As demonstrated by our ability to read this printed page, the human visual system is really quite marvelous. The key optical device contained within that system is the eye. Often referred to as the camera that allows us to take pictures for processing by the brain, it is worth noting that our eye is the original, while all cameras represent attempts (usually in vain) to copy the function and performance of that original instrument.
This book will deal with the basic principles that are involved in the design of an optical system to be used in conjunction with the eye. Such systems are often thought of as improving upon the performance of the eye. In reality, while eyeglasses, contact lenses, and surgery can improve vision, the introduction of an optical system is generally intended to permit the designer to trade off certain performance characteristics of the eye, such as field of view vs. resolution, spectral bandwidth vs. sensitivity, or depth of field vs. image brightness. In each case it is the optical designer's responsibility to produce a system design that will function well with the eye, taking advantage of its unique characteristics, while maximizing overall performance in those areas that are critical to the particular application. In order to do this the designer needs to have a basic understanding of the eye, its structure, function, and performance characteristics. In the next chapter we will present a simplified model of a typical eye that is suitable for computer analysis. By establishing the performance of this model (primarily image quality), it may then be combined with designs of various optical systems to demonstrate the impact that they will have upon the performance of the eye.
One of the unique aspects of optical design for visual applications is the condition of the final system output, i.e., the light as it emerges from the optical system. In most other cases, it is the optical designer's task to generate a system that will produce a real image that is compatible with the ultimate system sensor, such as film or a video CCD chip. This type of optical system (one that forms a real image) has come to be referred to as a focal system. In the case of a system to be used with the eye, it is required that the light emerging from the optics project a virtual image that appears at some substantial distance from the eye (usually between 10 inches and infinity).