Before continuing and demonstrating the method outlined in my article Measuring the Entrance Pupil, I first need to introduce the calculator I use to determine the entrance pupil position.
Calculating The Entrance Pupil
Simply enter the top, T, bottom, B, and the distance, X, for each measurement position. Once both positions are entered, the calculator will display the entrance pupil, K, portrait position field of view, and the lens’ true focal length.
Calculating The Entrance Pupil Distance From The Lens Flange
To determine the entrance pupil position referenced from the bottom flange of the lens, simply enter in the flange to lens cap measurement.
Calculating The Nodal Rail Positions
A nodal rail is used to offset the camera and position the pivot point about the entrance pupil. Therefore, the entrance pupil position on the lens must be translated to a position on the nodal rail.
The RRS Nodal Rail and Nodal Ninja calculations are a function of camera body, camera bracket, and nodal rail. This calculator is determining the results for Sony A7 series camera, RRS (Really Right Stuff) camera bracket, RRS Nodal Rail, and a modified Nodal Ninja rail. It will only give correct answers for this setup.