Diffraction Limit

Every lens has an absolute upper performance limit dictated by the laws of physics. This limitation is controlled by the working f/# of the lens and the wavelength(s) of light that pass through the lens. Known as the Diffraction Limit, this limitation is given in line pairs/mm and determines the theoretical maximum resolving power of the lens. Even a perfect lens that is not limited by design will be diffraction limited. This limit is the point where two Airy patterns (Limitations on Resolution and Contrast: The Airy Disk) are no longer distinguishable from each other. To calculate the diffraction limit, a simple formula that relates it to the f/# of the lens and the wavelength of light can be used. Learn more about f/# in f/# (Lens Iris/Aperture Setting).

After the diffraction limit is reached, the lens becomes incapable of resolving greater frequencies. The diffraction limit detailed in Table 1 shows contrast at 0% for given frequencies. These numbers may appear rather high, but are strictly theoretical – a number of other practical factors must also be considered. First, as a general rule, imaging sensors cannot reproduce information at or near 0% contrast. Due to inherent noise, contrast generally needs to be above 10% to be reliably detected on standard imaging sensors. To avoid imaging complications, it is recommended to target 20% contrast or higher at the application’s critical lp/mm resolution. Additionally, lens aberrations and variations associated with manufacturing tolerances also reduce performance. Modulation Transfer Function (MTF) curves are used to determine whether a lens will effectively utilize a sensor's capabilities and fulfill the desired application’s requirements.

Table 1: The diffraction limit calculated at different f/#s for 0.520μm light (green light).