I'm currently very pleased with Helicon Remote v 2.6, which is a free app. It works with a long list of Canon and Nikon DSLRs, my own being a Nikon D90. You'll get more from it if your camera has the equivalent of Nikon's 'Live View', i.e. the ability to swing up the mirror, open the shutter and show you the electronic version of the viewfinder. In this way, you can transfer the 'Live View' to your tablet in real time. Other facilities include the abilty to shoot 'time lapse' with the interval user-variable and such features as 32 (!) minute shutter speeds. You can operate the focus manually and set any number of exposure settings, all from your tablet. Of course, you can also take the shot! In case you're wondering why you'd want this when the camera already has a full set of variables on board and two viewfinders, being able to see what you're doing during 'table-top' work, maybe with the camera facing vertically downwards on its subject is quite an attraction. I suppose with a long enough USB lead, you could check self-portraits before firing it. The list of practical applications interfacing with other hardware just gets longer all the time.