Originally posted by onlineflyer Where an I wrong?

Where you are wrong is by overthinking it.

Draw a small square and a larger square (to represent sensor size) on a piece of white paper on a table under a light.

Select one of your lenses and hold it above the paper and focus the light on the paper and observe.

You do not have to change the lens to paper distance to focus on one square rather than the other do you(obvious) ? So the focal length is the focal length independent of format.

Observe that the light intensity is the same over both the squares (obvious). So aperture is independent of format.

Observe that the image of the light takes up a bigger proportion of the small square compared to the larger square and that reflects the increased telephoto effect of putting a lens onto a smaller format.

Think of this is a slightly different way and observe the the smaller square restricts the views out on the edges of the larger square - your field of view has changed.

Apply these observations to the real world of cameras and it all gets simple.