I'm a little ADD about my bags - I've done a ton of research and "field tested" various options and adding a DSLR to the mix introduces some new challenges. The biggest problem Ive discovered with DSLR backpacks is that they go on your back ! At SDCC, I am constantly reaching for my camera and stopping and unzipping a pack has caused me to miss a great shot more than once. Ive found the best solution is a cross body bag that holds your DSLR on your hip. The best I've found is the Wolfspur from Maxpedition You are not allowed to view links.
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It fits a an ipad mini, a few key supplies and a water bottle plus my Lumux DMC-FZ1000 easily and can withstand any Hall H line overnight. The downside is that you can't carry any additional stuff you might buy or get so I actually bring a backpack too that I'll wear to Preview night to carry loot. The right bag can make or break your SDCC experience so I recommend to NOT go cheap - get good quality and it will last forever. Enjoy the con !