The Sky View Factor (SVF) is a critical parameter for studying the thermal environment of buildings theoretically. However, there are two main definitions of SVF, one based on the visible area ratio and the other based on the view factor. When utilizing fisheye images for SVF calculation, various issues often arise, such as conceptual confusion and algorithmic errors, significantly impeding the accurate scientific analysis of thermal environmental problems. Therefore, in this study, we employed methods such as theoretical model deduction, field analysis, and software simulation and validation to investigate the calculation methods for fisheye images with equidistant projection, equisolid angle projection, and stereographic projection. We proposed the Image Distance Ratio Transformation Method (IDRTM), a correction method for calculating the SVF of fisheye images with the three projection types and corresponding calculation formulas. Finally, two cases demonstrate the computational advantages of simplicity and high accuracy associated with the orthogonal transformation formulas applied to the three types of fisheye images. In this study, the error rates of SVF calculation using three non-orthographic fisheye images directly are all higher than 29 %. After applying IDRTM, the average error rates of the transformed IDRs of three types of fisheye images are all less than 0.58 %, and the error rates of the corrected SVF of three types of fisheye images are all less than 1 %. This study elucidates the scientific definition and algorithm of SVF and provides a scientific basis for the accurate selection of SVF calculation methods corresponding to fisheye image types.