Absolute limite to z-buffer (depth buffer) value

Question

I'm trying to use the z-buffer to measure distances, but I found a unexpected issue.

I'm aware that the z-buffer returns a 0..1 float number through my code:

    zImageData = new osg::Image;
zImageData->allocateImage(720, 576, 1, GL_DEPTH_COMPONENT ,GL_FLOAT); 
osgCam->setProjectionMatrixAsPerspective(45.0, 1.0, nearplane, farplane);
osgCam->setViewMatrixAsLookAt(osg::Vec3(0,0,camera_high), osg::Vec3(0,0,0),osg::Vec3(0,1,0) ); 
osgCam->attach(osg::Camera::DEPTH_BUFFER, zImageData);
//osgCam is my master camera.

I then postprocess the z-buffer value using a callback function:

  z = ((float*)sonar->zImageData->data())[1];

  float true_distance = farplane*nearplane/(farplane - z*(farplane-nearplane));

Everything works just fine until I reach 24 distance units, as shown below.

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I have 3 variables, nearplane (the viewport), farplane and camera_high. The last one is the distance of the camera from the ground. It's looking at a plane (which is meant to be the ground). That's the program output:

const float nearplane = 0.2; const float farplane = 200;

const float camera_high = 5;
z value : 0.960961
z distance value : 5
*** Exited normally ***

const float camera_high = 10;
z value : 0.980981
z distance value : 10
*** Exited normally ***

const float camera_high = 20;
z value : 0.990991
z distance value : 20
*** Exited normally ***

const float camera_high = 23;
z value : 0.992297
z distance value : 22.9999
*** Exited normally ***

const float camera_high = 25;
z value : 1
z distance value : 200.003
*** Exited normally ***

const float nearplane = 20;

const float camera_high = 25;
z value : 1
z distance value : 200
*** Exited normally ***

const float camera_high = 23;
z value : 0.144928
z distance value : 23
*** Exited normally ***

const float nearplane = 24;

const float camera_high = 25;
z value : 1
z distance value : 200
*** Exited normally ***

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I just can't get any difference in the z-buffer beyond 24, despite the position of my nearplane and farplane.