Focal Length Explained - Introduction

It might sound a bit complicated to start with, but there really isn’t anything to be scared of when it comes to defining lens focal lengths and their relationship with your photographs

Focal Length Explained

Have you ever wondered how a camera can take a picture of a tree in the distance and have the subject fill the whole of the frame despite how far away it is?

Or how a picture can be taken close to a subject yet include a whole host of things either side of it? Stay with us and we'll tell you all you need to know.

In the world of photography the focal length describes the distance in millimetres between the lens and the image it forms on the sensor (or film) when it is sharply focused at infinity - the farthest possible distance. This distance, simplistically, determines the angle of view - how much the lens sees - which controls what portion of the scene will be captured. And this portion in turn is dependent on the size of the sensor (or film). The lens's focal length determines the magnification, that is, the size of the image that the lens forms. And to achieve the pictures you want you will have to employ lenses with different focal lengths - but more on that later.

A note on Sensor Size

When digital photography hit the scene, focal length on lenses played a major part in the final outcome of the images. The reason? Sensor size. Rather than having the historical 24x36mm recording area of 35mm film, the image sensors on most cameras were, and still are, smaller. The outcome of this is that the equivalent focal length of digital camera lenses is numerically longer (more telephoto) than their film equivalents. For example, a 50mm lens, having an angle of view of about 47į, is considered to be a normal lens for a 35mm film camera, ie. it produces an image that through the human eye would be recognised as normal, not distorted in any way. So for a digital camera with a sensor that is smaller than the customary 24x36mm recording area, the lens will in effect become more telephoto, creating a smaller angle of view. As digital photography has progressed, so more companies have made DSLRs with full frames whose image sensors are equivalent to the 35mm film size, making their angle of view the same as† the 35mm film cameras.