The Lens Bible

Essential Lens Knowledge

When you own an interchangeable lens camera you soon discover there's a lens for every scene and situation you find yourself in - the ability to switch between them is the true joy of a system camera. However due to having so many options available choosing which lens to buy can be a bewildering task, which is why we've designed this guide to help you choose the correct one to fulfil your photographic needs.

Focal Length

A top consideration when choosing a new lens is its focal length as this dictates how much of a scene you can capture in your photos. The first number you'll see used to describe a lens is it's focal length (50mm). The focal length, in addition to your camera's sensor size, determines the angle of view covered by the lens. A single number on the lens means it's a prime lens: fixed and operates only at that one focal length. Those with two numbers (24-70mm) are zoom lenses and their numbers refer to the extremes of the range they cover.

Wide Angle, Standard or Telephoto?

Wide Angle lenses are excellent for landscapes. With focal lengths of less than 50mm, they have a wide field of view. Standard lenses are used for portraiture, photojournalism and street photography as they give a similar perspective to the human eye.

When you get to lenses with focal lengths greater than 50mm, these are called telephotos and are used from general everyday use to wildlife and sports photography depending on their focal length range.


A lens aperture is the measure of how much light it can gather. There are several benefits to be had from choosing lenses with large apertures (small numbers) including the ability to shoot indoors and in low light without using flash. A decreased depth of field allows you to take shots where your subject is in focus and other parts of the frame are blurred.

Image Stabilisation

Image stabilisation prevents blurry images due to hand shake and becomes particularly more important with longer focal lengths.


Another factor in your lens choice will naturally be the camera that you own. Digital SLRs and mirrorless system cameras either have APS-C or full frame sensors. APS-C sensors are found in most affordable models - these have a cropping factor, which reduces the field of view you would achieve with a 35mm camera - in effect, the higher the crop factor, the less you get into the frame when using the same lens focal length.

Professional level cameras are equipped with full frame sensors. These are the same size as a traditional 35mm film frame, so there's no cropping or reduction in field of view.

When buying lenses you need to be aware that lenses designed for full frame cameras will work perfectly on APS-C cameras, but those designed for APS-C won't work properly on full frame models.

Lens Mount

All of the manufacturers have their own lens mount, which means you can't pick and mix as they won't fit unless you buy from a third party manufacturer who supply a variety of sizes. The exception is with Olympus and Panasonic who use the Four Thirds mount for DSLRs and the Micro Four thirds mouth for their mirrorless.

Your choice of lens has an equal bearing on the image quality you'll achieve as your camera body and know that lenses tend to last a lot longer than cameras therefore it really is worth buying the best you can afford.

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