Depth of focus describes the image location range where the image is clear when focused by an optical system. Outside this range, the image will be significantly blurry. However, within this few millimeter range, the image appears quite sharp. When working with a multiple lens system, it is essential to first calculate the position of the image formed by the first lens. Only after locating the first image is it possible to calculate the vergence of light as it reaches the second lens. This is the method by which any number of lenses can be analyzed. Always remember to locate the image formed by the first lens and use it as the object for the second lens to calculate the vergence of light as it reaches the second lens. Repeat the process for each subsequent lens.