When you are rendering something, like the apple, it is important to pay attention to the relationships between different values. When you pick out the location of your darkest dark, remember that every other place on the apple with the same value (in life) as the location of your darkest dark must be just as dark in your drawing. The same applies to your lightest light, and all the values in between. I don't know how clearly I explained this, so if you have questions, ask. I'll see if I can dig up a good post somewhere on the subject.