Lenses
- Angle of view - Wideangle lenses have a wide angle of view (hence the name), telephotos have a narrow angle of view. It's more usual to use the lens's focal length to judge its angle of view.
- Aperture (lens) - This is the size of a 'hole' in the lens used to control the exposure. Larger apertures let in more light, smaller apertures let in less. Aperture values are standardised across cameras.
- Aspherical lens - Aspherical lens elements are used within camera lenses to improve the optical performance. They are more expensive to manufacture and are often quoted as a selling point.
- Barrel distortion - Barrel distortion is an effect where straight lights near the edge of the picture appear to bow outwards. It's extremely common with zoom lenses at their wideangle settings.
- Bellows - Bellows were once used to extend the distance between the camera body and the lens for extra-close focussing. They're also used on large format cameras for adjusting perspective.
- Bokeh - Bokeh is a Japanese word which describes the visual quality of out-of-focus areas in a photography. Some lenses are considered to deliver better 'bokeh' than others.
- Chromatic aberration - A lens defect which produces colour fringing around objects near to the edge of the frame. It's largely unavoidable in zoom lenses, though some are better than others.
- Depth of field - Depth of field is the extend to which nearby and distant objects are both sharp. The greater the depth of field, the greater the near-to-far sharpness.
- Diffraction - Diffraction is an optical effect causing a loss of definition at small lens apertures. It's unavoidable, and worse with smaller format cameras/sensors.
- Distortion - Distortion is a lens defect which makes straight lines near the edge of the frame looked bowed outwards (barrel distortion) or inwards (pincushion distortion).
- Element (lens) - Lenses consist of more than just a single lens. In fact there are many of them inside the lens mechanism, and individually they are called lens 'elements'.
- Equivalent focal length - A lens's focal length doesn't necessarily tell you its angle of view because it also depends on the sensor size, so manufacturers quote 'equivalent' focal lengths instead.
- Extension tube - An old-fashioned accessory for extending a lens's close-focussing ability that's not really compatible with today's complex electronic lens/body connections.
- F-stop - The lens aperture setting. The numbers may appear arbitrary, but they follow a mathematical sequence where each one lets through half the light of the one before.
- Fisheye - Fisheye lenses are extreme wideangles which produce a characteristic circular distortion. They have both creative and scientific applications.
- Flare - Flare is the bright spots or reflections which sometimes appear in the image when you shoot directly into the light. It may also produce an overall drop in contrast.
- Focal factor - This is the adjustment needed to work out the equivalent focal length of a lens used on a digital SLR with an APS-C sized sensor.
- Focal length - The focal length of a lens gives you an indication of the lens's magnification or angle of view, though you also have to take the size of the camera's sensor into account.
- Group (lens) - Some of the individual lens elements within a lens are cemented together, and these form a lens 'group'. Lenses may be described as having 14 elements in 10 groups, say.
- Image stabiliser - Image stabilisers use gyroscopic sensors and motors to detect camera movement during the exposure and shift either lens elements or the sensor itself to counteract it.
- Kit lens - A kit lens is one sold with a digital camera body as part of a package. The quality of kit lenses is not particularly high, but an all-in package is convenient and good value.
- Lens cleaning - Dirt or dust on a lens can reduce its sharpness and contrast, and while some dust can be ignored, smears should be removed.
- Lens Correction filter - Photoshop's Lens Correction filter has controls for fixing many common lens aberrations, including distortion, chromatic aberration and vignetting.
- Lens hood - Lens hoods are designed to keep the sun off the front of the lens and reduce the risk of lens flare. Some lenses need them more than others, and the conditions play a part.
- Lens mount - The lens mount is what fixes the lens to a digital SLR's body. It consists of a twist-lock mechanism and a series of electrical contacts.
- Lens testing - Magazines and manufacturers often publish lens tests. These may be carried out in a lab using complex optical rig or consist of a series of photos of test charts.
- Lenses: the basics - Lenses are a whole field in themselves. Their key features, though, are their focal length, their maximum lens aperture and the sensor size/body they're designed for.
- Macro lens - A macro lens is one designed specifically for extreme close-ups. It will focus much closer than a normal lens and a fixed focal length.
- MTF - MTF stands for Modulation Transfer Frequency and it's a way of measuring a lens's resolution and, ideally, should rule out variables like the camera sensor.
- Optical zoom - This is a lens which offers a range of focal lengths. On compact cameras the zoom action is controlled by motors and buttons; on a digital SLR, it’s a ring around the lens.
- Pancake lens - A super-compact fixed focal length lens for digital SLRs and compact cameras with interchangeable lenses. You sacrifice the versatility of a zoom for size.
- Pincushion distortion - Pincushion distortion is the opposite of barrel distortion, and here straight lines near the edge of the frame appear to bow inwards.
- Prime lens - Put simply, a 'prime' lens is one which isn't a zoom, i.e. it has a fixed focal length. Prime lenses are less versatile, but they're lighter, smaller and may offer better quality.
- Reversing ring - An old-fashioned accessory for reversing a lens when mounted on a camera to improve its performance for close-ups.
- Screw mount - Up until the late 1970s most SRLs used a universal 42mm screw mount for attaching lenses. This was replaced with proprietary bayonet mounts which are faster to use.
- Shift (perspective control) lens - Shift (perspective control) lenses have special movements which can be used to correct converging verticals and other perspective distortions which ordinary lenses can't.
- Stop - This is the old-fashioned term for 1EV (exposure value) difference in exposure, so that you might open up the lens aperture by 'one stop', for example.
- Superzoom cameras and lenses - A 'superzoom' is generally reckoned to be a camera or a lens with a focal range of 10x or more. In other words, its maximum focal length is 10x its minimum focal length.
- Teleconverter - Teleconverters fit between the lens and the body of a digital SLR and extend the lens's focal length by a factor of 1.4x, 2x or 3x.
- Telephoto - Telephoto lenses magnify distant subjects in proportion to their focal length. They range from around 90mm equivalent (a 'short' telephoto) to 600mm or more.
- USM (UltraSonic Motor) - This is an autofocus mechanism designed by Canon and used in many of its lenses. The USM autofocus motors are faster and quieter than those of many rival lenses.
- Vignetting - Vignetting (also called corner shading) is where the lens doesn't produce the same brightness at the edges of the frame as it does in the middle.
- Wideangle lens - This is a lens which captures a wider angle of view that normal or, in the case of a zoom, one which offers a 'wideangle' view at its shortest focal length.
- Zoom lens - This is a lens with a variable focal length or angle of view. You can 'zoom' in or out to change the angle of view or the magification (the same thing).