24-70mm f/2.8

50mm f/1.4

300mm f/2.8

Those are just a few examples of some common DSLR/mirrorless interchangeable lens cameras (MILC) lenses that you might see on store shelves or on Amazon. But what does it all mean? In this article I will explain the meaning behind the focal length, which is usually measured in terms of millimeters (mm) for DSLR/MILC cameras today (I’ll be posting another article on the aperture, or f-number, soon).

The focal length of a lens is a physical measurement of how fast parallel light entering from one end, converges to a single point on the other. Note that this definition does not generally apply to all lenses or optical systems, but it does apply for consumer camera lenses. The idea is that, given a theoretically infinitely distant subject (and therefore a subject who’s light rays are entering the lens in parallel), a lens may contain optics that may strongly or weakly converge the light to a single focus point. Weaker optics with a long focal length generally provide higher magnification, while stronger optics with a short focal length are the opposite. This is the reason why a long 200mm lens seems “zoomed in” compared to a short 25mm lens, which seems very “zoomed out”.

An interesting side note, our eyes naturally see the world in what looks like a 40-45mm lens (in 35mm equivalent terms). This may be a reason why 35mm and 50mm lenses are so popular in photography.

It is important to understand that focal length is a physical quality of a lens (system), meaning that for a given lens the focal length is the same whether the lens is mounted on a crop sensor camera (like Nikon DX, Canon EF-S, or MFT) or a full frame camera. Often manufacturers will tell you the ‘35mm equivalent’ focal length, but this is only so you can compare what the field of view is to the equivalent lens on a full frame camera.

For example, an 18mm lens on a Nikon FX (full frame) camera is, of course, an 18mm lens. Now take that lens and mount it on a Nikon DX (crop sensor) camera. It is still 18mm. However, when you look through the viewfinder you will notice that the field of view (how “zoomed” the picture is) looks quite similar as if you had mounted a 27mm lens on a full frame camera. This means the full frame equivalent focual length of an 18mm lens mounted on a Nikon DX crop sensor camera is 27mm.

Notice in our example that the full frame equivalent is 1.5x the focal length of the lens for Nikon DX. This is called the crop factor. The crop factor is directly related to the size of the sensor, and for Nikon DX and Canon EF-S, the crop fractor is 1.5x. For MFT, the crop factor is 2x.