Is anyone using a Flipside and a sling such as the Black Rapid or Luma Loop at the same time? I had the idea to try my Canon backpack (which has a waist strap) and my OpTech strap, which I have lengthened to use as a sling. I put the camera "sling" on first, over my left shoulder, with the camera at my right hip. I then put on the backpack and buckled the waist strap. I had no problem spinning the backpack around for access to the rear. Had my camera strap been a sling where the camera slides along the strap, I would have had no problem getting the camera to eye level. It seemed like it would work well, unless I'm missing something.

If yours does, consider making the sling a little
