actually go for something big old and rough with adequate padding. maybe avoid something that says i have a shiny new dslr inside. maybe something that isnt specifically a camera bag but with a sturdy strap and an opening that if unzipped will not mean everything falls out in fact velcro and zipper fastening.
and something that doesn't look like you've just arrived in London to take photos of the guard outside Buckingham Palace!
and something that doesn't look like you've just arrived in London to take photos of the guard outside Buckingham Palace!