A-mount (Sony) The lens mount used by Sony’s Alpha SLT cameras. Because these cameras have a mirror in the body, even though it’s a non-moving one, the rear of the lens is further from the sensor, so Alpha mount lenses are physically different to Sony’s E-mount lens range. You can use Alpha lenses on an E-mount camera with a lens adaptor, but not the other way round.
Aberrations These are optical flaws produced by camera lenses and which are largely unavoidable except in the most expensive or the simplest lens designs. They include distortion, chromatic aberration (colour fringing), vignetting (corner shading) and edge softness.
AF-P lens (Nikon) This is a new autofocus technology being introduced by Nikon in its consumer-level lenses. It used different autofocus actuators to its existing AF-S (Supersonic Wave) lenses. The AF-P system uses stepper motors for a fast, quiet and smooth autofocus action that's especially well suited to video, where you don't want fast, sharp focus movements or audible motors that you can hear in the video.
AF-S lens (Nikon) This is the most common autofocus system in current Nikon D-SLR lenses it uses Silent Wave ultrasonic autofocus actuators to produce very fast and quiet focusing well suited to stills photography.
Angle of view This is quoted in degrees, and indicates how much of a scene a lens takes in. Wideangle lenses have a wide angle of view, telephoto lenses have a narrow angle of view. Angle of view is directly related to a lens’s focal length, though the different ‘crop factors’ of smaller sensors means that the relationship between angle of view and focal length is different on different-size sensors.
Aperture (lens) Aperture is the adjustable lens hole controlling light passage and exposure, with set values across cameras. It influences depth of field and lens 'bokeh'.
Aperture blades The adjustable hole in the lens diaphragm is created by a set of overlapping metal leaves, or ‘blades’. The greater the number of blades, the rounder the hole created and the better the lens’s ‘bokeh’ in out of focus areas. Aperture blades are often curved, too, to enhance that circular shape.
Apochromatic (APO) lens An apochromatic (APO) lens is designed to offer improved correction of chromatic aberration and spherical aberration using specialised materials and combinations of lens elements. It’s a selling point for lenses, though only indicates the lens design used and isn’t really a guarantee of good performance on its own.
Axial chromatic aberration Axial chromatic aberration is colour fringing that appears around out of focus objects. It happens when the lens defocuses different colours differently, e.g. blue goes out of focus more quickly than other colours and creates a soft blue fringe round out of focus objects.
Barrel distortion This is where straight lines near the edge of the picture appear to bow outwards, and you see this a lot with zoom lenses at their wideangle setting. It’s most noticeable if the horizon is near the top or bottom of the picture. Barrel distortion is very difficult to eradicate completely from the lens design, but it can be fixed using software, and some cameras now have distortion correction built in. It’s one of a number of common lens aberrations. Telephoto lenses often show the opposite effect, ‘pincushion distortion’.
Bayonet mount A twist-lock mechanism used almost universally for mounting lenses on camera bodies. You line up two dots, one on the lens barrel and one on the camera body and insert the lens, then twist the lens in the mount until it locks into place. The lens is released again by pressing a button on the camera body to release a catch, then twisting and removing it.
Bokeh This is a Japanese word to describe the particular visual quality of out of focus areas in a picture. You might think it hardly matters what things look like when they’re out of focus, but there’s a bit more to it than that. ‘Bad’ bokeh produces unnatural-looking outlines and highlights, while ‘good’ bokeh looks ‘creamy’, smooth and natural. Good bokeh is associated with the shape of the diaphragm in the lens – more aperture blades and rounded aperture blades produce a more circular shape and better bokeh. Some photographers confuse bokeh with how out of focus a subject is, but that’s not the same thing. A lens with a wide maximum aperture can make background objects extremely defocused, but that doesn’t mean they have good ‘bokeh’.
Chromatic aberration This is a lens aberration that produces colour fringing around the outlines of objects near the edges of the picture. It’s very hard to eradicate completely from lens designs without making them extremely complex or expensive, but it is possible to correct chromatic aberration using software and many cameras will now correct it automatically as they process the image.
Circle of confusion This is a concept used in calculating depth of field. When a point is slightly out of focus it's rendered as a circle instead, but as long as this 'circle of confusion' is small enough, it still looks like a point.
Corner shading Corner shading is another term for vignetting, where the edges of the picture are darker than the center. It's because the lens is illuminating the sensor unevenly, and most camera lenses have optical designs that reduce or eliminate this vignetting effect.
Crop factor Used to work out the effective focal length of lenses on cameras which don't have full frame sensors. You multiply the actual focal length by the crop factor to get the effective focal length. The crop factor of an APS-C camera is 1.5, so a 50mm lens has an effective focal length of 75mm.
