The inverse square law of light holds true for a point source of light, according to the definition in physics, but in photography we don’t really use the point source of light since it is too harsh.

A number of budding photographers often confuse depth of field with depth of focus. A number of cheap photography manuals also use the terms synonymously, but this is not correct.

Focus is one of the key parameters for judging a good photograph. If your photograph is out of focus then no matter how good it is, it will never be a quality picture, unless done so purposely and to some effect.