![]() ![]() The size of the image of a point formed by a perfect lens (i.e. If you force it through a small hole, it spreads out on the other side of Squeeze the end of the hose and make it small, the water sprays out in a fan. If theĭiameter of the hose is large, the water flows out in a narrow stream. An analogy (though oneīased on entirely different physics!) is water running through a garden hose. Light squeezes through a small opening it tends to spread out. Diffraction is the name given to the observation that when Obviously an in-focus image will be sharper (smaller) than an out-of-focus image. There are two things which affect the size of the image of a point source. States that if you want a sharp image, it has to be made up of well focused (i.e.small) Which will allow a reasonably sharp 8x10 print to be made from the image. The circle of confusion value is the maximum diameter of the image of a point source With in some depth in the article on Digital Depth of Field, The first concept to understand is that of the circle of confusion value. So why the complaints? Are these just bad lenses? Are the results of stopping down worse on DSLRs than on film Such apertures are often used in macro work in order to maximize depth of field. Performance of lenses on DSLRs when they are stopped down to f22 or smaller apertures. But how far down can you go before the image really starts to suffer from an overall loss of sharpness? The next section of this article deals with that in more detail.įrom time to time you see statements posted on web forums complaining about the 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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