Many times while I am out in the field, I am carrying 400mm telephoto lens on a Canon 1 Series Body, which is large and after a while becomes heavy.  So I usually do not carry a second body or second wider lens. But I do not want to miss opportunities if I come across a great shot that is not right for the 400mm. I usually have a Panasonic LX1 compact camera in my pocket for those instances where I want to take a few wide or detail shots while my main camera with long lenses. I have used the LX1 for years and it still works well even though it is getting a little dated in todays fast paced technology feature updates. It has the ability to shoot raw files which is important for my workflow and the images it produces are fine for prints up to 16×20 prints & even up to double page landscape spreads in my ebooks. Newer cameras like the Sony RX100 II & similar cameras are also excellent & handy backup cameras. Sometimes if I think I might need  a wider zoom range I use a Panasonic GH2 m43 camera with a 14 – 140mm lens (equivalent to 28 -280mm on full frame cameras). Even though it is smaller than a DSLR and easier to carry, it still would not fit in a pocket and might get in the way. I have also tried images from my iPhone, but I do not think it is a viable option yet. All examples here are with the Panasonic LX1.