18-250mm F3.5-6.3 DC Macro OS HSM

Found on the barrel of some Sigma lenses, this button can be assigned to various functions to widen the range of operations available on the lens (available functions depend on the camera model).

Allows users to change aperture using the lens rather than the camera. The ‘Auto’ button on the ring switches aperture control to the camera.

Some lenses with an aperture ring have a click switch, which allows users to remove the ring’s clicking action. This means the aperture can be changed completely seamlessly rather than in stepped 1/3-stop increments. The function is ideal for film-makers.

This switch locks the aperture ring in either manual or auto, ensuring it can’t get knocked out of position when shooting.

One of the three lines of Global Vision lenses. The Art range includes a mixture of primes and zooms, which boast fast apertures, superb optics and exceptional build quality.

A type of lens element found in most Sigma lenses. Aspherical elements compensate for spherical aberration and distortion, which cannot be completely eliminated using conventional spherical lens elements alone. They are also key to reducing the size and weight of high-power zooms and other large lenses while improving image quality. Hybrid aspherical elements are made by bonding two elements together, one of which is glass and one of which is a polymer. Precision-molded glass aspherical elements are made by direct forming.

All Sigma lenses have a brass mount. It combines high precision with rugged construction and its treated surfaces and enhanced strength contribute to the long-term durability of the lens.

A light but strong material used on some Sigma lenses. It is also used in the interior and exterior fittings of aircraft, among many other applications.

Designed with size and weight in mind, these highly portable primes and zooms are designed for photographers who need to travel light, but without sacrificing image quality. Contemporary lenses typically don’t have such wide maximum apertures as Art lenses in order to keep their weight down, but do not compromise on optical performance. The I series range is part of the Contemporary line-up, sporting have an all-metal build and a manual aperture ring.

Designed for crop-sensor cameras. They can also be used on full-frame bodies, but only in crop mode.

Designed specifically for mirrorless cameras. Some non-DN lenses also fit mirrorless cameras, but these were designed originally for DSLRs and later adapted. DN lenses tend to be smaller and lighter.

This is found on some longer Sigma lenses, such as the 60-600mm F4.5-6.3 DG DN OS | Sports. It allows the user to zoom either by turning the zoom ring, which is very accurate, or by pushing and pulling the end of the lens, which is very fast. This makes the lens very adaptable to different types of fast-action subject.

A series of weather seals around the mount connection, manual focus ring, zoom ring and cover connection to keep out dust and water. Although this construction allows the lens to be used in light rain, it is not the same as being waterproof, so please prevent large amounts of water from splashing on the lens.

The three-digit code printed on the surface of the lens is to indicate the year the lens was first released. 019, for example, denotes 2019.

A now near-defunct designation used to denote Sigma’s higher-end lenses. The only remaining current EX lens is the 105mm F2.8 EX DG OS HSM MACRO.

The degree to which light is refracted by glass depends on the light’s wavelength. This fact causes different colours of light to focus at slightly different points. The result is chromatic aberration, the colour fringing that is particularly noticeable in telephoto lenses. ELD glass is a type of glass used in Sigma lenses with low dispersion qualities, which helps to keep chromatic aberration to a minimum.

The degree to which light is refracted by glass depends on the light’s wavelength. This fact causes different colours of light to focus at slightly different points. The result is chromatic aberration, the colour fringing that is particularly noticeable in telephoto lenses. FLD glass is ultra-low-dispersion glass that offers performance of the highest level. Highly transparent, its refractive index and dispersion are extremely low as compared to conventional types of glass. It offers characteristics very similar to those of fluorite, which is valued for its anomalous dispersion. These characteristics minimise residual chromatic aberration (secondary spectrum), which cannot be corrected by ordinary optical glass, while helping to produce sharp, high-contrast images.

This system adjusts the distance between lens groups during focusing, thereby reducing the amount of lens movement required. The result is less aberration at different shooting distances.

This switch, found on selected Sigma telephoto lenses, allows users to limit the focusing range to either near distance or far distance. This helps speed up AF, and also ensures the camera won’t attempt to focus on an unwanted area of the frame. For example, when photographing zoo animals through a wire fence a photographer could set the focus limiter to, say, 10m–infinity, to stop the camera continually trying to focus on the fence, which is nearer to the camera than the subject. On some Sigma L-Mount lenses, the distances on the limiter can be customised via the USB Dock.

