Digital camera lens

How are lenses named?
Looking at the lens pages on manufacturers' websites can be a little intimidating for a new user. Lens names often include long lists of letters and numbers, which certainly sound impressive but can also be thoroughly confusing to the newcomer. Luckily you can safely ignore most of them to start off with, and concentrate mainly on just a few things:

• Focal length - defines the lens's angle of view
• Aperture - describes how much light the lens gathers
• Image Stabilization - some lenses include optical stabilization units to counteract the blurring effects of hand shake
• Format - describes the sensor size the lens is designed to work with.
• Lens mount - determines whether the lens will physically fit your camera

We'll look into each of these in more detail below.

Focal Length 
The first number used to describe a lens is its focal length; in combination with the camera's sensor size, this defines the angle of view covered by the lens, with smaller numbers indicating a wider angle. Zoom lenses are named using two numbers which indicate the extremes of the range, for example 18-55mm for a typical kit zoom lens. Fixed focal length lenses which don't zoom (also widely known as 'primes') just have a single number (e.g. 50mm).

Here, we can see this lens' key specifications expressed in terms of its focal length span ('zoom range) which is 18-35mm, and its minimum aperture range, which is F3.5 at 18mm, and F4.5 at 35mm. Other information here is specific to the manufacturer. 'AF-S', describes the type of autofocus motor, 'ED' means Nikon has used Extra Low Dispersion glass in the lens design, and 'G' denotes automatic aperture selection (rather than mechanical in earlier lenses).

The image below shows how the field of view varies with focal length on a camera with the most common sensor size, APS-C (as used by Canon, Nikon, Pentax and Sony). The conversion table shows how these relate to two other standard sensor sizes, namely 'full-frame' which is the same size as a 35mm film negative, and Four Thirds which is used by Olympus and Panasonic.

For the sake of convenient comparison, lenses are often referred to by their '35mm equivalent' focal length, for example a 18-55mm kit lens may be described as a 28-90mm equivalent. It's important to understand that this means simply that an 18-55mm lens on APS-C covers the same angle of view as a 28-90mm does on a 35mm camera, not that the focal length of a lens changes on different formats.

Lens type	35mm 'full-frame'	APS-C / DX	Four Thirds
Ultra wide angle	24mm and wider	16mm and wider	12mm and wider
Wide angle	28mm	18mm	14mm
Standard (Normal)	50mm	30mm	25mm
Telephoto	80mm and longer	55mm and longer	42mm and longer

Aperture 
The aperture of a lens is the second major parameter used in its specification, and describes how much light it is capable of gathering. Apertures can be expressed in several different ways, with F4, f/4, 1:4 all meaning the same thing. A smaller number means the lens has a larger maximum aperture and therefore gathers more light; an F2.8 lens collects twice as much light as an F4, for example

A lens with a larger maximum aperture allows you to shoot in lower light, and (for example) take pictures indoors without using flash. Larger apertures also give decreased depth of field (i.e. how much of the picture in front of and behind the focus point appears sharp), which is an important aspect of creative photography.

A large aperture such as F2.8 gives a shallow depth of field, allowing the isolation of one element in a picture	Large aperture lenses also allow you to shoot indoors without having to resort to flash

Image Stabilization
Image stabilization has become widespread across camera systems over the past few years, but the various manufacturers implement it in different ways. Pentax and Olympus incorporate it into the camera body, whereas Canon, Fujifilm, Nikon, Panasonic and Samsung use systems built into the lens. Sony (slightly confusingly) uses in-body 'Super Steady Shot' for its Alpha SLT cameras, but in-lens 'Optical Steady Shot' for its Nex system cameras. Image stabilization is especially useful with telephoto lenses, so is worth bearing in mind when comparing the available options.

Image stabilization systems reduce the blur caused by camera shake, allowing sharp pictures to be taken even in low light or at long focal lengths. If you own a camera that doesn't have stabilization built into the body, you'll probably want to consider buying stabilized lenses, especially telephotos.

