The Magic Numbers of Photography - How Lenses Work

While you probably know that you should put your money into your glass and you probably know that a longer focal length translates to a more telephoto/zoomy lens you may not know how the numbers written on the outside of your lens relate to the underlying physics or even how the lens ensure that images end up on your sensor/film rather than somewhere else in space. These things are worth knowing as they feed into deeper concepts further down the line. Luckily, they're actually very easy to understand.

A Basic Photography Lens

A basic photography lens has 2 key properties:
- It is a converging lens
- It has a focal length
Both of these properties are shown in the diagrams that are commonly used to represent lenses

Photography lenses are converging lenses

Photography lenses are converging lenses. This means that light entering the lens bends (or converges) toward the middle of the lens. This is an important property of lenses as it is this convergence of light that allows images to be captured on the film or sensor that sits behind the lens. This convergence also contributes to the circle of confusion associated with light coming from any single source.

Photography Lenses have a focal length

The focal length is the distance between the lens and the point at which light entering the lens converges. In the case of cameras this doubles as the distance between the lens and the film or sensor as we want the light to converge on the film or sensor in order to be able to take well focussed images.

So far so good. But how do all the numbers and moving parts fit in?

The Role of the Focus Ring

The Focus Ring allows the photographer to adjust the point at which the light rays converge after passing through the lens.

Zoom lenses

Zoom lenses have a variable focal length. While this could be achieved by moving the lens further from the film or image sensor it is usually achieved (at least in part) through the inclusion of additional lens elements in the zoom itself. These additional elements bend light inwards (converging) or outwards (diverging) as required.

The 1:X number - The Aperture Number(s)

This number (or these numbers) show the F number associated with the largest aperture(s) for a given lens. In the case of prime lenses or zoom lenses with a constant maximum aperture only 1 value will be shown. In the case of zoom lenses that do not have a constant maximum aperture the range of aperture values will be shown. Aperture values themselves are a ratio of the diameter of the opening compared to the focal length of the lens (focal length/Aperture diameter). The 1 in the 1:X represents the focal length of your lens while the X represents relative size of the aperture. In a 1:1 lens the aperture's maximum diameter is equal to the lens's focal length. As the X gets larger the diameter gets smaller.

Zoom Lenses and Variable Apertures - Why?

Remember that the F number is the ratio of focal length to the diameter of the aperture. Unless elements inside the lens are compensating for the variation in the focal length the F number will have to increase as the focal length increases.