Quality Criteria of Lenses
The quality of a lens cannot be expressed with a single number or a simple statement. A number of factors must be evaluated in calculating the quality of a lens. Some of these factors include sharpness, contrast, color correction, relative illumination, spectral transmission, and distortion.
Modulation Transfer Function (MTF)
An ideal lens would be one which is able to produce an image which represents an object exactly. An ideal lens could transfer all of the details of the object to the image without any variations. Anideal lens, however, could only exist under ideal conditions, and in the real world ideal conditions can not be achieved. As such, we must deal with real lenses, and real lenses produce slight variations between the object and the image. In order to show this, we represent the ability of a lens to transfer information from the object to the image as a Modulation Transfer Function (MTF). Coarse structures (such as coarsely spaced lines) are usually transferred to the image with better contrast, or modulation. Fine structures (such as finely spaced lines) are usually transferred with poorer modulation. Therefore, it is useful to graph the way contrast varies with respect to spatial frequency. This is usually expressed in line pairs per mm. A lens’s MTF is a measure of how well the original contrast of the object is transferred to the image. MTF is expressed in percent (the image has X percent of the original contrast remaining), as a function of spatial frequency.