Having the right bag for the task at hand can make a huge difference. Anyone who has ever carried a lot of camera equipment in a shoulder bag or backpack for a long shoot probably knows the shoulder pain you get when you carry it around.

 ThinkTank's solution is the Think Tank Modular System V20, a customizable modular belt system that allows you to attach various lens accessories and pouches to your belt for quick and easy access while reducing some of the weight off your shoulders. You can easily fit a 24-70 lens and a DSLR body and  2 more lenses in a Strap with pouches and feel comfortable while walking and standing up for long periods of time.  

My favorite configuration is to combine the Think Tank Photo Pro Speed Belt V3.0 because its padded and it gives you support on your lower back  with the Think Tank Digital Holster 20 V2.0 which allows me to have a ready to shoot camera every time and it's easy to change lenses because you always can be free handed do it. 

In my case the change from a backpack to the modular system has been great, I feel safe and confident when I'm walking and shooting, my balance has improved and I'm not stomping around.  

Another good thing it's that you can fully customize which pouches you carry on the belt depending on the gear you are using. Sometimes a speedlight and some other an extra lens or a water bottle for hot summer days.