Lens focal length is measured from the rear nodal point to the film plane. In standard large format lens the rear nodal point is at or very near the shutter/aperture blades. Mount the lens on a camera and focus it wide open on an infinity target at least 5000 feet away, further distance is better. Measure from the aperture to the front of the ground glass, remove the cells and hold the shutter open on B if necessary to get a good measurement. This is your lens approximate focal length. Next open the shutter on B or T, set the aperture wide open and measure the diameter of the aperture opening as viewed through the front element/cell. Divide the approximate focal length by the diameter of the aperture opening to get the maximum aperture. Additional apertures can be calculated by dividing the focal length by the desired aperture to get the opening diameter. Example: 190mm / f8 = an opening of 23.75mm as viewed through the front of the lens for f8. 190/16=11.875 for f16.