You still need to tell me how many volts you traversed to get that risetime. Risetime is meaningless unless you know the voltage swing.
For example : they specify on logic gates always what is the power supply voltage at which a particular risetime was measured.
Take a simple 4011 for example: Risetime 180nS @ 5v , 90nS @ 10v 65 nS @15v ...
Risetime is 10% to 90% of the output swing. But, without knowing what the output swing is, risetime is a useless number.
A 1 hz triangle can have a risetime of 1 picosecond... if its vpp is 1.2 picovolt ...
That's why, for opamps we dont talk about rise time but slew rate. faster slewrate = higher bandwidth ...
so if you are after testing a scope's bandwidth ... you need SLEWRATE ! . risetime is meaningless.
to establish slewrate you need the plateau's,
I am afraid that you are wrong here.
On an oscilloscope, the rise time and bandwidth are directly related (though less closely now with digital scopes than with old analog scopes). It is generally accepted that this relationship is BW = 0.35 / risetime. So a 200 MHz scope will have a rise time of approximately 1.75 ns. This rise time will be observed regardless of the magnitude of the pulse (until the slew rate limit is encountered).
For opamps, the slew rate is a prime specification, yes. But slew rate for op-amps is not directly related to bandwidth. The GBP is a more useful specification for that. Only when the output swing is very large does slew rate limiting come into play. For high voltage measurements with an oscilloscope, it is quite possible to come up against the slew rate limit before the rise time becomes the limiting factor, just as for an op-amp circuit you can be limited by slew rate before exceeding the bandwidth of the system, when outputting a very large voltage swing.
Rise time for an oscilloscope is not useless without a magnitude as you contend. The magnitude is, of course proportional to the input voltage. When thinking in the domain of digital logic levels, slew rate and rise time are related, by the logic levels themselves (the rise time is essentially how long the logic gate takes to slew at full speed from one level to the other). For an oscilloscope, you need to think differently. There is no fixed voltage. It is proportional to the input. And for that reason, the rise time is constant regardless of the input magnitude. If the scope has a rise time of 1 ns to a 10 mV pulse, it will have a rise time of 1 ns to a 10 V pulse too. This is despite the fact that the effective slew rate is three orders of magnitude higher in the latter case.
Google "oscilloscope rise time and bandwidth". Read.