FoCalc by [Gx]Ace Is the internets best 35mm Crop Factor Calculator. Ever wonder what the 35mm equivalent of a 23mm f2 on APS-C would be? Ever wonder what that 120mm f8 8x10 lens would be if it were put in 35mm terms? Well now you can know it all with FoCalc! FoCalc allows you to input a focal length and aperture as well as a focal reducer or teleconverter and automatically calculates the equivalent focal length and apparent aperture on 35mm Full Frame.