Physics lensmaker's equation vs real life

Purely out of curiosity. I don't understand it. What's going on?

I have an old stereo 6x6 camera with an 8 cm focal length lens (Sterelux).
I can adjust this between 2.5 meters and infinity. But the lens moves extremely little. If I measure it with a calliper, it is circa 0.5mm displacement.

If I calculate it with the lens-maker's equation the lens should move 2.6 mm.

I grabbed another camera (35mm) with a classic telephoto lens of 9 cm. With this lens, the measured distance that the lens moves further from the negative exactly overlapped with the calculated distance with the lens formula.

What is going on with the old camera (from around 1935) ?

The camera's lenses are not unit focusing. They don't focus closer by moving forward from the infinity position. They have front cell focusing, focus closer by reducing focal length.

Dan Fromm said:

The camera's lenses are not unit focusing. They don't focus closer by moving forward from the infinity position. They have front cell focusing, focus closer by reducing focal length.

Click to expand...

I've done a couple designs like this (although the focus group was in the back), for exactly the reason that the focusing group doesn't move as far to set focus. Faster to refocus and some other project-specific reasons.

Not just inexpensive 'folders;' some of the most expensive currently available Medium Format AF lenses focus that way. Like Schneider 50mm 2.8 AF for Rolleiflex.

Thanks for the answers. I did calculation and indeed if I lower the f value voor 2.5 meter I get sharp at 8.0 cm (lens to film).
Als tried to see it on the screen, but the change was too small to see with the naked eye.
Looked at my other old cameras also. Just fort fun.
The Ihagee 6x9 10,5 cm lens (~1930) the hole lens unit change for 3.6 mm (2.5m to ∞).
The Zeiss 6x9 10,5 cm lens (~1960) the front lens change for 1.0 mm (2.5m to ∞).
The Certo 6x6 8 cm lens (~1960) the hole lens unit change for 2.5 mm (2.5m to ∞).
The Sterelux 6x6 8 cm lens (~1960) the front lens change for ~0.3 mm (2.5m to ∞).