Allows user to toggle between focusing modes. On most Sigma lenses the options are AF and MF, but some lenses also have a Manual Override (MO) button, which allows manual focus by rotating the focus ring even during continuous AF.

HLA is SIGMA’s proprietary linear motor. It drives the focus lens directly without going through gears or other mechanical parts, resulting in quiet, high-precision and fast autofocus. The first lens to have an HLA motor was the 60-600mm F4.5-6.3 DG DN OS | Sports in early 2023.

Some Sigma lenses have a lockable lens hood to ensure they aren’t accidentally removed during use. Some work with a simple release button, while others have a screw action.

The Hyper Sonic Motor (HSM) is an original Sigma development that uses ultrasonic waves to drive the autofocus mechanism. It’s extremely quiet operation helps avoid disturbing photographic subjects. High torque and speed assure rapid autofocus response. Sigma uses two types of HSM: ring HSM and micro HSM. The Ring HSM configuration permits manual fine tuning of focus (Manual Override) by turning the focusing ring after autofocus is complete.

This sensor can detect the position of the focus lens with a high degree of precision by using a magnetic signal. This is done in real time to improve AF accuracy.

To increase stability, this lens configuration uses movable internal lens elements that adjust focus without changing the length of the lens barrel.

Some Sigma lenses incorporate an Inner Zoom mechanism, meaning the barrel of the lens doesn’t change length when zooming in or out. This ensures the lens remains balanced, and since the front of the lens does not rotate, polarising filters can be used with extra convenience.

This is a stabilisation algorithm that enables photographers to pan more effectively. It uses information collected by an acceleration sensor to detect a panning movement (horizontal, vertical or diagonal) and deliver effective stabilization. This feature is available on all Sigma telephoto lenses that have OS switches 1 and 2 (with the exception of the SIGMA 120-300mm F2.8 DG OS HSM | Sports).

Based on the optical characteristics of the lens, this function performs in-camera corrections of peripheral illumination, chromatic aberrations, distortion, and more, to further enhance image quality. It is recommended to leave all corrections turned in the camera’s menu. Not available on all camera models.

This is a protruding lip built into the front of the lens to help keep a lens heater firmly in position. Lens heaters are used by astro photographers to prevent condensation forming on the front element.

Linear focusing simply means that the movement of the focus ring is consistent with the change to the focusing distance, regardless of how quickly the ring is rotated. This is common to all mechanical focusing systems. Non-linear focusing, which is found on Sigma’s mirrorless DN line of lenses, is an electronically controlled focus system where the sensitivity of the ring changes depending on how fast it is rotated. On some of Sigma’s DN lenses, users can switch between linear and non-linear focusing using the USB Dock, and also change the focus throw of the lens.

The magnification ratio of a lens describes the maximum size at which an object can be reproduced on the camera’s sensor. For example, if an object is 1cm long, and a lens can photograph it so that the impression on the sensor is also 1cm long, the magnification ratio is said to be 1:1. If the impression on the sensor were 0.5cm, the magnification ratio would be 1:2. Sigma’s macro lenses are true macro because they have a 1:1 magnification ratio.

The MFL switch disengages the focus ring on the lens. After manually focusing the lens to the desired position, users can set the MFL switch to ‘LOCK’ in order make the focus ring completely inactive. This prevents the focus of the lens from being moved by accident (such as when attaching a lens heater or changing the composition). The focus ring can still be rotated but will have no effect.

When Focus Mode switch is set to the MO position, the lens may be switched to manual focus simply by rotating the focus ring, even during continuous AF. The MO function is only available on some Sigma lenses.

An MTF chart helps show the optical performance of a lens. On the horizontal axis (x-axis) the figure represents the distance in mm from the centre of the lens. On the vertical axis (y-axis) the figure represents the transmission of light that travels through the lens, with 1 being 100% of the light (which is not possible on any lens). The higher this number, the better. At the centre of the lens (0 on the bottom axis), the transmission of light should be highest, and then it falls off slowly towards the edge. There are two types of MTF chart. One considers the diffraction quality of light, which is called ‘Diffraction MTF’. The other, ‘Geometrical MTF’, does not.

