A guide to buying lenses for your interchangeable lens camera

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Interchangeable lens cameras are often sold in a body-only configuration so you can choose the best lens for your purposes, as most customers don’t want to be stuck with using a bundled “kit” lens. Although kit lenses are decent all-purpose lenses to begin with, they fall behind in optical performance and are generally poorer in terms of their low-light shooting capabilities when compared to after-market options. There are several lenses to choose from, all catering to specific photo-taking requirements, and each designed to enable you to better photos depending on what you intend to shoot.

The 18-55mm lenses, most commonly bundled as kits with today’s DSLRs have very limited telephoto capabilities, and if you have any interest in shooting wildlife, for example, they’re far from ideal. The focal length range on offer may be what you’re likely to use most frequently, but it is also likely that you’ll soon be frustrated with the limitations posed by a kit lens, especially with the limited zoom range and the sub-par low-light performance.

Buying lenses for your interchangeable lens camera can be quite a daunting task, especially if you’re new to the world of photography. Lens nomenclature doesn’t help matters and often adds to the confusion. If you’re considering buying a lens for your interchangeable lens camera, it a good idea to be reading up on the subject. We’ve tried to simplify some of the basic things you should to know about before buying a lens, and if you’re reading this, you should hopefully be better informed when you’re through with this article. Before we get started, one critical question you ought to have an answer for is what you intend to shoot with your camera.

What is focal length?

A simple definition of the focal length of a lens is the distance between the lens and the image sensor when the subject is in focus. Modern camera lenses usually comprise of several “lens elements,” each of which directs the path of light rays to recreate an image of your subject as accurately as possible on the digital sensor. In cameras, the focal length of a lens, usually stated in millimetres (e.g. 28mm, 50mm), is actually the distance from the lens’ optical centred to the image plane (digital sensor/film) in the camera when the lens is focused at infinity.

angle-of-viewIf you’d rather not bother with understanding the underlying physics, you only need to understand that focal length dictates your field of view. Changing the focal length changes your field of view. The shorter the focal length (e.g. 18mm), the wider the angle of view, and the greater the area captured. The longer the focal length (e.g. 100mm), the smaller the angle of view, and the larger your subject appears to be. If you’re photographing the same subject from the same distance, the apparent size of your subject decreases as the focal length gets shorter, and increases as the focal length gets longer.

Based on angle of view, lenses with focal lengths less than 35mm may be classified as wide angled lenses, and ones with focal lengths from 35-50mm are considered standard or normal because they offer a field of view a little like our own eyes. 50mm and upwards can be considered as telephoto territory. Focal length, therefore, is one of the most important factors to consider when you buy a lens. Deciding on the angle of view you’d like to work with is perhaps the most important decision you need to make when selecting a lens. Another factor that affects your field of view is the size of your camera’s image sensor. Different cameras have different sized sensors, causing an effective change in the field of view that a lens offers.

Figuring out your field of view

Full-frame or 35mm sensors, APS-C sized sensors and micro four thirds sensors are image sensors of different sizes. Full-frame is a term used by DSLR users as a shorthand for an image sensor format of the same size as 35mm format (36mm x 24mm) film. An APS-C sized sensor is much smaller than a 35mm full-frame sensor, and micro four thirds sensors are even smaller. For the sake of uniformity in nomenclature, the focal length of a lens is often discussed in relation to 35mm film or full-frame digital sensors. 35mm film or a full-frame sensor is used as a reference format due to mass adoption and popularity of the format. High manufacturing costs and other technological challenges led to manufacturers introducing smaller sensors in digital SLR cameras. This allowed manufacturers to keep camera mounts and lenses from the days of film SLRs still usable, requiring users invested in a particular camera system to simply replace their film camera bodies without having to repurchase lenses and accessories. Smaller sensors, or crop sensors as they are sometimes called, however resulted in a narrower field of view.

commonly-used-image-sensor-sizes
By MarcusGR [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
For this article, we are going to leave out the merits and demerits of these sensor systems, and discuss how they affect your field of view.

Crop factor and its impact on your pictures

As camera sensor size is independent of a lens’ focal length, we often speak of the different field of view produced by a smaller sensor as a “35mm equivalent” field of view or focal length. In order to allow for a better understanding of field of view of a lens, manufacturers came up with a way to calculate the focal length of a lens relative to 35mm film or a full-frame sensor. Multiplying the focal length of a lens with the “crop factor” of a camera’s sensor gives you the focal length of the lens if it was used on a full-frame sensor, allowing for a better estimation of the field of view of a particular lens across different sensor formats. Different sensor formats have different crop factors. APS-C sensors typically have a crop factor of about 1.5x. This simply means that a lens with a focal length of 50mm, for example, when used on a camera with an APS-C sensor will yield a field of view equivalent to what a 75mm lens would on a full-frame sensor camera. Similarly, micro four thirds sensors have a crop factor of 2, implying that a 50mm lens when used with a micro four thirds sensor yields a field of view equivalent to what a 100mm lens would on a full-frame sensor camera.

Angle of view of a 24mm lens on a full-frame and an APS-C sensor
Angle of view of a 24mm lens on a full-frame and an APS-C sensor

Understanding aperture and how it affects your images

Another important factor to consider aside from focal length is a lens’ maximum aperture. Maximum aperture values determine how much light is allowed to fall onto the sensor. The size of your lens’ aperture is inversely proportional to the F numbers used to denote aperture size. For example, F2.8 is larger than F11, and lets in more light onto the camera’s sensor, allowing for the use of faster shutter speed.

