Often, on a casual walk, I'll carry my Olympus Pen F with the 38 f1.8 mounted and stuff the 20 f3.5 and 100 f3.5 in a small case. I actually have the 50~90 f3.5 Zoom for that camera but compared to the 100 it is large and heavy so is left in the camera drawer at home.
Occasionally however I'll see a shot where the 38 is too short, the 100 too long and I cannot back up or go foward to frame what I want. Thats why I keep that zoom around. I'll return with a tripod and the zoom so I can get the framing I want. With a 18X24mm negative you sure don't want to crop to adjust composition.