A VERY BASIC GUIDE TO OPTICS IN PHOTOGRAPHYNote - this is a very basic guide, not for optics experts. Simplification often results in telling the truth, but not the whole truth. If it told the whole truth, it wouldn't be simple!
A perfect lens would produce a perfect image. However lenses are not perfect. Their defects are called abberations.
There are 3 aberrations which directly lower the sharpness of an image, Spherical Aberration, Astigmatism and Coma. Some lenses use aspheric elements to better correct spherical aberration at wide apertures. Spherical aberration affects the whole image, astigmatism and coma mostly affect the edges and corners.
Another aberration, Distortion, affects the shape of the image (pincushion and barrel distortion) , but not the sharpness. It makes staight lines look "bowed" near the edges of the image. "Pincushion" means the lines bow inward, "Barrel" means the lines bow outward.
A fifth aberration, Field Curvature, results in the image from the lens being formed as a curved suface, not flat like film, so when the center of the image is in focus, the edges are not, and vice-versa.
All of the above are so called "monochromatic" aberrations, i.e. they exist even for light of a single color. Real light is polychromatic, i.e. made up of many colors which results in a sixth aberration, Chromatic Aberration . Chromatic aberration (actually transverse chromatic aberration) leads to colored fringes in images (especially at the corners and edges of the image) and lowered overall sharpness. Apochromatic or "APO" lenses use special glasses to minimize chromatic aberration.
In theory, with enough effort and expense, all these aberations 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.
Stopping down a lens greatly reduces spherical aberration and coma. 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.
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.
© Copyright Bob Atkins All Rights Reserved