A beginner's guide to choosing a lens

Looking for a new lens? Perhaps you're stepping up from a compact camera to one with an interchangeable lens, or possibly you are aiming to expand your shooting potential beyond the kit lens that came with your camera. Here's what you need to know to find the perfect lens for you.

The name of a Canon lens includes all the key information about it. Take the Canon RF 100mm F2.8L MACRO IS USM for example:

• RF = the lens type, corresponding to the different types of lens mount on Canon cameras.
• 100mm = the focal length of the lens.
• F2.8 = the maximum aperture.
• L means this lens is one of Canon's L-series "professional" (higher-quality) range.
• MACRO = includes the capability to capture images at macro magnification.
• IS = the lens has a built-in optical Image Stabilizer to reduce blurring caused by camera shake.
• USM = Ultra Sonic Motor, a fast and quiet focusing motor.
• Specialist lenses may have other elements in their names, relating to the design or technologies inside such as Fisheye, DS (Defocus Smoothing) or DO (Diffractive Optics).
• Some lenses also have II or III in the name. This denotes the model number, rather than any properties of the lens. For example, the EF 75-300mm f/4-5.6 III USM is the third version of this EF zoom lens with a focal length range of 75-300mm, aperture range from f/4 to f/5.6 and a USM motor.

Before buying any lens, it is important to check that it's compatible with your camera. Each Canon EOS camera uses one of four different lens mounts. Each type of lens is designed to fit a particular lens mount:

• RF and RF-S lenses are for use with EOS R System mirrorless cameras – see below.
• EF, TS-E (tilt-shift) and MP-E (macro) lenses are designed for the EF mount on Canon EOS DSLR cameras. They can also be used on EOS R System cameras via an EF-EOS R Mount Adapter.
• EF-S lenses are for APS-C DSLRs. They cannot be used with full-frame DSLRs, but they can be used with (full-frame or APS-C) EOS R System cameras with a mount adapter.
• EF-M lenses are for EOS M mirrorless cameras and cannot be used with any other type of camera.

All EOS R System cameras have an identical RF mount, which means that any camera in the range can be paired with any RF or RF-S lens. RF-S lenses are designed for APS-C cameras but can also be used with full-frame EOS R System cameras, although the camera will automatically crop the image area to match the smaller APS-C coverage of the lens.

It is also possible to use a full-frame RF lens (or EF lens, with an adapter) on an APS-C EOS R System camera such as the EOS R7, EOS R10 or EOS R50. However, APS-C sensors are smaller than full-frame sensors, which means that the sensor won't use the full field of view of the lens. Instead, it will crop the image to the sensor's smaller active area. In Canon cameras, the "crop factor" is 1.6x. This has the effect of zooming into the image, giving a 50mm lens the same field of view as an 80mm lens on a full-frame camera (50 x 1.6 = 80), a 100mm lens the field of view of a 160mm lens, and so on.

To calculate the effective focal length of a lens on an APS-C camera, simply multiply the focal length of the lens by 1.6x. Or, to make things easier when you're using a zoom lens with variable focal length, the free Canon Photo Companion app contains an Effective Focal Length calculator.

The focal length of a lens is literally the distance between its optical centre and the camera sensor. You can usually find this written on the lens. If it is a prime (fixed focal length) lens, this will be a single number, such as 24mm or 135mm. If it is a zoom (variable focal length) lens, it will be a range, such as 15-35mm or 24-70mm.

The focal length of a lens determines its field of view, and lenses can be divided into three broad categories: wide-angle, standard and telephoto.

• Wide-angle lenses – those up to around 35mm – will include more of the scene you're photographing, making them ideal for landscape photography. Lenses below about 24mm (full frame equivalent) are sometimes referred to as "ultra-wide".
• Standard lenses typically have a focal length of around 50mm. These are said to have a "natural" perspective approximately matching that of the human eye, making them great all-purpose, travel and portrait photography lenses.
• A telephoto lens is one with a focal length of around 85mm or more. You'll find lenses up to 600mm, 800mm or even 1200mm. Lenses above 300mm are sometimes called "super-telephoto" and are typically used in genres such as wildlife and sports/action photography where photographers usually need to shoot subjects from a distance or pick out a small part of the scene.

Some zoom lenses cover more than one of these categories, with some going all the way from wide-angle to telephoto – for example, 24-240mm.

Is it better to have a prime lens or a zoom lens? Zooms offer greater versatility, meaning you don't need to carry as many lenses around in your kitbag to be prepared for different shooting situations. However, primes commonly have better optical quality and are often available with wider apertures – see below.

