The glass taxi by think tank can either be used as a shoulder bag or a backpack and is designed to fit a pro dslr with 300mm f28 attached or a 500mm f4 lens unattached the specially designed zippers hold their position to stop the main compartment from accidentally opening and both a foam collar and a foam ring are included to prevent large lenses from wobbling quickly transform this backpack into a shoulder bag with the tuck away shoulder strap and via the front rail fasten modular att