Gav, this is so simple. The object distance (lens to subject distance) is the distance from the front nodal point to the object in focus. The image distance is that between the rear nodal point and the image plane. The object to sensor distance (which is what is marked on the distance scale of most lenses, for practical reasons) is the sum of the object distance, internodal distance (which may be negative) and image distance. You can have lenses with the same focal length (hence the same image distance for the same object distance) but very different internodal distances.
The entrance pupil is the surface the lens 'sees' the world from. It is very easy to find.
Do you need more of this explaining or do you know enough to fill in the gaps? You haven't explained why your answer is correct, but you expect me to explain my answer - which should be obvious to anyone who has some knowledge of optics. I would like to read your explanation of why you believe you are correct.
Now that you point that out, I am not correct about merely lining up the sensors and objects.
However,
neither are you correct about merely lining up the entrance pupils and objects.
1/FL = 1/(front node to object) + 1/(
back node to sensor)
So lining up the objects and the front nodes is exactly as insufficient as lining up the sensors is. They both only end up holding constant 2 out of the 4 numbers that matter (object and sensor, or object and front node). No amount of lining up anything in fact will therefore work to automatically take care of such math for us, because one variable can't solve for two unknowns. It would require measuring the additional values and computing ratios no matter what.
However, we don't know where the rear nodal point is, so that entire method won't work, either, if you want to actually know the exact FL (or do we? Please explain how to find the rear nodal point if so). You can't just do arithmetic with the distance scale markings, because
the entire point of the thread is that we don't trust the manufacturer's markings (Also lots of lenses don't have those anyway). So since we can't trust them not to have rounded off or fudged those numbers either, how else can we calculate the rear nodal distance?
You might be able to measure only the sensor, and then use back focal as your unknown to solve, to simple answer whether the FL's are the same (not what they are). But if you're going to measure something and compute anyway, then starting by lining up sensors, and then measuring the front nodal point and calculating would ALSO work, and would take exactly as much effort.
Instead of all of that, I suggest an entirely different, empirical, guess and check method:
1) Get a piece of pegboard
2) jam some dowel rods in a rectangle, making them fairly far apart (several inches at least)
3) Tape off a straight perpendicular line from the dowel rod on the ground and place camera on a stool or tripod centered over that line at a height amidst the height of the dowel rods.
4) Put on lens #1, and take photos and adjust along the tape line until you get the front two dowel rods right at the edges of the frame.
5) Put on lens #2, and do the same thing, until they're right at the edges of the frame.
6) Measure the parallax distance between, for example, the front right dowel and the back right dowel in the two photos. If the effective focal lengths are equal, then parallax should be equal when the framing is matched. If not, then not.
