All internally focusing lenses do this by shortening their focal length. If you go all the way to 1:1 macro, your focal length would be cut in half. This means your beautiful 200mm macro lens with internal focusing would turn into an 100mm macro lens at 1:1 magnification. Pretty pointless, isn't it?

The distance between image plane and object is 4 times the focal length at 1:1 or otherwise: the focus distance at 1:1 will give you the focal length if you divide it by four:

EF-S 60 MFD 200mm Calculated Focal length at 1:1 50mm

EF 100 MFD 300mm Calculated Focal length at 1:1 75mm

EF 180 MFD 480mm Calculated Focal length at 1:1 120mm

Ok, it is not a true "single lens" but there is a good chance that you do not loose 50 % of the initial

focal length.

Modern lenses bend the light by crazy ways so there is a chance that you simulate lens positions to get

1:1 without loosing any focal length. Think about EF-S10-22 at 10mm which leaves 35mm space between back lens and image plane!