Focal Length and Angle of View

The last time we had a look at the picture to the left, we learned how to read the aperture specification from how it is mentioned on the lens. As you would remember, there is also another set of numbers on the lens. This represents the focal length of the lens. In this case the figures say 5.8 - 17.4, the readings being in mm.

The layman can translate this directly to the amount of zoom that the camera provides in terms of magnification(2x, 3x etc). The figures indicate that the cameras can zoom from a minimum focal length of 5.8mm to a maximum focal length of 17.4mm. So the magnification that the lens provides is 17.4/5.8 = 3x. This again reflects what is mentioned in the specification of the camera.

Now that we understand the significance of the figures mentioned, we can have a look at what the numbers exactly mean. For this we have to understand what is focal length. Without being too bookish, the focal length is the distance from the lens to the focus of the lens. The focus of the lens is the single point where the light converges after refracting from the surface of the lens. In the case of a fixed lens, this focus point is constant and is determined during the manufacture of the lens and depends on the curvature of the lens. You can read more about focal length and focus from these links - Focal Length and Focus.

Now lets move away from the geometrics and optics and focus on what it means for us as a photographer. As mentioned earlier, each lens has a fixed focal length and this cannot be changed. But cameras have a range of focal lengths as mentioned in the specification. This is achieved by using a combination of lenses. By using multiple lenses, in a straight line, and varying the distance between each of them, one can achieve different focal lengths.

Now most of the focal lengths are compared using the 35mm equivalents. The 35mm film was the most popular film format available and hence all focal lengths are translated into their respective 35mm equivalents. This is because digital cameras have varying sizes of sensors and there has to be a standard for comparisons. In case of compact cameras, the 35mm equivalent focal lengths are mentioned in the cameras specifications and in the case of SLR cameras a focal length multiplier is mentioned. In the case of the W80, the 35mm equivalent focal lengths for 5.8 - 17.4mm are specified as 35mm - 105mm. What this means in other words is that the 5.8mm lens on the W80 provides the zoom equivalent to a 35mm lens on a film camera.

We will take a more detailed look at what this 35mm equivalent means in a later post.