The various manufacturers all call lens-based optical image stabilization by different names, with corresponding initials in the lens names, so here's what you need to look out for when buying:

• Canon - Image Stabilization (IS)
• Fujifilm, Panasonic and Samsung - Optical Image Stabilization (OIS)
• Nikon - Vibration Reduction (VR)
• Sony (NEX system) - Optical Steady Shot (OSS)
• Sigma - Optical Stabilization (OS)
• Tamron - Vibration Control (VC)

Format Coverage
Most affordable SLRs and mirrorless cameras use APS-C sensors, which are approximately 24mm x 16mm in size, or less than half the size of the old 35mm film negative (Nikon calls these cameras 'DX format'). However high end Canon, Nikon and Sony cameras sport so-called 'full frame' sensors, which means purely that they are about the same size as a 35mm negative, i.e. 24mm x 36mm (Canon's older high speed professional cameras used an intermediate sensor size, called APS-H, which provided a 1.3X crop). Panasonic and Olympus, meanwhile, use the slightly smaller Four Thirds sensor format in their interchangeable lens cameras. All of the major manufacturers (aside of course from Olympus and Panasonic) now make a range of lenses specifically optimized for APS-C cameras, and these generally provide the best choice for general-purpose and wide angle zooms in particular. Lenses designed for full frame will also work just fine on APS-C cameras. However APS-C lenses won't work properly on full-frame cameras, and in the case of Canon, it's physically impossible to attach an APS-C-optimised EF-S lens to a full-frame camera. This is something worth bearing in mind if you are thinking of upgrading to a full frame system in the near future. The manufacturers label their APS-C format SLR lenses as follows:

• Canon - EF-S
• Nikon - DX
• Pentax - DA
• Sony - DT
• Sigma - DC
• Tamron - Di II
• Tokina - DX

Sigma and Tamron also have specific designations for their lenses for mirrorless cameras - 'DN' and 'Di III' respectively. At the time of writing, Tokina doesn't make any lenses of this type.

Lens Mount 
Each camera maker uses its own proprietary lens mount, meaning that lenses can't be swapped across brands; a Canon lens won't fit on a Nikon body, for example, and you'll cause damage to lens and camera if you try. There are a couple of exceptions - Olympus and Panasonic both use the Four Thirds mount for DSLRs, and the Micro Four Thirds mount for their mirrorless interchangeable lens compacts (ILCs). Samsung's now-obsolescent SLRs were essentially re-badged Pentax KAF-mount models, however the company is now concentrating on its NX ILC series. A number of third party manufacturers, most notably Sigma, Tamron and Tokina, also make lenses in a number of different mounts to fit the multiple camera brands. The table below lists the currently available lens mounts.

Manufacturer	SLR	ILC	Notes
Canon	EF,  EF-S	EF-M	EF-S lenses are designed for APS-C DSLRs and cannot be used on cameras with larger sensors. However all EF lenses can be used on APS-C DSLRs. All EF and EF-S lenses can also be used on the EOS M via Canon's own adapter, which maintains autofocus and image stabilization functions.
Fujifilm	F	XF	Fujifilm's long-discontinued line of SLRs used the Nikon F mount. Its X-system mirrorless cameras use the entirely different XF mount.
Nikon	F	1	Nikon's entry-level DSLRs will not autofocus with many older lenses which don't have a built-in AF motor. Nikon makes an adapter to fit F-mount lenses to its 1 System mirrorless cameras, but again only lenses with built-in motors will autofocus.
Olympus / Panasonic	Four Thirds	Micro Four Thirds	Four Thirds SLR lenses can be fitted to Micro Four Thirds cameras via an adapter, but autofocus may not function well, if at all.
Pentax	KAF	Q	Some of Pentax's latest lenses with built-in autofocus motors will not autofocus on older DSLR bodies that lack the contacts to power the AF motor.
Samsung	KAF	NX	Pentax K-mount DSLR lenses can be used on NX cameras via an adapter.
Sigma	SA	-	Only Sigma makes lenses to fit its SA-series cameras
Sony	Alpha (A)	E	Alpha mount lenses can be used on NEX E-mount cameras via an adapter. Autofocus is dependent upon the lens type and the adapter used; with the original LA-EA1 adapter only lenses with built-in focus motors (SAM and SSD) will work, whereas all lenses will autofocus on the LA-EA2.

The new generation of interchangeable lens compacts all offer a degree of compatibility with the respective manufacturer's SLR mount via lens adapters, but in general performance is compromised to some extent - in particular autofocus is often slow and hesitant.

