In the following equation, the object distance (O) is the distance between the mask and the imaging lens. The imaging distance (I) is the distance between the imaging lens and the image plane (work piece). F is the focal length of the lens. The relationship between these three terms is described by this fundamental optical equation, known as the thin lens equation:
The terms magnification and demagnification are used to describe the ratio of the size of the object (mask) to the size of the image (feature on the target materials). Magnification indicates an image size that is larger than the object size, and demagnification indicates an image size that is smaller than the object size. Imaging systems that provide demagnification are most commonly used because compression of the beam is required to achieve fluence levels that achieve ablation of the target. Demagnification is described by the following equation:
As described in the previous section on beam compression, the fluence of the beam is dependent upon demagnification. In an imaging system, ri denotes the fluence at the imaging plane, and ro denotes the fluence at the mask.
Due to the loss factor term (Lf), the fluence at the image plane for a given demagnification is somewhat reduced by losses through the imaging optics. These losses typically run about 5% per optical element. It is therefore important to minimize the number of optical elements in the design of excimer imaging systems.
Often, the size constraints placed upon optical systems limit the achievable demagnification factor for a given focal length lens. It is therefore useful to be able to determine the optical path length as a function of lens focal length. Substituting the demagnification equation into the thin lens equation, optical paths are given a function of focal length:
| d | Demagnification factor |
| f | Focal length |
| O | Object distance |
| i | Image distance |