Put simply, the aperture of a lens is the opening through which light passes. The size of the aperture is expressed in f-stops, in the form f/2.8, f/4 and so on. The slash isn't always included in lens names, but it's important, because f-stop is actually a fraction:

    aperture diameter = focal length (f) divided by f-number

Once you grasp this, you'll understand why a smaller f-number means a larger aperture – it's the denominator of the fraction, and 1/4 is larger than 1/16. The smaller the number, the wider the maximum aperture the lens is capable of: f/1.4 is very wide, f/4 is narrower, and f/16 even narrower.

The wider the opening, the more light can pass through to your camera's sensor, which means better results particularly in low light conditions. A lens with a wide maximum aperture (small f-number) is known as a "fast" lens – because it lets in more light, you can use a faster shutter speed than you'd need with a smaller-aperture lens to achieve the same exposure.

In addition, the aperture also determines the image's depth of field – that is, how much of the scene you're photographing is in sharp focus. A wide aperture (smaller f-number) produces a shallow depth of field, giving portraits an attractive blurred background, for example. A narrower aperture (bigger f-number) results in a deeper depth of field, which is ideal for front-to-back sharpness in a sweeping landscape. Find out more in our comprehensive guide to aperture.

Some zoom lenses have a variable maximum aperture, meaning that the maximum aperture you can set is different depending on which position the zoom is set at. For example, the RF 24-240mm F4-6.3 IS USM lens has a maximum aperture of f/4 at the 24mm end and f/6.3 at the 200mm end.

Prime lenses offer the widest apertures, making them a great choice for shooting in low light as well as creating a shallow depth of field. Because of the optical precision required, lenses with very wide apertures can be costly, but not always – affordable choices include the popular Canon RF 50mm F1.8 STM lens.

With zoom lenses, selecting a lens with a reasonably wide maximum aperture, such as f/2.8, will give you an excellent balance between the benefits of a wide aperture and the flexibility of different focal lengths.

All lenses are products of complex optical and mechanical engineering, but some components, materials and technologies are more expensive than others. The cost of a lens may be affected by numerous factors such as the type of glass used, special coatings or processes used in optical elements to reduce problems such as ghosting and flare, or the precision construction and sealing materials used to enhance dust- and moisture-resistance. Canon's L-series lenses, which have an L in the lens name after the aperture, are designed and built to meet the exacting performance and durability requirements of professional users.

Some lenses have built-in image stabilisation (IS) to counteract image blurring caused by camera shake. These lenses have IS in their name. They may or may not have a switch on the side to activate IS, and a separate switch to toggle between different IS modes for different shooting situations. IS will benefit you if, for example, you're shooting in low-light conditions and need to use a slow shutter speed – with IS, you can still get a sharp shot, even if you're shooting handheld. IS also helps if you don't have a tripod, it's not practical to use one, or you're somewhere tripods are not permitted. This could include all kinds of photography from motorsports to mountaineering. The specs of an IS-equipped lens (for example on its web page on the Canon site) will include how much benefit you can expect from the IS, although this will vary with focal length, shutter speed and other factors.

Want to photograph insects, flowers or other small subjects close-up? You can do this with different setups, but a macro lens – one with MACRO in its name – will help with a close minimum focusing distance and a magnification of 0.5x or more. Bear in mind that a macro lens can also be used for all kinds of photography – the RF 24mm F1.8 MACRO IS STM, for example, is a compact, lightweight wide-angle lens ideal for landscape photography and vlogging as well as macro photography.

The size and weight of a lens may be important to you, depending on factors such as how many other lenses you already have in your kitbag and what you feel comfortable carrying around. Even though EOS R System cameras can work with EF lenses (with a mount adapter, which adds a little to the weight), it's worth looking at RF lenses in preference – in general, RF lenses are smaller and lighter than their EF counterparts. They also provide numerous technical benefits such as much faster communication between the camera and lens.

Even among RF lenses, though, you may find lenses that seem very close in specs but differ in size. The RF 24mm F1.8 MACRO IS STM and the RF 28mm F2.8 STM, for example, have similar focal lengths and both make good "walk around" lenses for street, travel and everyday photography. However, the latter is what's known as a "pancake" lens, meaning it is very compact – just 24.7mm long and 120g in weight. The former is 63.1mm long and weighs 270g. If size and weight are your top priority, then the 28mm lens is a great option. However, the 24mm lens offers a wider maximum aperture and gives you the option of macro photography at 0.5x magnification. So, if greater flexibility is the priority, then the extra size and weight are the trade-off.

If you need help choosing a lens, Canon's handy lens finder tool will tailor recommendations based on your camera, the genre you want to shoot, and which features you value most. If you have a particular genre in mind, take a look at our guides to the best lenses for portrait photography, landscape photography, action photography and wildlife photography to help you decide.

Ready to take the next step and push your photography even further? Our advice will help you make smart lens choices and start shooting the kind of images you've always dreamed of creating.