Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: What is the weight of the empty backpack
Document: i switched from a generic shoulder camera bag to a backpack because i do a lot of traveling, mostly hiking, and nature trips. this bag very comfortable with lots of shoulder straps and locks to distribute the weight equally around your shoulders. since the product ad doesn't specify the brand, its a vivitar backpack and comes with its own raincoat. padding appears sufficient enough to protect the camera and other equipment as well as keep it snug in the bag.
I gave this camera bag one star because it is not what the product description says. This is what the seller claims:Backpack or Sling - You DecidePadded tuck-away shoulder straps allow you to wear the backpack on one or both shoulders by easily tucking the extra strap into the luggage handle slot. Also includes a waist strap for bag weight management and extra support.BUT, there are NO EXTRA STRAPS!
its structure is not rigid, does not support either the weight of the camera, the compartments are weak, the second time you use it, the left strap broke, do not recommend it for heavy cameras or to bring many things ...
I like to hike, snowshoe, kayak generally be outside and this backpack case works great. Easy access and comfortable to carry
A rain cover, tripod spot even attach it to rolling luggage should make this backpack just perfect for the things I do. Get one.
Ultra Portable DSLR Camera Backpack Sling Case with Cleaning Kit. I was able to use the case like a backpack or or a sling. carried my camera and lenses.
I love this backpack. I carry my Canon T3i inside. It has so much storage space and is lightweight even when full of equipment and snacks for a day at theme parks, etc.
I have had this bag a couple of months now and have carried it everywhere. It fits my camera and 3 lenses perfectly as well as lots of little odds and ends. I feel that it works best as a backpack as opposed to the other options it has, but that is a purely personal opinion.
I've only had this backpack for a few days. The good on this backpack is that it's a nice size for a day outing, comfortable straps, has a tripod holder, and has better zippers than some i've seen.The bad:  I wanted to like this backpack, but I had a few issues:1. I couldn't get my equipment organized ideally and safely protected into this thing.
I love this backpack because it seems to be perfect for what I needed. It holds my stuff easily (I'm just a hobby shooter) and is small and lightweight. Took it to the Smokey Mountains and hiked around and it was very comfortable. What I really liked is that you can sling it or back pack it, and it has easy access to the camera with the lense on it, without taking the pack off.
Answer: I did not weigh the bag before we filled it with stuff, but it was fairly light. Now, after stuffing it with equipment, not so much.