Crop sensor A 'crop' sensor is one that's smaller than a full frame sensor. This means that it captures a smaller area and a narrower angle of view with the same focal length lens. In effect, smaller sensors make lenses look as if they have a 'longer' focal length, and by a specific factor – or 'crop factor'.
Depth of field Depth of field refers to the sharpness of an image, both near and far. Factors influencing it include lens focal length, aperture, and focus distance.
Depth of field preview Usually you view the scene with the camera lens wide open and it only stops down to your chosen aperture the moment you press the shutter button, so it’s hard to judge just how much depth of field the final photo will have. The depth of field preview stops the lens down to the taking aperture, though, so you can judge the effect in the viewfinder or on the LCD display.
Diaphragm Mechanism inside a lens which uses interlocking metal leaves, or 'blades', to produce a variable-sized aperture within the lens. This is used to control the amount of light passing through and hence the exposure.
Diffraction Diffraction is a softening effect you see at very small lens apertures. It's caused by the way light bends when it passes a sharp edge (the edges of the aperture diaphragm blades), and it's worse at small lens apertures because a higher proportion of the light is bent compared to a relative small proportion passing unchanged through the center of the aperture.
Distortion An optical effect in some lenses where straight lines come out slightly bowed. You often see ‘barrel distortion’ with wideangle lenses or ‘pincushion distortion’ with telephoto lenses at their longest zoom setting. More expensive lenses tend to have less distortion but, generally, the longer the lens’s zoom range the more likely you are to see distortion creeping in.
Distortion correction Software correction carried out either in the camera during image processing or later on in software to correct bowed edges caused by lens distortion.
E-mount (Sony) This is the name of the lens mount used by Sony mirrorless cameras. Regular E-mount lenses fit its APS-C format cameras, like the Sony A6500, while FE lenses fit its full-frame mirrorless cameras, including the A7 series and Sony A9. Sony also makes A-mount lenses for its Alpha SLT cameras, but these are not the same.
Effective focal length The angle of view of a lens changes according to the size of the sensor in the camera. A smaller sensor captures a narrower angle of view and makes it look as if the lens has a longer focal length. So in addition to the actual focal length, the manufacturers will usually quote the 'effective' focal length too.
Electromagnetic diaphragm A system introduced by Nikon for some of its lenses where the lens aperture diaphragm in the lens is controlled electromagnetically rather than by the traditional mechanical linking. This gives more accurate and consistent exposures, especially during continuous shooting, where the lens diaphragm may be adjusted many times a second.
Element (lens) Camera lenses are made up of not just one single lens but many different lens 'elements', sometimes cemented or fixed together in 'groups'. A comparatively simple prime (non zoom) lens may have 6-7 elements while a complex zoom lens might have 17 or more. The different lens elements are needed to compensate for a variety of common lens aberrations and offer autofocus and zoom capabilities.
This is a fine screw thread cut into the front of almost all DSLR and mirrorless camera lenses. This is where you screw in round filters, or the adaptor rings for square filter holders. The size of the filter thread varies, so you need to make sure you buy filters or adaptors the right size for your lens.
Fisheye lens A fisheye is an ultra-wideangle lens that no longer attempts to render straight lines as straight and instead produces images with strongly curved edges and a characteristically surreal look. It's a striking effect, though one to be used occasionally.
Flange distance This is the distance between the mounting plate on a camera that takes interchangeable lenses and the sensor itself. Mirrorless cameras have a shorter flange distance because there's no mirror inside the body, and this makes the cameras slimmer. DSLRs have a longer flange distance because there needs to be space inside the body for the mirror that this design gets its name from. This makes DSLR bodies thicker. This difference in flange distances means that it's sometimes possible to use lens adaptors to fit lenses of a different type, brand or lens mount to a camera. This generally works one way only – you can mount a lens with a longer flange distance (e.g. a DSLR or old film SLR lens) on a camera with a shorter flange distance (e.g. a mirrorless camera) but not the other way round.
Focal length This tells you a lens’s magnification or angle of view (it’s the same thing really) and it’s quoted in millimetres. Sometimes the makers quote actual millimetres and sometimes they quote the ‘effective’ focal length, which is what the lens would be equivalent to if it was a 35mm camera.
Focus limiter A switch found on some telephoto and macro lenses to restrict the autofocus to a specific range. This speeds up the autofocus for situations where you know you won't need the lens's full focus range.
Group (lens) Camera lenses used complex configurations of different optical elements, often cemented or fixed together in 'groups'. Lens groups may be designed to counteract common optical aberrations and you may have autofocus 'groups' and zoom groups. Lens elements and groups often move relative to each other in complex ways as the focus and zoom settings are changed.
