The focal length is an optical measurement that indicates how strongly the light converges or focuses. When parallel rays of light strike a lens focused at infinity, they converge to a point called the focal point. The focal length of the lens is the distance from the middle of the lens to that focal point. A simple way to think about it is that the focal length equals image distance for a far subject (infinity). To focus on anything closer than infinity the lens must be moved farther from the sensor. That is why the lens becomes longer when you turn the focusing ring.

Simply put, the focal length is how much the camera can see. A focal length of around 50mm mimics what the human eye sees and gives the most “natural” view. A focal length of 35mm or less is considered wide angle which allows more of the subject to be seen. A focal length of 70mm or greater is considered telephoto with less of the subject showing. The table below put it in perspective. 

One other term that is common in photography and in lens descriptions is the f-number. Because of the letter “f,” you might think that this would refer to the focal length, but instead the f-number refers to the aperture of the lens as well as the focal length. The f-number is the ratio between the diameter of the aperture and the focal length of the lens. The f-number is usually written as something like f/8 which means the aperture diameter is 1/8th the focal length. It is often also written with a colon. So f/8 would be the same thing as 1:8.

Aperture is another common photographic nomenclature. The aperture of a lens is the amount of light that the lens allows to reach the sensor. In SLR lens, the aperture is usually a range that indicates the smallest amount of light to the largest amount of light that the lens can open and/or close to accommodate.

Contrary to what may seem logical to the amateur, larger apertures have lower f-numbers and smaller apertures have higher ones. Lenses with larger apertures are often referred to as faster because the shutter speed can be made faster for the same exposure. Smaller apertures mean that the objects can be focused over a wider range of distance which is also referred to as the depth of field.

Here’s what you need to remember. Higher f-numbers have smaller aperture size, require slower shutter speeds and accommodate a wider depth of field. Lower f-numbers have larger apertures, require faster shutter speeds and have a narrower depth of field.

The numbers you see on the side of a lens usually look similar to the Olympus lens shown in Figure 1. 14-42mm is the focal length. 1:3.5-5.8 is the aperture.

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Figure 1: Markings on a Lens.