MTF and Lens Design Quiz: Aberrations and Optical Quality

An MTF chart plots contrast reproduction as a function of spatial frequency. High contrast at low frequencies indicates strong overall rendering; high contrast at fine detail frequencies indicates a lens that resolves small features with good acuity.

A lens with high MTF at low spatial frequencies transfers large-scale tonal differences faithfully, giving images their fundamental sense of contrast and pop. A lens weak at these frequencies produces images that look veiled or flat even at fine detail frequencies.

High MTF values at fine spatial frequencies indicate the lens can distinguish and render tiny details with strong contrast. This is the quality that makes fine textures, small type, and intricate patterns appear crisp rather than blurred together.

Field curvature means the lens focuses a flat subject onto a curved surface instead of a flat one. When the center is in focus, the edges are focused slightly in front of or behind the sensor, so they look soft.

With field curvature, the edges are not out of focus because they are poor quality; they simply focus at a slightly different distance. Refocusing a little brings them sharp, which usually softens the center in turn.

Decentering is a manufacturing or damage-related misalignment of lens elements. It shifts the optical axis off-center, producing an image where one corner or side is noticeably softer than the opposite side at the same aperture.

Coma smears off-axis point sources into comet-like streaks. In portrait or landscape photography, subjects are rarely small enough points of light for this to matter, but astrophotographers working near the frame edges see it immediately in star images.

Longitudinal chromatic aberration happens when different colors focus at slightly different distances. Areas just in front of the focus plane show one color fringe, often purple, and areas behind it show another, often green. It can appear anywhere in the frame, not just the edges.

Lateral chromatic aberration shifts different wavelengths by different amounts across the radius of the image, displacing color channels relative to each other. Because the shift scales from center to edge, it can be corrected precisely by scaling each color channel independently.

Many aberrations are caused by the outer zones of the lens, which are excluded when aperture is reduced. Stopping down limits the cone of light to more corrected central rays, reducing fringing, coma, and spherical aberration, though it cannot fix lateral CA through this mechanism.

Focus shift is when the point of best focus moves as the lens is stopped down. Many cameras focus with the lens wide open, so the image taken at a smaller aperture can end up focused slightly off. It is caused by spherical aberration.

Focus breathing is a change in a lens's field of view as focus moves. The framing appears to zoom in or out slightly during focusing, which is most noticeable in video.

Lens profiles contain measurements of a specific lens's distortion, vignetting and often lateral chromatic aberration. Applying the profile automatically corrects those predictable faults. It cannot fix missed focus, subject motion or perspective.

Vignetting is strongest wide open and lessens as the lens is stopped down. It is also easy to fix in editing with a lens profile or vignette slider, which brightens the corners.

Sharpness in real photos depends on lens quality, accurate focus, camera stability and how the image is processed and sharpened. A great lens cannot overcome missed focus or camera shake.