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Sabtu, 11 Disember 2010

Bagi yang baru nak beli lenses

What lens should I buy for my digital camera?

Once you've bought a new interchangeable lens camera - either a digital SLR or one of the new breed of interchangeable lens compacts typified by Micro Four Thirds - you'll inevitably start thinking of adding an extra lens or two to your arsenal. This is the first step to realizing the flexibility of a system camera, but with the bewildering variety of options available on the market, you could be forgiven for wondering just why you left behind the relative simplicity of a compact camera. In this guide we'll lead you step-by-step through the process of understanding the different kinds of DSLR lenses, and choosing the right one for your needs.

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).
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 (see our DSLR Buying Guide for more information about sensor sizes).
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
Four Thirds
Ultra wide angle
24mm and wider
16mm and wider
12mm and wider
Wide angle
28mm
18mm
14mm
Standard (or 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 (see our glossary for more detail). 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, Nikon, Panasonic and Samsung use systems built into the lens. Sony (slightly confusingly) uses in-body 'Super Steady Shot' for its Alpha SLRs, 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)
  • Nikon - Vibration Reduction (VR)
  • Panasonic and Samsung - Optical Image Stabilization (OIS)
  • Sony (Nex system) - Optical Steady Shot (OSS)
  • Sigma - Optical Stabilization (OS)
  • Tamron - Vibration Control (VC)

Format Coverage

Most affordable SLRs 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 DX). 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 high speed professional cameras use an intermediate sensor size, called APS-H). Panasonic and Olympus, meanwhile, use the slightly smaller Four Thirds sensor 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, which 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 lenses as follows:
  • Canon - EF-S
  • Nikon - DX
  • Pentax - DA
  • Sony - DT
  • Sigma - DC
  • Tamron - Di-II
  • Tokina - DX.

Lens Mounts

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. 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 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-S lenses are designed for APS-C DSLRs and cannot be used on cameras with larger sensors (1.3x or full frame). However all EF lenses can be used on APS-C DSLRs.
Nikon
F
-
Nikon's entry-level DSLRs will not autofocus with many older lenses which don't have a built-in AF motor.
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
-
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 SLR lenses may be used on NEX E-mount cameras via an adapter, but autofocus will not work.
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

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

Other lens features

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 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.
  • Ultrasonic-type motors are becoming increasingly popular, 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).

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