Image circle All lenses produce a circular image on the camera sensor or film, and this 'image circle' must be at least large enough to cover the full film/sensor area. Different lenses designed for different sensor sizes and formats have different-sized image circles. Lenses designed for APS-C format cameras, for example, have a smaller image circle than lenses for full frame cameras. Some specialised perspective control or tilt-shift lenses have larger image circles to allow for lens movements relative to the camera.
Image stabilizer A mechanism that counteracts camera movement during the exposure. Lens-based stabilisers use a moving lens element, while sensor-based stabilisers move the sensor itself. Image stabilisers are used to get sharper telephoto shots and low-light shots without camera shake.
Keystoning Where the tops of tall buildings appear to converge. This happens when you're so close you have to tilt the camera upwards to get everything in. You can correct it by choosing a more distant viewpoint and keeping the camera level, or by using keystone correction tools in software.
Lens The lens is a fundamental part of any camera. It's what creates the image on the camera sensor (or film). Some cameras have a fixed, non-removable lens while others offer interchangeable lenses. Your choice of lens has a major impact on the appearance of your pictures, including the lens's focal length (angle of view) and its aperture setting (which you may or may not be able to adjust). At a simple level the lens is just the thing on the front of the camera, but on a more advanced level lenses open up a whole world of photographic choices, buying decisions and technical comparisons.
Lens adapter In principle, you can't mix and match different types and brands of lenses with different camera bodies. Each camera maker uses its own bespoke lens mount and different mechanical and electronic connections between the camera body and lens. However, it's often possible to make lenses fit different brands and types of bodies with lens adaptors. These are usually from third-party makers and designed for users who don't mind a few compromises in camera functions. For example, you may lose autofocus functions and have to use manual focus only, and it's likely that you'll have to use manual exposure and lens aperture control rather than the camera's full range of exposure controls.
Lens corrections Lenses aren't perfect – they all have optical aberrations of one sort or another. Now, though, many software applications have lens correction to correct these digitally, either with manual controls or automatic lens correction profiles.
Lens hood Lens hoods can reduce lens flare and improve contrast when there's a bright light source just outside the edge of the frame, but they won't help if the sun, for example, is in the frame. Lens hoods are usually 'petal' types that allow for the fact the image frame is rectangular.
Lens modulation optimiser (LMO) A processing algorithm used by Fuji in some of its cameras to counteract the softening effects of diffraction at small lens apertures, and image softness at the edge of the frame. It seems likely the LMO is simply applying some intelligent sharpening.
Lens mount This is the physical connection between a lens and the body of a DSLR or mirrorless camera. It consists of a twist-lock bayonet mount and electrical connectors. The lens mount is specific to a camera brand – you have to make sure you get lenses in the right fitting for your camera.
Lens profile Almost all lenses suffer from aberrations, including distortion, chromatic aberration and vignetting. These are difficult to eliminate optically in the lens design, so software publishers are increasingly offering lens correction profiles to do this digitally. The software can identify the lens used from the image’s EXIF data and then find and apply the correct profile automatically.
Macro lens Strictly speaking, macro photography where a real-life object is captured at the same size on the sensor. So a bee 10mm long would form an image 10mm long on the sensor. True macro photography needs dedicated 'macro' lenses.
Manual focus Useful when you want to make the most of depth of field – with often means focusing between two objects rather than on one or the other. It’s also handy for ‘zone focusing’ in shoot-from-the-hip style street photography, where you want an instant shutter response.
Maximum aperture The maximum light-gathering power of a lens and a major selling point.It lets you use faster shutter speeds or lower ISO settings in poor light.This lens has a maximum aperture of 1:2.8. This is the same as f/2.8 – different makers use slightly different terminology.
Mount adapter A lens mount adaptor which lets you mount a lens designed for one camera or brand on a different make or type of camera. For example, you can get adaptors for fitting DSLR lenses on some mirrorless cameras. Mount adaptors (lens adaptors) are used widely in videography.
Optical stabiliser Image stabiliser which moves physical elements within the lens, or the sensor itself, to keep the image steady during the exposure. This is superior to ‘digital stabilizers’ which use image processing techniques to reduce blur, but which also lead to a loss in quality.
Optical zoom A zoom function produced by changing the magnification of the lens rather than by simply blowing up a central part of the image (digital zoom). Makers will always specify optical zoom and digital zoom separately in their specifications.
Pancake lens A fixed focal length (‘prime’) lens designed to be as slim as possible so that the camera/lens combination is lighter, more compact and more unobtrusive. Their only real concession compared to a regular prime lens is maximum aperture – typically f/2.8 for a pancake lens.
