This article provides a glossary of useful terms, written in such a way as to flow from one concept to the next. The first section covers basic lens terminology. The second section provides capsule descriptions of the most common lens aberrations, defects that the lens designer struggles to overcome.
In the interest of time I haven't provided illustrations. Please consult other sources after familiarising yourself with this primer.
Lens Glossary
The image plane is the surface on which incoming light rays form a virtual image of the subject. This is where optical film or a digital sensor is placed.
The optical centre is the point at which light rays from different sources cross. This point is usually (but not always) within the extent of the lens itself.
The focal length is the distance between the optical centre of the lens and the image plane, when focused at infinity. This is typically measured in millimetres (mm) but traditionally also in inches, for makers in the UK and USA. A shorter focal length bends light rays more sharply and so brings light to focus in a shorter distance. This allows a wider field of view. Traditional focal lengths from the 135 film era include 18mm, 28mm, 35mm, 50mm, 85mm, 100mm, and 135mm. Today 24mm often replaces 28mm in a lens family, while 135mm is uncommon.
A prime lens has a fixed focal length and can achieve excellent optical quality (generally speaking). A zoom lens conveniently allows a range of focal lengths by compromising on image quality, size, and/or cost.
The field of view is the angular extent of the scene captured by the lens. A wide angle lens has a focal length less than "normal". Historically this included 35mm and below, though by contemporary standards 35mm is not considered wide. Lenses with narrow fields of view (85mm and higher) are typically called telephoto, though technically this is a misuse of the word.
A normal lens has a focal length approximating the diagonal of the image plane. For 135 film this is 43mm, but typically lenses from 40 to 60mm are described as normal with 50mm being by far the most common.
The plane of focus is the distance at which a subject is rendered with precision. Everything in front of or behind this plane is out of focus, to different degrees. Depth of field (DOF) refers to the distance in which the photograph appears acceptably sharp. This is a perceptual affect that depends on the focal length and distance to subject. For more details read my article on 3D pop.
Most lenses are rectilinear, accurately reproducing the geometry of the subject, with straight and parallel lines. Exceptions include the fish-eye lens, which projects a panoramic or hemispherical image, capturing a wider angle than would otherwise be possible. The special effect can be desirable for its own sake.
The iris is a mechanism that allows control over how much light can enter the lens. Several mechanisms have been designed for this purpose, including a circular wheel with a gap in the arc (common in cinematography) or a simple blind that moves horizontally or vertically (common in early cameras). Contemporary photographic lenses use an overlapping system of curved blades that provide for a more-or-less circular opening at all sizes. The number of blades is sometimes taken as a measure of quality but other factors are equally (or more) important.
The aperture is the lens opening itself, the word being Italian for window. The terms iris and aperture are used interchangeably and often incorrectly. Aperture is more common in photography while iris is more prevalent in cinematography.
An f-stop (also f-number) is a measure of aperture size and hence how much light is gathered. This dimensionless value is calculated by dividing the focal length by the diameter of the entrance pupil. As the number gets larger, the aperture gets smaller, since it is on the denominator of the expression. Each doubling of the denominator halves light. These increments are called a stop. Typical f-stops for 135 cameras include f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, and f/22.
Lens speed is indicated by the highest f-stop (the lowest number). A lens that can open to f/2 or greater is generally considered to be “fast.” These terms relate back to optical film development.
Magnification is a dimensionless quantity that expresses how large the subject appears on the image plane. A magnification of 1:1 reproduces the image size without a change in size. Generally a shorter focal length provides a wider field of view but offers less magnification. Conversely, longer focal lengths provide less field of view but greater magnification.
Lenses specifically designed for close-up photography are termed macro, which, strictly speaking, is reserved for those that attain 1:1 magnification. However, a lens labelled “macro” might in fact only magnify 1:2 or less.
The close focus distance specifies the closest a lens can focus on a subject. This is measured from the film (or sensor) plane, not from the front of the lens. Typical values for normal lenses are 50 to 80 cm. As focal length increases, so does this distance, with lenses in the 200mm range typically having 1 to 2m close focus distances. Lenses which can focus unusually close but which are not macro are sometimes labelled "close focus", though this practice is inconsistent.
Lens Aberrations
The perfect optical lens does not exist. The various imperfections are termed aberrations and fall into various defined categories. The most important of these are distortion, spherical aberration, coma, field curvature, astigmatism, and chromatic aberration. There are further "higher order" aberrations that actual designers need to be aware of, but which consumers rarely mention.
Distortion describes the deviation from the correct geometric representation of the subject. Barrel distortion occurs when the lines defining the rectangular frame bend outwards, objects hence becoming barrel-shaped. Pincushion distortion is the opposite: the centre of the lines bends inwards. In the digital era both of these can be corrected in post-production; indeed, many manufacturers now rely on this fact. However any digital correction reduces image quality and might require cropping the original image. Moustache distortion describes wavy distortion and is the most difficult to correct, since such is not a simple geometric operation.
The next two aberrations are especially noticeable at high (open) apertures.
Spherical aberration is apparent from a blurred (less-than-sharp) image, caused by a spread of light rays from an ideal point. This can be corrected by pairing a converging lens element with a diverging lens of less power. The combination will be still converging and the shapes and materials can be tuned to reduce this aberration.
Coma is the unsymmetrical rendering of points of light, so that they appear to have a tail, like a comet. This is due to off-axis rays travelling through more (or less) glass than those that are on-axis.
Field curvature describes when a virtual image is rendered onto a curved field rather than a flat plane. This curve is typically concave, curved forwards at the edges relative to the centre. Designers specify this with a positive Petzval sum. But in certain telephoto designs the focus field is convex (a negative Petzval sum).
Astigmatism describes how one set of parallel lines is out-of-focus compared to another set. This is especially noticeable at low (closed) apertures. In radial astigmatism, circular lines are blurred progressively from the centre of the image. In tangential astigmatism, radial lines are blurred. It is not possible to fully correct for both field curvature and astigmatism; this is always a compromise.
Higher order problems include oblique spherical aberration, seen as a symmetrical light fringe. Unlike spherical aberration this increases with distance from the centre of the field.
Newton’s famous prism demonstrated that white light could be separated into different hues, each of which represents a different frequency band. This experiment demonstrated that light frequencies bend to different degrees when travelling from one medium to another. This dispersion produces several problems grouped under the heading of chromatic aberration. Achromatic lenses correct this for two of the three additive primary colours, whereas apochromatic lenses correct all three colours. However, these names do not indicate the degree of correction, and should be treated with scepticism.
Axial chromatic aberration (CA) is visible as colour fringing (often called “purple fringing”) in areas of high contrast. Lateral chromatic aberration (LCA or LoCA) is seen as colour changes within the depth of field, the blue end of the spectrum being either closer or distant, depending on the type of aberration.
The effects of diffraction must now be noted, even though this is beyond the control of lens designers. As the lens is stopped down, the effects of light bending at the aperture becomes increasingly evident. This unavoidable effect (not a lens aberration) reduces sharpness at low (closed) apertures. In this regime aberrations become less important than diffraction itself.
Decentring describes when elements are shifted from the perfect axis through the lens. This produces skewed images, exacerbating one or more of the above problems. Decentring is typically due to either manufacturing defects or damage over time.
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