What Do the Numbers on Camera Lenses Mean

Michele Wanke
Do the best camera lenses have lots of numbers?

Don't be afraid to ask: "What do the numbers on camera lenses mean?" Many amateur photographers get confused when looking at the tiny digits and letters that encircle camera lenses.

Getting Started

If you are branching out in the world of digital photography, you might be considering buying a lens kits for your DSLR camera. If so, you will need to know what the numbers, letters, and symbols that dot the lenses mean. Understanding these markings will ensure you purchase the lens that best suits your photo needs.

Fortunately, deciphering the numbers on a camera lens is not that difficult. The numbers provide information about your camera's functions and inform you about the lens itself. Understanding how to manipulate the lens and the numbers can make or break a great photo opportunity.

What Do the Numbers on Camera Lenses Mean? The Answer

The majority of DSLR camera lenses will have at least a few numbers and letters printed on them. Some of these numbers denote the basic technical specification of the lens, while others indicate the features and capabilities that the lens is equipped with. Generally, the numbers indicate terms that are common for all lenses, but some may be manufacturer specific. The following information will help you answer the question: "What do the numbers on camera lenses mean?"

MM Number

The "mm" letters on a camera lens refer to the focal length in millimeters. Focal length is the length or distance from the front of the lens to the sensor inside of the camera. The focal length is one of the most important considerations you need to take into account when buying a new lens. Larger mm numbers, such as 200mm or 300mm provide a magnified telephoto view or a telephoto shot. The larger numbers are ideal to use if you are taking a lot of pictures from a long distance away. For example, if you want to take a photo of a bird in a tree, you would want to take the shot with a focal length of 300mm or greater depending on where you are situated in relation to the bird.

Conversely, the smaller the mm number the wider the angle of view within a photograph. Smaller mm numbers are better employed for wide shots. In addition, if a lens has two mm numbers on it, for example 17-85mm, this indicates that you can take a shot with a 17mm focal length or zoom in all the way to a 85mm length. With zoom lenses, you might also see a series of numbers next to a slider or a rotating grip, with an arrow or line pointing to one of the numbers. This indicates the current focal length of the lens.

Finally, be aware of the null symbol on a camera lens. It looks like this: Ø. It is a symbol for a screw of that mm number, which fits the lens should you need to replace it. For example, if you notice a Ø58mm written on your lens, it means that a screw-on filter that is 58mm in diameter will fit that particular lens.

USM Number

USM stands for Ultrasonic Motor, which is typically built for faster and silent focusing. If your lens features USM and a number, it means you don't need to focus manually. Rather, the camera will focus automatically when you press the shutter button. The number that precedes "USM" simply indicates that the lens is one in a series. For example, 2USM means that the lens is the second version in a particular series. Some camera manufacturers use Roman numerals to indicate a series number, so instead of 2USM you would see IIUSM.

Ratio Numbers

This is a set of numbers shown as ratios, which indicate the widest possible aperture for a given lens. A camera's aperture is the opening where light comes in. This opening can have a range of sizes for a particular lens. The range is expressed as a ratio. The ratio determines how your camera is able to handle light, since the aperture size affects how much light enters the camera sensor. In the case of a zoom lens, you will typically see one or two values, such as 1:2.8, 1:4-5.6. In the first example, the f/2.8 refers to the widest aperture available for that lens. In the second example, the value varies between f/4 and f/5.6 depending on the focal length.

F Number

When you are shopping around for a DSLR camera lens, you will notice that it will feature an "f" number. The f number indicates the aperture value. For example, on a Canon EF 75-300mm f/4-5.6, the f number (f/4-5.6) reveals whether the lens is considered fast or slow. A slow lens has a maximum aperture value (f number) of 3.5 to 5.6 or higher. The higher the f-number, the slower the lens. Faster lenses allow more light to enter at faster shutter speeds. Consequently, you will be able to take sharper photos in low light without the need for a tripod or flash.

Faster lenses also let you take pictures in low-light situations without increasing the ISO. Generally, the lower the ISO setting, the better the photo quality. In addition, with a fast lens (one that features a lower f number) you can shoot subjects at closer range while blurring the background. If you were shooting the same image with a slower lens, you would need to be farther away in order to blur the background.

The downside of faster lenses is that they are more expensive, larger and heavier than slower lenses. An f/2.8 lens will cost more than a 4.0 or 5.6 lens. Plus, it will take up more space and be harder to carry around.

What Do the Numbers on Camera Lenses Mean