As you can see, the focal length of your lens has an effect on how your model looks when shooting a portrait. This is especially true for very close shots. Longer lenses tend to flatten the face, and wider angle lenses tend to give a fish-eye type distortion.
Another thing to consider when using the class issued Nikon D40’s is that they do not use a full 35mm sized sensor, so all of your lens focal lengths have to be multiplied by what is called a “crop factor”. The crop factor for most cameras like the D40 is 1.5. So a 100mm lens would work as a 150mm lens on this camera. The article suggests a 135mm lens for portraits, but that would only apply to full frame DSLR cameras or 35mm film cameras. 85 mm is a popular focal length for digital photographers.
Hopefully this isn’t too “tech” and my explanation shed some light on why sometimes portraits look kind of funny when shot with extreme wide angle lenses.