Therefore there is some optimum aperture for each lens, where the best balance between aberrations and diffraction is reached.
On the other hand, stopping down increases diffraction. 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. Stopping down a lens greatly reduces spherical aberration and coma. The two phenomena aren't really related (different mechanisms apply), but the analogy helps to "get the picture" of what's happening. 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 smaller the aperture the more the light spreads out. Without getting technical, diffraction is the spreading out of a light beam when its "squeezed" though a small aperture. Diffraction reduces image sharpness and as you stop down more and more, diffraction effects get larger and larger. The smaller the aperture, the more diffraction and the lower the sharpness of the image. However, there is another limit on image quality (sharpness) which cannot be beaten even with the best possible optics and that is Diffraction.ĭiffraction is the spreading out of light when it passes through an aperture (such as a lens). 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. 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. Apochromatic or "APO" lenses use special glasses to minimise chromatic aberration. 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.Ĭhromatic aberration (actually transverse chromatic aberration) leads to coloured fringes in images (especially at the corners and edges of the image) and lowered overall sharpness.