Last Updated on 03/03/2018 by Mark Beckenbach
If you’re into photography, you most likely know the answer to the question “What do the letters ‘mm’ on a lens stand for?” But for the uninitiated and for lots of Americans who don’t know the metric system or what millimeters are, it can be pretty confusing. I mean, we can understand that eight inches is larger than two inches, right? So bigger, must mean better, right? No, not exactly and that couldn’t be further from the truth.
Essentially, the wider the field of view, the smaller in focal length number. The more narrow (telephoto) views are larger in focal length numbers. So, a 14mm lens will give you a wider field of view than an 85mm lens. But then things become even more confusing as pretty much every standard of focal lengths are then more or less based off of 35mm film.
You see, there are things called crop factors–and they are created in regards to how much smaller or larger a sensor or film plane is to 35mm film. So on a Four Thirds sized sensor, there is a 2x crop factor. So a 12mm lens will give you the equivalent field of view of a 24mm lens on a full frame 35mm camera. But nonetheless, the standard stays the same. The smaller the number of millimeters, the wider the field of view and focal length. The larger the number, the more narrow it is.
But then consider the geekier side of it. what does the millimeter actually stand for. To put it simply, it stands for the distance between the back element of the lens and the camera’s sensor or film plane. With a zoom lens, you can expect that to change accordingly. With an option like a 24-105mm lens, the back element of the lens starts out at 24mm and as you zoom further into the subject (increased the millimeter numbers) you move the back element further away from the sensor and film plane. Part of this is why we call them “longer focal lengths” or “shorter focal lengths” because longer lenses need to be made, well, longer in order to accommodate the design.
Of course, it can get more complicated than this, but why worry?