Isn't the maximum sweep speed of the 2225 5 nS/div, so that rise time is more like 4.5nS at best?
Looking at a picture of the front of a 2225, the x50 switch has the word "except" on it bracketting the 0.5, .1 and .2 uS/div speeds. In other words, you do not get a x50 MAG at 0.1uS/div.
Richard