Joe,
It is more or less backwards from the way you ask the question. The
scope input C was determined by the available technology and design
skill at the time. Reducing the C at the scope input has a direct
effect on reducing the C at the scope probe tip, which has a direct
effect on reducing the load on the circuit being observed. In a
perfect world, the scope input C would be zero and the input R would
be infinite, and when you touched the probe to the circuit there
would be absolutely no effect. So one goal of scope design has always
been to lower the input C.
In the real world touching the scope probe to the circuit always has
some effect. In the case of high frequency oscillators, the scope
probe C of a few pF is too much to allow probing the tuned circuit
without affecting the frequency. In the case of high speed logic the
scope probe C of a few pF will cause the rise and fall times to be
degraded by as much as a few nanoseconds. It is not only that a scope
doesn't give the 'big picture' when troubleshooting a pentium
computer, the real problem is that there are very few circuit nodes
that can handle the effect of the probe without causing the circuit
to malfunction.
- Bill
