Lens Aberrations Quiz: Chromatic Aberration and Distortion

Chromatic aberration is color fringing at edges of high-contrast areas, caused by the lens focusing different colors at slightly different distances. It is most visible in wide-angle lenses and at the edges of the frame. It is easily corrected in post-processing.

Vignetting is a darkening of the image toward the corners caused by the lens barrel blocking oblique rays. It is most pronounced at wide apertures and can be removed with a lens correction profile or used deliberately to draw attention toward the center.

Barrel distortion makes straight lines bow outward from the center of the frame, as if the image were wrapped around a barrel. It is most common with wide angle and zoom lenses at the wide end.

Pincushion distortion makes straight lines bow inward toward the center of the frame, like the sides of a pincushion. It is most common with telephoto lenses and zooms at the long end.

At maximum aperture, many lenses are softer in the corners because their aberrations are worst at the edge of the image. Stopping down a stop or two cuts off those outer rays and usually sharpens the corners. Past a certain point, diffraction begins to soften the whole frame again.

Coma is an optical aberration that smears off-axis point sources of light, such as stars, into comet-like streaks radiating outward from the frame center. It is most visible at wide apertures and near the edges of the image.

Spherical aberration happens when rays through the edge of a lens focus at a different distance than rays through its center. The result is a soft, low contrast glow, most visible wide open, which usually improves when the lens is stopped down.

Flare happens when bright light, often from the sun or a lamp just outside or inside the frame, bounces between lens elements and the barrel. It shows up as bright spots, streaks or a washed out haze that lowers contrast.

A lens hood blocks light coming from outside the frame from striking the front element at an angle. That stray light would otherwise bounce around inside the lens and cause flare and a hazy loss of contrast.

An MTF chart plots how faithfully a lens reproduces fine spatial frequencies at various apertures and field positions. Higher MTF values across the frame indicate a lens that renders detail with strong contrast and maintains sharpness to the corners.

Corner softness in wide-angle and fast lenses is common at maximum aperture. Stopping down a stop or two increases the depth of the focal plane and allows the lens to use the more corrected central rays, often producing noticeably sharper corners.

Field curvature means the lens focuses a flat subject onto a curved surface rather than a flat one. The center can be sharp while the edges are slightly out of focus, or the reverse, even though the subject is flat.

Microcontrast describes how well a lens differentiates small, adjacent tonal values. High microcontrast makes textures and fine detail appear crisp and three-dimensional; low microcontrast produces smooth but slightly flat or hazy rendering.

Decentering means one or more lens elements are slightly tilted or shifted off the optical axis, from manufacturing or a knock. Sharpness is no longer even across the frame, so one corner or side looks noticeably softer than the others.

Lens profiles are correction maps that record a specific optic's distortion, vignetting, and chromatic aberration at various apertures. Applying the appropriate profile in post-processing reverses these optical faults automatically.