7. Camera lenses and focal length

Lenses play an important part in photography — some may even argue that they’re equally, if not more important, than the choice of camera.

As we touched on in chapter one of this course, different lenses produce different results depending on the configurations within the lens barrel. These configurations determine the magnification and angle of view and allow us to create different effects, depending on the lens.

How camera lenses work

Lenses focus the light onto the recording medium, which allows an image to be recorded. As you’ll see from the illustration of a DSLR camera below, light enters the lens and passes through the front element (2) before reaching several lens elements (3, 5 & 6). These elements serve to direct the light to a single focal point so that it can accurately be recorded by the sensor (9) once the mirror (7) flips up (which is triggered by pressing the shutter button). Mirrorless cameras work the same way, but do not have the mirror or prism.

How camera lenses work infographic

Lens quality is determined by factors such as its maximum light gathering ability (f-stop number written on the lens), its resolving power (how sharp the images produced are), the colors it can focus (quality of the glass), the contrast it achieves (also the quality of the glass) and the type of material the lens is made from - all part of lens design.

What the numbers on a lens mean

Now that you understand the basics of how a lens works, you might be wondering what all the numbers (and letters) on a lens mean. Typically, lenses are written in a format like this: Canon EF 24-70mm f/2.8L II USM Lens or Nikon AF-S 50mm f/1.8G. Each of these numbers and letters tell you essential information such as focal length, maximum aperture, lens version and focusing motor. Other features specified can include stabilisation, filter diameter and focusing distances.

Image of lens annotations

When looking to buy a lens, it’s important to consider each of these points as these will, in part, tell you about the characteristics and capabilities of the lens.

Types of camera lenses

When it comes to lenses, there’s no shortage of options. Quite simply though, lenses can be categorised into two main groups — prime or zoom lenses, both of which come in different focal lengths.

Prime vs zoom lenses

Prime (or fixed) lenses feature a fixed focal length. Available in a range of focal lengths, prime lenses are known for delivering high quality images. They also generally offer much wider apertures compared to zoom lenses.

Zoom lenses, on the other hand, provide a variety of focal lengths in one lens. This makes them very versatile and also reduces the need for multiple lenses.

Focal length

The focal length is one of the defining characteristics of a lens as this determines the angle of view as well as magnification. How focal length is determined requires an understanding of the physics of light and concave or convex lenses, but very simply, when light enters a lens, glass elements within the lens serve to converge the light to a single point, known as ‘focus’. The distance between this point and the centre of the lens is the focal length.

lens focal length and field of view

The focal length is usually indicated both on the side of the lens barrel and sometimes on the front of the lens, along with the lens diameter. Ranging anywhere from 8mm to 2000mm, there is a wide variety of focal lengths for photographers to choose from, each of which are better suited to different genres of photography. Lenses with shorter focal lengths provide a much wider angle of view and are therefore better suited to landscapes than product photography as well as less magnification. The opposite is also true for longer focal lengths, which may be better suited to sport or wildlife photography than architectural photography. Lenses with shorter focal lengths also provide less magnification than those with larger focal lengths.

Camera lenses

In addition to influencing the angle of view and magnification, focal length can also impact camera shake. Longer focal lengths are more susceptible to camera shake due to greater magnification — the tiniest movements become magnified and can result in blurred photos. Generally, I recommend shooting with a shutter speed equal to or greater than the focal length as this can help prevent this from happening.

Aperture

Although not specifically a type of lens, aperture is another important feature of lenses.

As you’ll already know from the previous chapters, aperture refers to the opening in the lens that controls how much light reaches the sensor. This is indicated in the format 1:2.8, for example, with the second set of numbers indicating the maximum aperture. Some lenses will feature two maximum apertures (shown as 1:4-5.6). What this means is that as you zoom, the aperture capability changes. So at the shortest focal length, you may be able to shoot at f4, but once you zoom to the longest focal length the widest aperture you’ll be able to shoot at will be f5.6. Generally lenses with wider apertures are favourable due to their increased light capturing capabilities.

There is also a category of lenses that feature a fixed aperture. Catadioptric, or mirror lenses, which used to be fairly common, usually feature a longer focal length, such as 500mm at a fixed aperture of f6.3. Most telescopes are catadioptric.

aperture blades
An example of aperture blades that are partially closed to approximately f8. © Karl Taylor

Pinhole cameras

Throughout this course, we’ve referred to six essentials of photography. However, at this point, it is important to mention that this could in fact be five. This is because images can be recorded without a lens. These cameras are known as pinhole cameras (or camera obscuras). However, recording an image without a lens is far more complex and doesn’t afford the photographer the same creative freedom as when using lenses. Essentially, without a lens, you're shooting at a very small fixed aperture. Lenses, due to the variations in magnification, allow us to shoot many different objects, which simply wouldn’t be possible with a camera obscura. For more information about this, refer to the PDF guide.

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