Understanding the Millimeters

Categories: Photography
Comments: 2 Comments
Published on: 2011.02.23

New photographers are often lambasted with technical information that’s difficult to understand. There are dozens of numbers that apply to each camera and lens. Several of those numbers have “mm” after them. Hopefully, I can sort this out a little for you.

If you’ve been to school, you know that “mm” is the abbreviation for “millimeters”, a metric measurement that’s 1/1000 of a meter. But that doesn’t help you if you’re not familiar with photography.

Types of photography specs that are measured in millimeters:
1) Focal length
2) Filter thread size (diameter)
3) Sensor / film size
4) Minimum focus distance
5) Extension Tube length

Focal Length

In my experience, focal length is the measurement most-often mentioned. Your camera may have come with a “kit lens” in the package, and the kit lens’ name had something like “18-55mm” as part of a long string of letters and numbers. For example, the most common Canon kit lens is the Canon EF-S 18-55mm f/3.5-5.6.

There’s a relatively complex explanation of focal length on Wikipedia (here), but put more simply: “The focal length of a lens is … the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject … is in focus” (source).

Even more simply, for the beginner: a lens with a larger number (example: 200mm) is a “long” or telephoto lens, while a smaller number (example: 20mm) is a wide-angle lens.

A telephoto lens will have a narrow field of view, and therefore increased magnification. A wide-angle lens has a wide field of view. In between are “normal” lenses, which attempt to approximate the field of view of the human eye.

If your lens has two numbers, like the 18-55mm example given above, that means it’s a zoom lens. Its focal length is variable. You can take pictures at 18mm or 55mm, or any point in between. If your lens has only one number in the name (example: “Canon EF 50mm f/1.8 II”) then it’s a “prime” lens and cannot change focal length.

Most lenses will have these numbers printed on the side of the lens, usually near the camera mount.

Note: Your camera does not have a focal length. This is purely a lens measurement.

Filter Thread Size

Almost all lenses have a thin metal or plastic ring around the front, to which you can attach filters (polarizing filters, neutral density filters, UV filters, or various color filters, all purchased separately). The thread depth is pretty much universal, but the diameter of the ring will depend on your lens.

Again, using the “kit lens” example, the spec sheet shows its filter size is “58mm”. This is unrelated to the focal length, and only tells you what size filter to buy. You won’t need to know this number unless you’re about to buy a filter for your lens (which isn’t really necessary for beginning photographers), or when purchasing a replacement lens cap.

If you are about to purchase a filter or lens cap, you absolutely need to know this number, or your filter/cap won’t attach to the front of your lens.

There are also “step-up” and “step-down” filter rings, which are very inexpensive, and allow you to use a filter that isn’t the right size for your lens. For example, if you have the kit lens mentioned above, with a filter thread diameter of 58mm, and someone gave you a filter that’s 62mm in diameter, then you would need a step-up ring that matches both measurements. One side of the ring will be 58mm and screw onto your lens and the other side is 62mm in diameter, to which you can attach the larger filter.

Most lenses have this measurement printed on the front of the lens, right next to the filter thread.

Sensor / Film Size

Sensor size is less-often mentioned, but you will eventually see it in print or hear about it when talking to other photographers. The sensor is the light-sensitive device inside your camera that is used in place of film, and sends information to your memory card. Today’s digital cameras have varying sizes of sensors, some referred to as “crop sensors” and others referred to as “full frame sensors.” Neither is a very accurate description of their sizes, but both terms are very common.

A full frame sensor is one that’s approximately equal in size to the old 35mm film. For example, Canon’s full-frame sensors measure 36mm by 24mm, while Nikon’s are 36mm by 23.9mm.

Crop sensors come in varying sizes, including 22.3mm by 14.9mm (Canon Rebel T3i) and 23.6mm by 15.8mm (Nikon D90). Today’s compact cameras often have even smaller sensors.

For practical purposes, you don’t need to know the physical measurements of your sensor, though you’ll want to know whether you’re buying a crop sensor or full frame camera. When it comes to your sensor, what’s usually more important are the pixel dimensions of the image it can produce and the quality of the processor behind the sensor, both of which will drastically affect image quality.

Minimum Focus Distance

The minimum focus distance of your lens is usually measured in meters, feet, or inches, though occasionally you may see it measured in millimeters, especially for macro lenses. This distance signifies how close you can get to an object and still achieve focus.

Extension Tube Length

This will be the rarest number of all the ones I’ve listed, and you may never encounter it unless you’re in the market for extension tubes. But I’ll list it here since it is yet another photography-related spec that’s measured in millimeters.

Extension tubes are hollow rings that can be attached between your camera and lens, reducing the minimum focus distance of the lens. They’re used with non-macro lenses to help achieve macro-like magnification. In this case, the measurement is a simple distance measurement, noting the length of the tube and how far it will move your lens from its normal mount on the camera.

My set of Kenko extension tubes, for example, includes three tubes, measuring 12mm, 20mm, and 36mm, respectively.


There’s no need to be confused by all these measurements. In normal photography talk, people are referring to focal length when they speak of millimeters. It won’t be long before you’ll instantly recognize the numbers, so when someone says they have an 85mm lens, you’ll already have an idea of what that lens can do.

Filter size is only important when buying filters or lens caps.

Sensor size isn’t as important as some make it out to be, though generally speaking you’ll get better image quality from a full-frame sensor, pretty good quality from a crop sensor, and less quality from the tiny sensors in compact cameras. There are, of course, exceptions to this.

Minimum focus distance is only important if you plan to shoot many closeups or are interested in macro photography.

Extension tube measurements are for specialized applications and you needn’t worry about it.

Questions and comments are welcomed below (especially if I’ve forgotten something).

  1. Tod Noens says:

    Work is a [expletive deleted] scam. What kind of world is this that some people spend 60 hours a week pulling fries out of hot grease? What does a $300,000 car say to the cosmos? Please, [expletive deleted] wipe us out with a giant asteroid already?, is what.

  2. Wil C. Fry says:


    I’ll give you the benefit of the doubt and assume you were posting on this blog entry by accident.

    However, this blog entry is about photography. Try to keep comments on topic. For future reference, check my Discussion Guidelines before commenting here again.

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