Zoom vs. Prime 
Zoom lenses have become almost ubiquitous over the past few years, and at first sight buying a lens which is restricted to a single angle of view might seem pointless. But prime lenses still have some very real advantages; compared to zooms they tend to be smaller and lighter, have faster maximum apertures, and give sharper images. These factors make them extremely useful for specific purposes, for example low light shooting where a large maximum aperture is advantageous.

Fixed focal length 'prime' lenses are often much smaller and lighter than zooms covering the same angle of view. This is Pentax's 15mm F4 lens alongside a typical wideangle zoom, the Tokina 12-24mm F4 - the size advantage is obvious.

Some popular lens types

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Standard Zoom
A standard zoom is a general-purpose lens that covers a range of focal lengths from wideangle to moderate telephoto. The most obvious example is the kit lens that comes with the camera (generally an 18-55mm F3.5-5.6 for APS-C), but this can be upgraded to something with slightly more range and better optical quality, or with a fast F2.8 maximum aperture.

Most manufacturers offer general-purpose upgrades to their kit lenses with expanded zoom ranges suitable for a wide range of subjects, such as this Sony 16-105mm. Typical lenses: 16-85mm F3.5-5.6, 17-55mm F2.8

Telephoto Zoom
Often the second lens that photographers buy, the telephoto zoom effectively allows you to get closer to your subject, and is therefore useful for photographing such things as sports, wildlife, or children running around playing.

Telephoto zooms such as this Nikon 55-200mm allow you to zoom in on your subject. Typical lenses: 55-200mm F4.5-5.6, 75-300mm F4-5.6.

Superzoom
Superzooms are all-in-one lenses which cover a full range of focal lengths from a moderate wideangle to long telephoto. They combine in one package the range of the kit zoom that came with the camera, plus that of a telephoto zoom, and therefore make perfect general purpose travel lenses. The technical image quality is often not quite as good as two separate lenses, but for many users this is more than made up for by the convenience.

Superzoom lenses such as the Tamron 18-270mm F3.5-6.3 encompass a wide range focal lengths from wideangle to telephoto. Typical lenses: 18-200mm F3.5-5.6, 18-250mm F3.5-6.3

Wideangle Zoom
The wideangle zoom extends the angle of view out beyond that captured with the kit zoom, allowing you to capture broad sweeping vistas. It's therefore a popular choice for landscapes, architecture, and interior shots.

Wide zooms such as the Sigma 10-20mm F4-5.6 let you fit more in the frame. Typical lenses: 10-24mm F3.5-5.6, 12-24mm F4

Macro Lens
'Macro' is used to describe a lens with extreme close-focusing ability, which allows you to take photographs of small objects such as insects or flowers. Some zoom lenses use 'macro' in their name to indicate closer-than-usual focusing ability, but true macro lenses tend to have fixed focal lengths. In general, the longer the focal length, the further away you can be from your subject. (Nikon calls these lenses 'Micro' instead.)

Macro lenses like the Olympus 50mm F2 allow you to shoot closeups in fine detail. Typical lenses: 60mm F2.8 Macro, 100mm F2.8 Macro

Fast Prime Lens
Fast prime lenses come in all focal lengths, from wide angle to ultra-telephoto, but what they share in common is the ability to capture a lot of light in a relatively small, discreet package with high optical quality. Once an endangered species, this class of lens has seen a resurgence in recent years, and undoubtedly the most popular is the 50mm F1.8, or the more expensive 50mm F1.4. On a camera with an APS-C sensor this makes for a short telephoto perspective, ideal for snapping pictures of friends and family using natural light.

Fast prime lenses such as this Canon 50mm F1.8 allow you to shoot indoors in natural light without having to use flash. Typical lenses: 50mm F1.8, 85mm F1.8

Pancake Lenses
The word 'pancake' is used to describe slimline lenses that are designed to make a camera as compact as possible. These have enjoyed a resurgence recently as a natural companion for Interchangeable Lens Compact cameras, but are also available for SLRs (most notably from Pentax and more recently Canon).

Three slimline 'pancake' lenses, from Olympus, Samsung and Pentax.

Other lens features

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There are a few other aspects of build and operation which you may wish to consider when buying a lens.

