Not entirely true dery .... there are many backpacks that come with a middle or lower body chain and a flap inside to convert the backpack into separate the two sections ( though not completely sealed off.... if one buys such a bag and ask the local tailor or whosoever to seal of these two sections completely - then the below section can be used as a direct access Camera bag.
An example from an Indian trekking goods manufacturer / retailer is below
There is a chain below the purple line at the bottom which opens up the bag from that side - Ideally that's meant to retrieve boots or rain wear on an urgent basis without disturbing the top load