Application Note

Lens Geometry Performance Comparison

Source: Edmund Optics

The ideal: each point on an object being imaged maps perfectly to a point on the image. The reality: aberrations blur perfect image points to larger spots. Though manufacturing tolerances also contribute to performance degradation, these aberrations occur even for a perfectly manufactured lens.

Spherical aberrations occur when rays at the aperture’s edge focus at a different axial position compared to rays at the aperture’s center (Figure 1a). These aberrations increase with pupil size, so smaller f/# lenses are affected more than larger f/# lenses. Aspheric lenses can be designed with a different curvature at the lens edge instead of the center to correct for spherical aberrations.

Due to the glass property of dispersion, different wavelengths of light experience unique indices of refraction thus are bent by different amounts. Chromatic focal shift (CFS) describes the change in the axial position of focal wavelength. The resulting chromatic aberrations lead to larger spot sizes (Figure 1b). Achromatic lenses use multiple materials with opposed dispersions to improve performance with multichromatic light.

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