Digital Photographer

Lens glossary

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APS-C

A sensor format that’s smaller than full frame, measuring 23.5x15.6mm. Cameras with these sensors can accept ‘digital only’ lenses or full- frame compatible optics.

DIFFRACTIO­N

A reduction in sharpness, which is induced by light rays being deflected by the aperture diaphragm blades at high f-numbers (small opening).

FULL FRAME

A sensor measuring 36x24mm, requiring a larger lens image circle. full-frame lenses are usually larger and heavier due to the physical requiremen­ts of the glass elements.

HYPERFOCAL DISTANCE

A predetermi­ned distance beyond which objects will be rendered ‘acceptably sharp’. Calculated as focal length² / (f-number x circle of confusion).

LOW-DISPERSION GLASS

Optical glass with superior refractive properties. LD, ELD or ULD glass converges light of varying wavelength­s more successful­ly, reducing fringing and chromatic aberration.

MTF CHART

A modular transfer function plot of resolution and contrast, which indicates the performanc­e of a lens in terms of lines per millimetre.

SPECULAR HIGHLIGHTS

In this context, the circular out- of-focus bright spots that are characteri­stic of bokeh. Circular aperture designs produce more rounded highlights, which is considered more attractive.

ASPHERICAL ELEMENTS

Glass elements in the lens system that do not form part of a sphere. These are designed to reduce the effects of spherical aberration for superior edge performanc­e.

FLANGE DEPTH

The distance between the rear element of a lens and the sensor position, which is required for accurate focusing. This differs between lens mounts.

F-NUMBER

Denoting the size of the lens aperture, calculated as focal length / pupil diameter. A low number indicates a larger opening, a high number identifies a smaller opening.

IMAGE CIRCLE

The image that is projected by the lens. The diameter of the projected image must cover the entire sensor area in order to avoid vignetting.

MAGNIFICAT­ION RATIO

The size of a projected image to the size of the real-world object. E.g. a coin measuring 22mm is magnified with a ratio of 1:1 (life size) when the projected coin is 22mm.

REAR FOCUS

A focusing method that moves the rear elements of a lens, rather than glass, further towards the front of the lens system. Allows for internal, not rotating, movements.

STANDARD LENS

A lens with a focal length of around 50mm (full frame) or around 35mm (APS-C), providing a field of view comparable to that of the human eye.

BARREL DISTORTION

A bending of lines within an image, which makes the lines appear to bulge at the centre of the frame. It is the opposite effect to pincushion distortion.

FLUORINE

A chemical element sometimes applied as a coating to the front element of a lens. Fluorine has anti- static properties that repels dirt and water.

GAUSSIAN OPTICS

When all of the optical components are flat or form part of a sphere. This is sometimes attributed to achieving superior out-of- focus areas.

IMAGE STABILISAT­ION

A system utilising movable lens elements to compensate for shift in camera position during exposure. This reduces blur and enables handheld exposures in lower light.

MERIDIONAL

A term used to refer to a lens’s horizontal axis (across the middle), which is perpendicu­lar to the diagonal plane.

SAGITTAL

A term that is used to refer to a lens’s vertical axis

(top to bottom), which is more or less parallel to the diagonal plane.

VIGNETTING

Darkening of frame edges, caused by imbalanced light transmitta­nce of lens elements; by an image circle smaller than the sensor; or objects extending beyond the front element.

CHROMATIC ABERRATION

Coloured fringing along highcontra­st edges within an image, caused by different light wavelength­s being differenti­ally converged by the lens at the same point.

FLUORITE

A chemical component often applied to lens elements as a coating. Fluorite has lowdispers­ion optical properties, which provide reduced chromatic aberration.

GHOSTING

Duplicated image detail, which is caused by light reflecting internally between the lens elements and back onto the sensor, but in a different location.

INTERNAL FOCUSING

A system that shifts the position of the lens elements within the lens barrel, so that the front element does not rotate or extend. Useful when using filters on the front thread.

MINIMUM FOCUS DISTANCE

The closest distance from a subject at which a lens can achieve focus. The closer the distance, the greater the magnificat­ion, but with a less practical workflow, potentiall­y.

SONIC DRIVE MOTOR

A focus motor type in higherend lenses. Uses a voltage to create waves of movement in a ring of elastic material (stator), which is in contact with a rotor.

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