One of the most common mistakes people make when trying to determine what size lens cap or filter they will need for their lens is mistaking the lenses focal length for its filter size. For example, both Canon and Nikon make an 18-55mm lens and many people assume that they will need a 55mm filter for these lenses. In fact neither of these lenses take a 55mm filter, the Canon lens takes a 58mm filter and the Nikon lens takes a 52mm lens. Why?

Plumtree Photo
EST. 2006



As you zoom the camera in and out the focal length becomes longer or shorter. The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position. Wide angle lenses have small focal lengths, while telephoto lenses have larger corresponding focal lengths. Larger focal lengths focus on a smaller portion of an object or scene and therefore make it appear to be closer.
The size of lens cap that you will need is actually determined by the lens diameter. If your lens uses screw on filters, the diameter, also known as the filter thread, determines the size of filter you will need. The diameter of the lens is the length of the straight line through the center of the lens touching two points on its edge.
Because the 18-55mm refers to the focal length of the lens. The focal length is the distance from the optical center of the lens to the focal point of the camera, the sensor in a digital camera or the film in a film camera.
Both the focal length and the diameter of the lens are usually printed somewhere on the lens, often on the face of the lens but sometimes on the outer edge. The focal length is listed in millimeters (mm) and the diameter size is usually preceded or followed by a symbol that looks like a circle with a line through it, ø.

If you still are having trouble please click here to view the lens charts that we have available to help you determine the lens cap or filter size you will need.