Perspective control (PC) lens Perspective control lenses have special tilt and shift movements for correcting converging lines (shift movement) in architectural images, for example, and adjusting the plane of sharp focus (tilt movements) for objects at an angle to the camera. By applying a vertical shift you can bring the top of a tall building into the frame without tilting the camera (this is what caused the converging vertical effect). By applying a tilt movement to the lens you can change the plane of sharp focus away from the perpendicular towards the plane of your subject – this increases the depth of field available.
Pincushion distortion This is where straight lines near the edge of the picture appear to bow inwards. It’s not as common as barrel distortion, but you do see it quite a lot with telephoto zoom lenses when the lens is set to its maximum focal length. You may not notice it with many types of subject, but it can be corrected with software later anyway.
Portrait lens As the name suggests, this is a lens particular suited to portraiture by virtue of its focal length and a fast maximum aperture.
Prime lens A lens with a fixed focal length, as opposed to a zoom lens. Prime lenses are more restrictive, but they tend to produce better optical quality with fewer aberrations and offer a wider maximum aperture. They also tend to be lighter and smaller, and many photographers find that having to change position to get the best framing leads to better pictures.
Reproduction ratio Reproduction ratio is a term used in macro photography to indicate the degree of magnification. A ratio of 1:1 is usually considered the minimum for 'true' macro photography. This means that an object is reproduced at exactly the same size on the sensor or film surface as it is in real life. If the first number is higher, e.g. the reproduction ratio is 2:1, it means the lens can reproduce objects at twice life size. If the second number is larger, e.g. 1:2, it means objects are reproduced at half their actual size.
Resolution This can mean one of several things depending on the context. Camera resolution is the number of megapixels on the sensor, lens resolution is how well the lens is able to resolve fine detail. Screen resolution is the number of dots on the screen and therefore how sharp/clear it looks.
Stepping motor As the name suggests, stepping motors move in small, incremental steps. They are simple, reliable and offer a good deal of control and precision. They're now being used by Canon (STM lenses) and Nikon (AF-P lenses) in a number of consumer-orientated lenses, where they offer fast, quiet and smooth autofocus.
STM (Canon) STM stands for stepper motor lenses, a new type of autofocus motor used by Canon in some of its lenses. Stepper motors offer fast, precise and quiet focus adjustments, so these lenses are well suited both to regular stills photography and to video, where autofocus noise can be picked up very easily by the camera's internal microphone. Canon's STM lenses work very effectively with cameras using Canon's Dual Pixel CMOS AF autofocus system.
Super telephoto An extreme telephoto lens with an effective focal length of 400mm or longer and most commonly used by sports, wildlife and press photographers. These lenses are big, heavy and expensive but provide high levels of magnification not possible with ordinary lenses. Some consumer-orientated 'bridge' and 'travel' cameras have very long zoom ranges that fit into this category, though the small size of the sensor and limited optical quality of the lenses stops them being taken seriously alongside professional cameras.
Teleconverter A special magnifier lens that fits between a telephoto lens and the camera body to increase its focal length. Teleconverters are often matched to specific lenses to ensure optical quality and performance. They typically come in 1.4x, 1.7x and 2x magnifications.
Telephoto A lens which gives a magnified view of the scene. The magnification is proportional to the focal length of the lens, so a 100mm telephoto gives 2x the magnification of a 50mm standard lens.
Ultra wideangle A lens with a much wider angle of view than your camera's kit lens. In 35mm camera terms, a super-wideangle lens is one with a focal length of around 20mm or less. Super-wideangle lenses are quite expensive and characterised by large, bulbous front lens elements.
Volume deformation A special type of distortion correction once built into DxO Optics Pro but now built into the separate DxO ViewPoint application. It fixes the distortion usually seen with wideangle lenses where objects near the edge of the frame appear disproportionately wide – it's most obvious with human figures.
Wideangle lens A lens that takes in a wider than usual angle of view. Wideangle lenses have an effective focal length of 28mm or shorter. The shorter the focal length, the wider the angle of view.
X-mount (Fujifilm) This is the lens mount for Fujifilm's X-series mirrorless cameras. These include the Fujifilm X-T2, X-H1, X-E3 and others. Any X-mount lenses can be used on any X-mount camera, though note that Fujifilm's medium format GFX 50S uses a different mount and different lenses.
Zoom lens A lens which can be adjusted to give a range of different focal lengths. Most lenses in use today are zooms because they're so much more versatile than fixed focal length (prime) lenses – you can adjust the framing without having to change the camera position. The disadvantages of zoom lenses are increased distortion and other aberrations, reduced maximum aperture and greater cost and weight.
Zoom range The difference in magnification offered by a zoom lens and its widest and longest focal lengths. The average kit lens has a zoom range of 3x, so at full zoom objects appear 3x larger than they do when you're zoomed right out. The Nikon P900 has a record-breaking 83x zoom.