I'm going on a trip soon, and I was thinking it'd be nice to leave the big, heavy, and oh so white 70-200/2.8 zoom at home if I could make sense of it.
As I was trying to sort out how the bag would be packed, I had both my FF body, and a crop backup in the bag. Then the light went on. On the crop body, the 135mm lens should frame quite a bit like the 200mm end of the big white thing. And since I was bringing a 24-105/4 zoom along I don't think I'd miss having the long zoom. So I decided to run a couple tests.
I tried to sort out how I could make this as unscientific as is humanly possible, and I'm fairly sure I've succeeded. I paid special attention to not framing things identically as I switched bodies/lenses as I didn't really want it to be that much easier to sort it out.
I did, however, fail by leaving the aperture @ 5.6 for every shot. Missed a perfect opportunity to help confuse the issue a bit more.
In all of these, the FF is on top.
First - the completely unnecessary comparison. The 17-40 @ 17 on the crop, and the same lens @ 29 on the FF. I selected 29 because it was what I got when I set the lens to about 27 on the scale. Works for me.
FF + 29 vs Crop + 17
Then, probably the most important of the 'tests' - the 70-200 @ 200 on the FF, the 135 on the crop. Here, the DOF differences are starting to show up.
FF+200 vs Crop+135
Lastly, and again a not very needed test as I wouldn't have been bringing my 300 along, I just wanted to throw it on the FF to compare against the crop with the 135 & 1.4x TC. This shot clearly shows how the DOF is going to be different (due to longer focal lengths) on the FF.
FF+300 vs Crop+135+1.4TC
So, my end conclusion is that I'm planning on bringing the 135 along instead of the big white thing. At least today that's what I'm planning. Saves me about 1/2 the weight of the zoom and some room in the bag.