Using a Canon 550D with a EF 100mm f/2.8 Macro USM lens, I can make a 1:1 macro photo at shortest film plane to subject distance of 310 mm, thus getting 22 mm of a scale on the APS-C sensor, which is 22.5 mm wide. Photo is here:
The focal length of this lens is 100 mm, so with a film plane to subject distance of 310 mm, I expect 210 mm from object to "cross" point, and another 100 mm (given by lens) from "cross" point CCD, as shown on this drawing:
(2015-01-25: Note that drawing was updated to fix wrong terms)
But with the ratio of 2.1 to 1 between distance from object to "cross" point and from "cross" point to CCD (210 mm to 100 mm), I would expect the scale to be 2.1 times smaller on the CCD, which is clearly not the case, since 22 mm of scale can show on the 22.5 mm wide APS-C sensor.
Where is the mistake in my calculation above ?