to the above resultsare proposed.IntroductionThe role of photographic objectives is to make images of objects on a film. This rolecan be separated into following functions. (1) To make the image of an object with finestructures. (2) To make the similar image of a two -dimensional object perpendicular to anoptical axis. (3) To reproduce the brightness distribution of an object. (4) To reproducethe color distribution of an object. (5) To project a three -dimensional space to a two -dimensional plane. (6) To generate ghost images and veiling glares.Evaluation methods on these functions are almost already established except (6). Forexample, Modulation Transfer Function, Distortion, Relative Illumination, ColorContribution and Focal Depth of photographic objectives are well known. Ghost images andveiling glares, in spite of their remarkable influences on image quality, have not beenstudied sufficiently. Purposes of this work are to predict the influence of ghost imagesin the designing stage, to study their generation process about several types ofphotographic objectives and to introduce some methods to reduce the influence of them.Generation processes of ghost imagesDefinition of ghost imagesGhost images are defined as images on a film surface generated by the internal multi -reflections of rays from small and strong light sources. Such internal multi -reflectionscould occur among lens surfaces, diaphragms and a film surface. Typical ghost images areshaped like diaphragms as shown in Fig.l.Generation processes of ghost imagesThe origin of ghost images is the secondary light sources generated by reflections ofrays from a primary light source on the various lens surfaces. Based on their generationprocesses, ghost images are classified into following four kinds as shown in Fig.2. Inthe process (1), a secondary point light source is focused as a point image and in theprocess (2) it is defocused as an areal image on a film. And a secondary point light sourceprojects a diaphragm or rims of lens elements on a film. The process (3) and (4) differfrom the process (1) and (2) only in the generation of secondary light sources. In theprocess (3) and (4), secondary point light sources in the process (1) and (2) are replacedby secondary areal light sources. As examples, ghost images and their generation processwhich correspond to the process (2) are shown in Fig.3(d), Fig.4(d) and Fig.5(d).In the process (1) and (2), rays which are reflected an even number of times on thevarious lens surfaces form ghost images on a film. There are many possible combinations ofreflections. Even if two times of reflections are taken into account, there aremC2= m(m -1)/2 combinations of reflections in the objective with m surfaces.It is usually difficult to find out the combination of reflecting surfaces generatingthe ghost image in a practical picture. A computer program to simulate the ghost imagesgenerated by the process (1) and (2) is developed. The outline of this program is given inthe next section.