7. Camera lenses and focal length
Lenses play an important part in photography. In fact, your choice of lens is arguably even more important than your choice of camera.
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.
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 (triggered when you press the shutter button). Mirrorless cameras work the same way, but do not have the mirror or prism.
The quality of a lens depends on various factors. One is its maximum light gathering ability (f-stop number written on the lens). Another is its resolving power (how sharp the images produced are). Yet another is the colors it can focus (quality of the glass). Also important are the contrast it achieves (also the quality of the glass) and the material the lens is made from.
What the numbers on a lens mean
Now you understand how a lens works, you might be wondering what all the numbers (and letters) on a lens mean.
Typically, lens names are formatted 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 relays essential information, such as focal length, maximum aperture, lens version and focusing motor. Other features specified can include stabilisation, filter diameter and focusing distances.
When you’re looking to buy a lens, it’s important to consider each of these points. Together, they will help you understand the characteristics and capabilities of the lens.
Types of camera lenses
Lenses can be categorised into two main groups: prime or zoom lenses. The main difference between the two relates to focal length.
Prime vs zoom lenses
Prime (or fixed) lenses feature a fixed focal length. Available in a range of focal lengths, these lenses are known for delivering high-quality images. They also generally offer much wider apertures than zoom lenses do.
Zoom lenses provide a variety of focal lengths. This makes them very versatile and reduces the need for multiple lenses.
Focal length
Focal length determines angle of view and magnification. 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.
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.
Focal lengths range from 8mm to 2000mm. Different lengths are better suited to different genres of photography.
Lenses with shorter focal lengths provide a much wider angle of view and less magnification. This makes them better suited to landscapes than to product photography.
Longer focal lengths offer a narrower angle of view and more magnification, which means they are better suited to sport or wildlife photography than architectural photography.
Focal length can also impact camera shake. Because of the greater magnification they offer, longer focal lengths are more susceptible to camera shake. This means that the tiniest movements become magnified and can result in blurred photos. Shooting with a shutter speed equal to or greater than the focal length can help prevent camera shakes ruining your images.
Aperture
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). This means 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.
Some lenses 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.
An example of aperture blades that are partially closed to approximately f8. © Karl Taylor
Pinhole cameras
Throughout this course, we’re referring to six essentials of photography. However, it is important to mention that this could in fact be five! Why? Because images can be recorded without a lens on so-called 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. Essentially, without a lens, you're shooting at a very small fixed aperture. Thanks to the variations in magnification they offer, lenses allow us to shoot many different objects, which simply wouldn’t be possible with a camera obscura.
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