We are constantly harping here on the need to carry every lens that you own with you at all times - in case a zeppelin is being moored near you and someone is smoking a cigar. It pays to be optically ready for anything. Landscape photographers who take sunsets also need to make sure they are ready in case the sun whips over to the east and then back again.
These people are well catered for by the bag manufacturers who make the equivalent of upholstered sea containers with straps. They can get several DSLR bodies, tele lenses, macro sliders, video rigs, and panorama gear in there, plus a pint of Voltaren. School children line up to see them go by and then run shrieking when they fall over.
Those who do not wish to shoulder the burden of centuries - mirror-less camera users or those who use the tiny video cameras - will welcome the Lowepro Photo Traveller 150 backpacks that we have on special. They are small, brown, and tough...like Gurkhas...and will hold all you need to carry for small system work. Your chiropractor will despair of you as the bag will not overstrain your back - or your budget.
There's a big stand of them at the front of the store for $ 49. Light on the lumbars and light on the pocket.