I recently bought an Arri BP-7 bridge plate to use with my SR3, and I was a little surprised when I noticed that the rods on the BP-7 are offset from the camera's optical center (compared to my 15mm lightweight rods which are perfectly centered). I knew that the width between 15mm studio rods was wider than between 15mm lightweight rods, however I didn't realize until I did a little research that the studio rods are, by design, offset from the optical center by 17.25mm.
Out of curiosity, does anyone know why the 15mm studio rod standard has a 17.25mm offset from the camera's optical center? The 15mm lightweight and 19mm studio rods are perfectly centered, yet the 15mm studio rods have the 17.25mm offset. On the surface, the offset seems kind of counter-intuitive, so I'd be interested to know why such an offset exists.
Also, in terms of studio rods, which format is more commonly used today - 15mm studio or 19mm studio?
In case anyone is interested, Arri has a nice chart which specifies the dimensions and offsets of the three rod variations.