Please read the section on how lenses work first. LINK here to this section.

Aberrations

First you have to understand that no lens is perfect. They all have aberrations which reduce their performance. Classically there are five so called "Seidel" aberrations. They are sometimes called third order aberrations based on the mathematics used to model them.

They are:

• Spherical aberration
• Astigmatism
• Field curvature
• Distortion
• Coma

All lenses have these aberrations and they are worse in fast lenses. Stopping down a lens greatly reduces Spherical aberration and to a lesser extent reduced the effects of Coma, Astigmatism and Field curvature on image sharpness. Distortion is unaffected by aperture.

The main tow groups of aberrations a lens can have are:

• Monochromatic Aberrations
• Chromatic Aberration

Monochromatic Aberrations

Monochromatic aberrations exist even for light of a single colour.

From our list these include:

• Spherical aberration
• Astigmatism
• Field curvature
• Distortion

They mainly consist of:

• Distortions affect the shape of the image (pincushion and barrel distortion) , but not the sharpness. It makes straight lines look "bowed" near the edges of the image. "Pincushion" means the lines bow inward, "Barrel" means the lines bow outward.

• Field Curvature, results in the image from the lens being formed as a curved surface, not flat like film, so when the center of the image is in focus, the edges are not, and vice-versa.

Spherical Aberrations

Spherical aberrations are the indistinct or fuzzy appearance of the outer part of the field of view of a lens, which is caused by the non-convergence of rays to a common focus.

Astigmatism

Modern lenses are amazing, but, because of the way optics work, all lenses still have at least a little bit of astigmatism. This means the focal plane is not the perfectly uniform two-dimensional invisible wall we like to imagine it to be. You can see this with large apertures especially.

Field Curvature

Field Curvature, also known as “curvature of field” or “Petzval field curvature”, is a common optical problem that causes a flat object to appear sharp only in a certain part(s) of the frame, instead of being uniformly sharp across the frame. This happens due to the curved nature of optical elements, which project the image in a curved manner, rather than flat. And since all digital camera sensors are flat, they cannot capture the entire image in perfect focus, as shown in the illustration

Distortion

Every lens suffers from lens distortion. High-end lenses will have less distortion than cheap lenses. But because of the physics of light, no lens can produce a perfect, distortion-free image.

To capture a wide field of view, the glass in the lens needs to have an extreme curve to project onto the camera sensor. Glass from wide-angle lenses have much more spherical glass than a telephoto lens.

The curved shape allows the lens to capture a wide view of the scene. But the curve also means the objects at the edge of the image are a different distance from the lens than those at the center. That difference means the objects at the edge of the frame aren’t enlarged as much as the objects in the center. Magnify a line at the edges and not in the center, and that line appears to curve at the edges.

Chromatic Aberration

Chromatic aberration (actually transverse chromatic aberration) leads to coloured fringes in images (especially at the corners and edges of the image) and lowered overall sharpness. Apochromatic or "APO" lenses use special glasses to minimise chromatic aberration.

You might think that if aberrations are reduced as you stop down the image would get sharper and sharper as the aperture got smaller and smaller. But you'd be wrong.

In theory, with enough effort and expense, all these aberrations could be eliminated, or at least reduced to a point at which their effect would be very small. However, there is another limit on image quality (sharpness) which cannot be beaten even with the best possible optics and that is Diffraction.

Diffraction is the spreading out of light when it passes through an aperture (such as a lens). The smaller the aperture, the more diffraction and the lower the sharpness of the image. There is no way around this. Diffraction reduces image sharpness and as you stop down more and more, diffraction effects get larger and larger. Without getting technical, diffraction is the spreading out of a light beam when its "squeezed" though a small aperture. The smaller the aperture the more the light spreads out. One analogy is that of a garden hose. When the end is open water flows out in a narrow stream, but if you squeeze the end to form a small opening, the water fans out. The two phenomena aren't really related (different mechanisms apply), but the analogy helps to "get the picture" of what's happening.

Stopping down a lens greatly reduces spherical aberration and coma. But it has a small effect on the effects of astigmatism and field curvature, little or no effect on chromatic aberration and no effect on distortion. On the other hand, stopping down increases diffraction.

Therefore there is some optimum aperture for each lens, where the best balance between aberrations and diffraction is reached. Usually this will be around f5.6 for good, fast, prime lenses and between f8 and f11 for "consumer" grade zooms. A reasonable "rule of thumb" is that you need to stop a lens down about 2 stops from wide open to get maximum sharpness. There are, however, no hard and fast rules - each lens is slightly different.

While sharpness may peak (at least in the center of the image) between f4 and f8, stopping down to smaller apertures may still improve image quality in the corners of the image. That's because Coma and Astigmatism don't affect the center of the image, so the main effect of stopping down in the center of the frame is the reduction in Spherical aberration and spherical aberration goes way very quickly for a small decrease in aperture (It goes down by the square or cube of the aperture, depending on exactly how you quantify it). The effects of Coma and Astigmatism are reduced slower with decreasing aperture, so stopping down past f8 may further reduce their effects at the edges and corners of the image

There are other lens defects such as flare and vignetting. Flare results from internal reflections within the lens and causes images to be "washed out", or causes bright spots to appear when a bright light (e.g. the sun) is in the picture. Vignetting is when the lens cuts off light in the corners of the image. Both are usually improved by stopping down the lens.

See Chromatic Errors HERE