The value that gets written into the Z-buffer is value of z/w after
projection, and it has an unfortunate shape that gives enormous
precision to a very narrow part close to the near plane. In fact,
almost half of the possible values lie within two times the distance of
the near plane. In the picture below it's the red "curve". The
logarithmic distribution (blue curve), on the other hand, is optimal
with regards to object sizes that can be visible at given distance.
Floating point depth buffer should be able to handle the original z/w
curve because the exponent part corresponds to the logarithm of the
number.
However, here's the catch. Depth buffer values converge towards value
of 1.0, or in fact most of the depth range gives values very close to
it. The resolution of floating point around 1.0 is entirely given by
the mantissa, and it's approximately 1e-7. That is not enough for
planetary rendering, given the ugly shape of z/w.
However, the solution is quite easy. Swapping the values of far and
near plane and changing the depth function to "greater" inverts the z/w
shape so that it iterates towards zero with rising distance, where
there is a plenty of resolution in the floating point.
I've also found an earlier post by Humus
where he says the same thing, and also gives more insight into the old
W-buffers and various Z-buffer properties and optimizations.
Update
I tried to compute the depth resolution of various Z-buffers for whole range from 0 to 10,000 kilometers. Here's the result (if I didn't make a mistake), showing the resolution at given distance (lower is better):
The red curve shows resolution of the logarithmic Z-buffer. It scales linearly with distance, what means that in the screen space it's constant. That's also an ideal behavior.
The blue curve shows the resolution of 32 bit floating point depth buffer with swapped far and near plane. The spikes are formed with each decrement of the exponent. Floating point depth buffer has worse resolution than the logarithmic one, but it should be still sufficient. The green line is for an ideal Z-buffer that has depth resolution of 0.12 millimeters at one kilometer. It also happens to be the worst case bound for the floating point depth buffer.