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-Grays of Westminster Glossary

Optical Aberrations

Axial Chromatic Aberration

Axial chromatic aberrations are errors that cause different wavelengths of light to be brought to focus at points either in front of, or behind the plane of the film or digital sensor. Typically light in the blue part of the visible spectrum (short wavelengths) is focused in front, and light in the red part of the visible spectrum (long wavelengths) is focused behind the intended plane of focus.

Axial aberrations tend to manifest themselves as a minor colour fringing dispersed evenly across the entire image. Most modern Nikkor lenses show very low levels of axial chromatic aberration, and even where it is perceptible it is controlled very tightly, particularly in those lenses that incorporate Extra-low dispersion (ED) glass. ED glass was developed exclusively by Nikon to reduce the level of error when focusing light from the full visible spectrum. Nikkor lenses make use of several types of ED glass, each matched for its optical properties to the specific design of the lens. The most recent development is Super ED glass as used in the Nikkor AF-S VR 200mm f/2G IF-ED.

Lateral Chromatic Aberration

Lateral chromatic aberration is caused by different wavelengths (colours) of light being diffracted by varying amounts as they pass through a lens element. The result is a separation of continuous spectrum light into constituent colours, which causes red, green, and blue wavelength light to be brought to focus at different locations in the plane of the film, or digital sensor. At the centre of the image it is rare for lateral chromatic aberration to be present but as you move away from the central axis along radial lines the effects of the aberration become more pronounced, particularly at any edge within the image that coincides with one of these radials. Thankfully, it is very unusual to find lateral chromatic aberration in professional grade Nikkor lenses but it can be found in some of the lower priced consumer grade wide angle and telephoto lenses. If you suspect that your lens is susceptible to this form of aberration try to ensure that no distinct edges occur on any radial extending from the centre of the image and that neither horizontal nor vertical edges intersect the central point.

Coma

Coma occurs when a lens is incapable of focusing a point source (small circle of light) that occurs away from the central axis as a distinct point. Instead what you see are clearly defined small point sources at, or close to the central axis but points closer to the edge of the frame exhibit a faint tail (think of a comet and its tail). To reduce the effect of coma you can try closing the lens aperture down but not by too much otherwise the effect of diffraction will become a problem.

Some lenses, for example the now discontinued manual focus Noct-Nikkor 58 mm f/1.2, are highly corrected in respect of the effects of coma, and are ideal for shooting any twilight or night scenes that contain multiple point light sources such as stars, and street lighting.

Spherical Aberration

Spherical aberration is the result of light passing through the curved surfaces of a lens element in different areas and being brought to different points of focus subsequently. Typically, light that passes through the centre of the lens element is brought to sharp focus at the plane of the film, or digital sensor. However, light that passes through the element close to its edge is focused either in front of, or behind this plane. A lens that renders images with a slight softness at, or near to its maximum aperture and yet produces crisp, well-defined images at f/8 – f/11 is exhibiting spherical aberration. Similarly, if the corners of an image are soft but the central region is sharp spherical aberration is affecting lens performance. Closing the lens aperture down one or two stops from its maximum value will normally cure the problem.

The effects of spherical aberration are, generally, more prevalent in wide-angle lenses due to the light passing through the lens being bent (refracted) at more acute angles at each element surface to air interface, compared with telephoto lens designs in which light passing through the lens tends to be more collimated. To help control the effect of both coma and spherical aberration Nikon use a variety of aspherical lens element types, especially in wide-angle Nikkor lenses.

Curvilinear Distortion

Curvilinear distortions cause straight lines to be rendered as a curve. Generally, the effect of such distortion becomes more apparent the further the line is from the central axis of the lens, where quite often the line appears straight. A typical example is a straight horizon placed close to the edge of the frame that appears to bow outward from the centre of the lens (barrel distortion), or alternatively it bows inwards towards the centre of the lens (pincushion distortion).

To determine whether your lens suffers from curvilinear distortion, examine any straight line in a composition, particularly those at or near the edge of the frame area to see if they are rendered as straight. If not your lens is causing distortion. Barrel distortion is often, although not exclusive demonstrated by short focal length (wide-angle) lenses, and pincushion distortion by long focal length (telephoto) lenses. It is not uncommon too see the both types of distortion present in zoom lenses, particularly high ration zooms that cover wide-angle to telephoto focal lengths. In these instances the distortion usually greatest at the extremes of the focal length range with a neutral zone toward its middle.

Diffraction

When light encounters an abrupt, sharp edge such as the blades of the iris diaphragm in a lens some light is deflected. As the size of the lens aperture is reduced the effect of diffraction increases, because the light wave strike the edge of the iris blades at a more acute angle. The effects of diffraction become visible first with light of a longer wavelength (red). Typically with lenses designed for 35mm film cameras and the smaller DX-format digital sensor diffraction starts to be come an issue at apertures of f/16, or smaller. The focused distance also affects the level of diffraction; diffraction is increased as the focus distance is reduced.

Diffraction is rarely, if ever, an issue with modern Nikkor lenses used with an aperture of f/16 or more, at normal focus distances, which is a good reason for using the hyper focal distance technique when you wish to maximise the depth-of-filed in a picture rather than resorting to setting the minimum aperture available on the lens (often f/22 on mainstream Nikkors but in some cases with Micro-Nikkors f/32, or even f/45). Close-up and macro photography frequently require small apertures to be used at very short focus distances, so the effect of diffraction can become an issue, and will manifest itself as an overall softness in the image.

© Simon Stafford
October 2005

SUNDAY
19
NOVEMBER