![]() ![]() 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. 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). 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. Most good lenses will peak in sharpness in the center of the imge field somewhere between f4 and f8. That's typically enough to significantly reduce spherical aberration, but not enough to make diffraction a problem. Typically this happens when a lens is stopped down a few stops from wide open. So if stopping down reduces the Seidel aberrations but also increases diffraction, I think you can see that maybe there's a "best" aperture which is a balance between reducing the optical aberrations and increasing the effects of diffraction. 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 it's "squeezed" though a small aperture. Diffraction reduces image sharpness and as you stop down more and more, diffraction effects get larger and larger. That's because of a phenomenon called "Diffraction". So 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. A 6th aberration, Chromatic aberration, is to a first approximation unaffected by aperture too. 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. They are:Īll lenses have these aberrations and they are worse in fast lenses. They are sometimes called third order aberrations based on the mathematics used to model them. Classically there are five so called "Seidel" aberrations. They all have aberrations which reduce their performance. Why is this?įirst you have to understand that no lens is perfect. the aperture at which it is sharpest, varies from lens to lens, but as a general rule it's between 1 and 3 stops down from the maximum aperture for the center of the field. Optimum Aperture - Format size and diffraction
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