Imaging lens with positive-negative-negative-positive refractive power lenses on object side

An imaging lens comprising:

• a first lens having positive refractive power;
• a second lens having negative refractive power;
• a third lens having negative refractive power;
• a fourth lens having positive refractive power;
• a fifth lens;
• a sixth lens;
• a seventh lens;
• an eighth lens;
• and
• a ninth lens having negative refractive power, arranged in this order from an object side to an image plane side,
• wherein said ninth lens is formed in a shape so that a surface thereof on the image plane side has an aspherical shape having an inflection point,
• the first lens to the ninth lens are arranged with air intervals between adjacent ones of the first lens to the ninth lens, and
• all of the nine lenses that compose the imaging lens are made of plastic materials.