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On Matters of Resolution! - part 3 of 1 2 3 4 5

by Mike McNamee Published 01/02/2009

The next issue is the pixel area that faces the incoming light rays of the image. The larger the area, the greater the electrical signal, which in turn reduces noise and thereby allows higher ISO ratings. Therefore fewer pixels spread over the same area is, within limits, a preferable solution to noise and ISO issues.

The final piece of the jigsaw is 'diffraction limiting'. This is worthy of more discussion because it is a completely inescapable effect and absolutely nothing can be done to prevent its effect in softening images. Diffraction occurs when light passes over a sharp edge, through a slit or through a small hole. In a lens, when the diffraction pattern reaches a similar size to the detail being imaged a loss of resolution occurs. Diffraction depends upon the size of the aperture and the wavelength (colour) of the imaging light.

How diffraction effects a digital image depends upon the pixel pitch. Here is the way it works:


Light from an aperture is distorted by diffraction so that its intensity varies in the shape shown in the diagram. For a circular aperture this shape is revolved into a cone, with outlying rings of decreasing light intensity. This pattern is called an Airy disc after the physicist George Airy. When two resolvable patterns in an image get close to each other, such that the Airy discs overlap, the limiting resolution has been reached.

In practical terms this might be when the image of two stars cannot be separated by a telescope (ie you cannot tell if you are looking on one bright star or two smaller ones) or you cannot separate two diatoms under the microscope. Closer to home, in a digital camera, the diffraction limit is reached when the Airy disc spans two pixel sites. If this sounds esoteric it is not - the diffraction limit for most cameras occurs between f8 and f16, so it is right in the territory that many photographers work. You can do nothing about this drop in resolution, it is not dependent upon either the cost or the quality of the lens. However, a poor lens will mask diffraction limiting because the shape and interaction of the Airy disc is further impeded by the residual aberrations. An additional complication is the design of the Bayer camera chip, which has twice as many green pixel sites as either blue or red. Red light starts to diffract first, then green then blue.


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1st Published 01/02/2009
last update 18/05/2017 12:32:26

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