Another property that varies with a change in aperture is depth of field. Wide apertures result in images with a shallow depth of field, and the narrower your aperture, the greater the depth of field. A shallow depth of field helps focus the viewer’s attention on a very specific plane in an image, allowing for images where the viewers attention can be deliberately drawn to specific areas in an image.

With the basics out of the way, it’s time to delve into the various categories of lenses available.

Zoom lenses

Any lens that allows you to change the focal length to alter the field of view available is called a zoom lens. There are both fixed aperture zoom lenses and variable aperture zoom lenses in the market. In fixed aperture zoom lenses, the maximum aperture remains the same across the range of focal lengths on offer, whereas with variable aperture zoom lenses, the maximum aperture varies across the focal length range. An 18-55mm F3.5-5.6 lens, for example, has a variable maximum aperture wherein it allows for an F3.5 to be used at the wide-angle end of the zoom range, and an F5.6 at the telephoto-end of 55mm.

zoom lens
By NG1604 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Prime lenses

Prime lenses offer a fixed focal length. So the field of view remains fixed and cannot be altered. It may seem a little ridiculous to first-time users that anyone would want a prime lens over a zoom lens. They do have several advantages, though. For one, prime lenses often feature wide open apertures compared to zoom lenses. Many photographers will also tell you of how they facilitate a more careful consideration of your frame, and help improve composition.

Prime Lenses
By TonyTheTiger (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

The kit lens

Interchangeable lens cameras, especially those falling in the entry-level and enthusiast segments, are commonly available with kit zoom lenses. Aside from the default kit zoom, camera manufacturers also often bundle a telephoto zoom lens as part of a dual-lens kit. While these kit lenses cover the most frequently used focal length range, they’re not the best quality lenses out in the market in terms of their imaging capabilities. The average kit lenses (even the ones bundled as dual lens kits) also commonly have a maximum aperture of F3.5-5.6 across the focal length range on offer, and don’t fare as well as lenses with fixed wider open apertures in low-light. Given their maximum aperture range, they’re not the most suited to shallow depth of field and low-light shooting.

kit lens
Kit lens (Pic used for representation purposes only)

In comparison, lenses with fixed wide-open apertures of F2.8 or larger work a lot better in low-light. They also often yield noticeably sharper images than the kit lenses bundled with your interchangeable lens camera. You’re likely to find quite a few interesting third-party alternatives to the default 18-55mm F3.5-5.6 kit lens with an angle of view equivalent to what a 28-88mm lens would allow for on a full-frame camera.

Wide-angle lenses

Sample Wide angle photo
By Wcarlisle4 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
As explained earlier, and as the name suggests, wide-angle lenses allow for a wider view than what your naked eye allows for. Thanks to this, they’re frequently used for landscape and other types of photography, adding a touch of drama to your perspective of the world. Wide-angle lenses have shorter focal lengths.

Telephoto lenses

telephoto-example
Sample telephoto image

Telephoto lenses let you get close to the action, making them especially suited to wildlife or sports photography. While moderate telephoto lenses are quite suited for portraiture, the greater focal length telephoto lenses (longer telephoto lenses) lend themselves very well to various types of sports and wildlife photography requirements.

Special purpose lenses

Macro lenses are suitable for taking photos from usually close to your subject, and are designed to magnify your subjects so they don’t appear tiny within your frame. A macro lens, typically, is capable of reproduction ratios of at least 1:1 which is the the ratio of the subject size on the image sensor to the actual size of the subject.

Tilt-shift lenses allow for the rotation of the lens plane relative to the image plane (called tilt) and the movement of the lens parallel to the image plane (called shift) in order to control the appearance of perspective in the image. This is helpful in avoiding the convergence of parallel lines, as when taking pictures of tall buildings, for example.

Fisheye lenses are extremely wide-angle lenses with a field of view of up to 180°.

Third party lenses

 Tamron 17-50mm f/2.8 XR Di II with Canon EOS 70D
Tamron 17-50mm f/2.8 XR Di II with Canon EOS 70D

Sigma, Tamron, and Tokina are among some of the popular third-party manufacturers of lenses. One of the main advantages of using third-party lenses is that they’re often cheaper than proprietary lenses, offering greater value for money. You’ll find lenses for just about every shooting requirement on offer, often available for a fraction of the amount of proprietary lenses.

Other points to consider

Image stabilisation in lenses comes in handy both for video and stills. When shooting stills, image-stabilised lenses are likely to give you an advantage of at least one stop in exposure, meaning you could likely use a shutter speed at least one stop faster. A single stop advantage might just be all that you’ll need to avoid a blurry picture, which a non image-stabilised lens won’t allow for. With video, image stabilisation helps reduce the effects of camera shake and allows for a much more stable video recording without abrupt and jerky motion in video.

Although not necessarily a deciding factor, if you intend to record a lot of video with your camera, it may be in your interest to invest in a lens that incorporates an ultrasonic motor as part of its autofocus system. Ultrasonic motors are not only faster but also less noisier than micro motors typically seen in camera lenses, and are especially useful in cutting out the noise typically made by autofocus motors. When recording video, the camera’s microphone often picks up the sounds made by a lens’ autofocus motor, and this is undesirable. Different manufacturers incorporate this technology under different names. While Nikon calls it a Silent Wave Motor or SWM, Canon calls it an UltraSonic Motor. Sony calls the same a Super Sonic wave motor or SSM.

Lenses, if taken care of well, can last you a lifetime. They often outlast camera bodies, and what you buy is quite likely to be alive and well, even after your camera body gives up on you. What’s more, lenses are often backward compatible. This means that there is even the possibility of being able to use old 50mm (or any other lens your father may have owned) with a brand new interchangeable lens camera.

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