Sigma’s Nano Porous Coating is a lens coating that is designed to make a lens less susceptible to strong incident light, such as backlight. It uses porous silica, which has nano-sized holes containing air, as the coating material. Having holes of this size enables a large reduction in the refractive index, allowing the reflectance to be lowered more than conventional anti-reflective coatings. As a result, reflected light causing flares and ghosting is sharply reduced, yielding clearer, higher contrast images.

SIGMA’s OS (Optical Stabilizer) function uses sensors inside the lens to detect motion, then moves specific lens elements in order to effectively minimise blur. Owing to the stabilised image in the viewfinder, it is possible to fine-tune composition and ensure accurate focusing.

A small number of Sigma wide angle lenses have a rear filter holder. This allows sheet-type filters, which are about the size of an SD card, to be inserted behind the rear element. Rear glass filters can be purchased, or filters can be cut from a special sheet using a guide plate as a template.

The polygonal shape of a conventional iris diaphragm causes out-of-focus points of light to appear polygonal. Rounded diaphragm blades are designed to make the aperture opening circular, producing circular out-of-focus highlights at wider apertures.

The degree to which light is refracted by glass depends on the light’s wavelength. This fact causes different colours of light to focus at slightly different points. The result is chromatic aberration, the colour fringing that is particularly noticeable in telephoto lenses. SLD glass is a type of glass used in Sigma lenses with low dispersion qualities, which helps to keep chromatic aberration to a minimum.

Sigma’s Sports line of lenses is made up of telephoto primes and zooms designed for fast-action photography, such as wildlife, sports, motorsport and aviation. Sports lenses tend to have a wide range of features, superb image quality, weather sealing and very fast autofocus.

Lenses with a stepping motor are capable of smooth, quiet and high-speed AF, and are compatible with Face/Eye Detection AF and video AF.

SIGMA’s Super Multi-Layer Coating suppresses flare and ghosting by minimising reflections within the lens. All lenses in the current Sigma range feature this technology. On digital cameras, flare and ghosting may also be caused by reflections between the image sensor and lens surfaces. Here too, the coating is highly effective, ensuring images with outstanding contrast.

An F stop, which is how the aperture value is expressed on a still lens, is measure by the size of the opening that lets through the light. On cine lenses, a T stop value is used instead, which also takes into account how much light is lost as it travels through the lens. This is why Sigma cine lenses have slightly different T stop values to the F stops values on the equivalent stills lenses.

Often called an extender, a teleconverter is a device that can be fitted to the rear of a lens to effectively increase its focal length. 1.4x converters increase focal length by 1.4 times, but at the cost of roughly one stop of light, and 2x converters increase focal length by 2 times but at the cost of roughly two stops of light.

Tripod socket Some longer Sigma lenses have a tripod socket attachment. Usually the socket is included with the lens, except on the 100-400mm F5-6.3 DG DN OS | C, on which it can be purchased separately. Some tripod feet have an Arca Swiss type shape, so can be attached straight on to a compatible tripod. The purpose of a tripod foot is to ensure the lens is properly balanced on a tripod. Without one, a tripod plate would need to be attached to the camera, which when used with a long, heavy lens, would place a great deal of strain on the lens and camera mount.

TSC is a type of polycarbonate with a thermal expansion rate similar to that of aluminium. This means that all parts that make up the lens barrel behave in a similar way as temperatures change, which ensures consistent results in all environments.

A special lens coating that allows water to be wiped away easily and prevents oils from sticking to the surface, even in challenging shooting conditions.

The Zoom Lock switch fixes the zoom ring at a specific focal length to help stop lens creep. Some lenses can only be locked at the widest focal length to keep the zoom locked during transportation, while others can be locked at two or more different focal lengths in the zoom range.

Introduced with the SIGMA 150-600mm F5-6.3 DG DN OS | Sports, the Zoom Torque switch changes the resistance of the zoom ring. At the ‘S’ setting, the ring has lower resistance and is easiest to turn. At the ‘T’ setting, the ring has greater resistance. At this setting lens creep is less likely. At the ‘L’ setting, the ring is locked in position.