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What do all of those numbers mean on my lens?

Perhaps one of the most mis-understood concepts in photography are how lenses are classified. The numbers you find on your lens - whether it be a compact camera or DSLR - may seem very crypic to the novice, but they do mean something.

In reality, the numbers found on the typical lens denote the model number (for interchangeable lens cameras), and the lens parameters (all cameras). There are two classifications of numbers you may encounter; Common (applying to all lenses), and Proprietary (applying to one brand of camera).

In the photo below, we can see the lens label has a lot of perhaps meaningless numbers. However, the actual model number published in the Nikon lens catalog has this model number:

Typical DSLR lens label.

In this photo, we see a typical compact camera lens (permanently attached). Most compact cameras have their lens name printed on the front of the lens - although some are on the side.

Typical Compact Camera lens label.

Deciphering the labels.

Focal Length: The most basic of all descriptions is the lens focal length. This more or less describes the "view" you should expect from a lens. For a better idea of what focal lengths look like, I refer you to this web page.

The definition of focal length is the physical distance, in millimeters, from the sensor to the optical center of the lens (not the physical end of the lens).

This reference provides a perspective on the various focal lengths and how they compare to each other:

Zoom Lenses: The definition of a zoom lens is a lens that has multiple focal lengths, or more accurately a range of focal lengths. When the lens is "zoomed out" it is said to be at the "wide angle" end of the lens. And most compact cameras will have a "W" in the LCD screen or on the zoom control to denote this. A wide angle lens opens up the scene so that you have a wide view, and is typically used for landscapes.

At the opposite end, when the lens is "zoomed in", it is said to be the "telephoto" end of the lens. A telephoto lens brings distant objects in close, similar to a telescope. And in a similar fashion, most compact cameras will have t "T" on the control for zooming in.

Zoom lenses are always designated with two numbers in the focal length position of their description, separated by a hyphen. The left-most number is also always a smaller number, and designates the minimum focal length at the wide angle end. The right-most number designates the maximum focal length of the telephoton end of the lens. Therefore, a lens labelled "18-300mm" means that the lens has a focal length of 18mm at the wide angle end, and a focal length of 300mm at the telephoto end. And as zooms are, if you position the lens somewhere in-between the two ends, the focal length will also be somewhere in-between.

Prime Lenses: Not all lenses are zoom lenses, but some only have a single focal length. They cannot be changed to zoom in or out. While the reason for this may not be immediately apparent, realize that any lens starts to have optical issues as the zoom range is increased. This is true for a $100 camera or a $10,000 DSLR lens. Simply put, prime lenses have a place in the photography world when the very best lens is needed.

As a prime lens only has a single focal length, they only have a single number. A lens marked "50mm" without a dash or a second number is a prime lens, with a 50mm focal length.

Zoom Power: Found chiefly on compact cameras, many zoom lenses have a zoom power as part of the description or model number. The zoom power is simply the maximum focal length divided by the minimum focal length. So, a 18-300mm lens would have a zoom power of 16.6x. However, realize that as a ratio, zoom power is an arbitrary figure, and provides no indication of the view you will obtain with your camera.

For example, a 50-200mm lens is a 4x zoom (200mm / 50mm). But a 10-40mm lens is also a 4x lens (40mm / 10mm). However, these two lenses provide completely different views. For this reason, zoom power is rarely listed for DSLR lenses as people that own them are generally more comfortable with focal length numbers.

And to enforce your understanding of prime lenses, a prime lens would have a "zoom power" of 1x, meaning the minimum and maximum focal length are the same... that is, it doesn't zoom.