Autofocus
The autofocus system used by a lens can have a large impact on its focusing performance, particularly in terms of noise and speed. The focus motor can be positioned either in the camera body or in the lens, and in-lens focus motors come in a variety of types with different characteristics. Here's an overview of the most important types:

• Screw drive lenses don't have an internal motor, and instead are driven from the camera body via a mechanical coupling, which tends to be fast but comparatively noisy. Many older Nikon, Pentax and Sony lenses use this approach, although all three companies are now moving towards in-lens motors. Entry-level Nikon bodies don't have built-in motors, and so can't autofocus with this type of lens.
• Micromotor drives use conventional DC motors to drive the focus group via a gear train. These tend to be found in cheaper lenses from the camera manufacturers and in many lenses from third party manufacturers (particularly Tamron and Tokina). AF performance is highly variable - at worst slow and noisy, at best reasonably fast and quiet.
• Linear Stepper motors have become common in lenses designed for mirrorless cameras, as they can offer fast and silent autofocus during movie recording. Canon has also made a couple of SLR lenses using this technology.
• Ultrasonic-type motors are very popular in SLR lenses, with the main attraction of being near-silent in operation. They come in two main flavors; the cheaper micro-type has similar characteristics to micromotors, while the more expensive ring-type has a number of advantages. In general, they are fast, silent, and enable full-time manual override of autofocus (see also below). Unfortunately not all of the manufacturers like to make the distinction between the two types clear in their marketing materials.

As usual, each company has a different name for its ultrasonic motors, and uses the corresponding initials in the lens name.

• Canon - Ultrasonic Motor (USM)
• Nikon - Silent Wave Motor (AF-S)
• Olympus - Supersonic Wave Drive (SWD)
• Pentax - Supersonic Drive Motor (SDM)
• Sigma - Hypersonic Motor (HSM)
• Sony - Supersonic Wave Motor (SSM)
• Tamron - Ultrasonic Silent Drive (USD) and Piezo Drive (PZD) - ring-tye and micro-type respectively
• Tokina - Silent Drive Module (SD-M)

Electronic manual focus ('focus by wire')
Most SLR lenses have manual focus rings that move the focus group using a direct mechanical coupling. In contrast most mirrorless cameras employ a 'focus by wire' system, which uses the built-in motor for manual focusing. At its best this offers highly responsive, accurate manual focusing while keeping the lens size to a minimum.

Manual Focus Override
On most cameras and lenses there is a switch to change between autofocus and manual focus, and turning the focus ring when in auto mode can potentially damage the motor or gearing. Some lenses, however, employ a clutch mechanism that allows the photographer to tweak focus manually at any time without risking damage. In general this is limited to the more expensive ultrasonic-type lenses, but Pentax deserves credit here, as almost all of its current range has this feature (which the company calls 'Quick Shift' manual focus)

Some Olympus lenses, like the 17mm F1.8 shown here, offer manual focus using a 'snap ring' manual focus system. Pulling the focus ring back (towards the camera) reveals a distance scale, engages manual focus mode and gives an impressively mechanical-feeling manual focus experience.

Most mirrorless camera systems also allow manual focus override, but rather than using a switch on the lens barrel, this is generally enabled by a menu setting on the camera.

Manual Focus Lenses
A few companies still make high quality manual focus-only lenses, even in this era of autofocus. These tend to be fixed focal-length lenses with metal barrels and premium optics. The principal names to look out for here are Carl Zeiss and Voigtlander. Certain specialist optics from the major manufacturers are also manual focus only, including Canon and Nikon's tilt and shift lenses.

Build Quality and Weathersealing

As a general rule, the more expensive a lens is, the better built it is likely to be. The kit lenses that come with cameras tend to rather lightweight and plastic in construction; spend a bit more and you can get something more durable. Some lenses incorporate environmental sealed against dust and water; in general this tends to be towards the top end of the price spectrum, but Pentax and Olympus in particular offer a decent range of mid-priced sealed lenses (Pentax even makes weather resistant 'WR' versions of its kit lenses to go with its top-end K5 series DSLRs). Special mention must also be made of Pentax's 'Limited' range of primes, which hark back to old fashioned manual focus lenses, with finely engineered and beautifully finished aluminium barrel construction.