No design task is complete until the tolerances have been evaluated. The lens design alone, while interesting to the designer, needs to be fabricated within some range of realistic tolerances in order to be of interest to the user. A full tolerancing of a lens may become a more difficult task than the original design of the lens. The specified tolerances must be sufficient to ensure that the image quality goals are likely to be met. The tolerances required for fabrication are the major drivers in determining the cost of actually building and assembling a lens.

Before proceeding to carry out tolerancing the designer must decide upon the allowable degradation in the image. Despite some claims to the contrary, no system will ever be built absolutely perfectly with no deviations from the specified parameters. Therefore the imagery produced by a real system will differ from that of a perfectly fabricated system. During the design stage the designer should have considered this problem and designed into the lens sufficient margin that some errors in the lens parameters can be allowed. The balance between design margin and allowable tolerance loss is frequently an important economic issue. It is also important to remember that some margin usually must be assigned to operational considerations, such as setting the focal position, or to environmental effects such as temperature changes.