How Aperture Affects the Image Quality of Your Photograph
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You're probably already aware of how your choice of lens aperture influences depth-of-field, but you may not be as familiar with the effect that your choice has on image quality. Every lens, no matter how costly or inexpensive, has a "sweet spot" of two or three aperture settings that produce the sharpest images (depth-of-field considerations aside). The settings are somewhere in the middle of the aperture range of the lens, and image quality gradually tails off as you reach the extreme settings. You're more likely to notice the difference on inexpensive lenses rather than professional lenses, but it's always there.
The job of a camera lens is to direct the light that enters the camera so that it comes into sharp focus when it hits the camera's sensor or film plane. But no lens performs perfectly, and all camera lenses suffer from several types of aberrations. Most of these faults are of more concern to lens design engineers than to photographers. However, some lens aberrations are more pronounced at certain aperture settings than at others. By knowing and using the sweet spot on your lens, you can minimize these aberrations.
Spherical Aberration
Spherical aberration happens when light passing through the edges of the lens elements comes into focus at a slightly different point than light passing through the center. This aberration softens the image. Spherical aberration is normally an issue only with prime lenses used at the widest aperture settings, such as f1.0, f1.2, f1.4, or f1.8.