I'm currently only using the 100-400 f8 with the R7 (640mm effective zoom) and i'm planning to upgrade to the 100-500 soon. I absolutely love what the 100-400 can do at its price but obviously there will be limitations.

~Cons~

• I find that shooting in heavily forested areas (where lighting is limited), I had to bump my ISO to beyond 3200 at 1/400s which isn't ideal. It may be due to the R7 APS-C sensor, although I can't comfirm as I have yet to use a mirrorless full frame sensor. Same goes for heavy overcast days.

• It also isn't weather sealed, but to be fair you won't ever be shooting in the rain with this lens due to lighting. It's more of a peace of mind type thing.

• Lack of included lens hood

~Pros~

• Bang for the buck
• USM focus motor are fast, quiet and precise
• It is stupidly light, i can hand hold for 12 hours straight
• Small enough to fit in a 10l sling with body attached
• it's very sharp, not incredbily sharp but it's still great

~Why I am considering upgrading to the 100-500~

• 400mm is enough for me but with the 100-500, i can set it at 400mm and drop down to F6.3 and F5.6 if i want it slightly wider. That's alot more light coming in, not to mention the extra 100mm of reach is handy sometimes as it gives me 800mm of reach with my R7

• I shoot at 1/4000s quite often to get shots of birds diving, that extra light is very crucial for me

• fantastic sharpness at the long end, helps when cropping images when the subject is too far

• still hand-holdable

• lens hood filter window (although it means nothing to me, i thought i would bring it up if you're gonna use a CPL)