Although this is a depth of field and focus problem (as I posted above) if you are using a longish shutter speed like 1/25sec camera shake could be a problem, unless you are using a tripod or have some kind of image stabilisation system (IS or VR etc). And if you use a higher f/number, of course the shutter speed will get longer still.
For hand-holding a camera, the rule of thumb is to use a shutter speed that is at least as high as your effective focal length (including crop factor). So if you are using at 100mm lens on a Canon crop camera (1.6x effective focal length) you should use 100x1.6=1/160sec as a minimum.
IS/VR will allow you to increase that to maybe 1/40sec or perhaps even longer if you have one of the latest versions and a naturally steady hand, but remember that those are only a guide to the absolute minimums and as always with these things, the higher the better.