I think
everyone here will have some experience with camera bags (and at least as many different opinions). This is just mine:
I have a
Lowepro Slingshot 100, which is relatively tiny, but at the moment I have in it:
- Canon 550D with 18-55mm kit lens attached and third party battery grip attached
- 55-250 lens
- 50mm 1.8 lens
- Three-part set of Kenko extension tubes
- three extra SD cards, four extra batteries, all the cables, chargers, about 4 filters, tripod plate, a couple of remote controls, spare caddy for the battery grip, instruction book for the camera
That's a lot of stuff in a little bag, and it isn't squeezed in by any means (though it is a tight-ish fit). I also have an extra lens case attached to the strap, but it spends most of its time empty.
Now, I don't cart all that stuff around all the time, and I don't find that I ever need to use all of it all at once - but it is nice that it all fits in, just for storage.
On city walkabout I'll ditch the bag and just take the camera and maybe a spare lens in the lens case on my belt.
On country walkabout I'll ditch the chargers/cables/batteries and put sandwiches in their place and use the lens case for water or coke.
For home and studio stuff I just bring the whole damn bag and pull out what I need.
I guess if I had an external flash I'd probably just cart fewer cables around (and since I bought the extra batteries last week, maybe the charger).
Thing is, you don't necessarily need a huge great bag unless you actually need all of that stuff all of the time.