What's teh best gadget to take pictures of an edge, my cell phone certainly won't do the job!
Canada's Sharpest Lefty
If you're doing parkour while taking pictures, I say GoPro. Otherwise, I like tintype....
Seriously, a manual focus camera is best.
Without knowing exactly what cellphone you have the general answer is cellphones are quite capable for this task. One of the factors that affects depth of field (the apparent area of focus) is sensor size. Cell phones, like point and shoot cameras with their tiny sensors are capable of capturing a huge depth of field even at minimum focus (macro) distance. They are so good at it in fact that one of the disadvantages of small sensors is that it is extremely hard if at all possible to produce an image that has a blurred or unfocused background (technically called bokeh).
Originally Posted by gic
What is likely happening with your attempt to capture the edge is a problem of focus. First, make sure that your subject is well secured and you are too. Leaning your body against something or planting your elbows on something solid will help you steady the camera. Second, you could be too close to the edge for your phone to achieve focus. Try taking the shot a little further back. At full resolution (again, depending on your phone) you should be able to crop in quite a bit without sacrificing too much detail. Third, remember that autofocus systems work using contrast. If there isn't enough contrast the AF won't lock where you want it to. You (or someone else) can temporarily hold something like the end of a ballpoint pen or piece of coloured paper at the same level as the edge and pull it away once your camera has locked focus, allowing you to take the shot.
Hope that helps you out a bit.