Some zoom lenses have an aperture ring and distance scale.
Camera lenses are complicated mechanisms involving precision in design. Most photographers understand them in terms of what they do to a photograph's magnification and focus. Understanding some basic terminology about them can help you when choosing a lens for a particular situation.
Lens Elements
A camera lens is actually composed of multiple pieces of glass. These pieces are referred to as the lens elements, consisting of a lens with one curved and one flat side or two curved sides. Each lens refracts the light in a particular way to bring the image into focus. A problem called chromatic aberration can occur with poorly-made lens elements; this is the inability of the lens to focus red, green, and blue color wavelengths on the same plane. The result is a halo of a particular color around the edges of the subject.
Focal Length
Focal length measures the distance between the film plane and the lens' rear nodal point when the lens is focused on infinity. For a 35 mm film camera, the focal length of what is called a normal lens that approximates what the human eye sees is 50 mm. A lens greater than 60 mm magnifies the image and is called a long lens. Lenses that measure less than 40 mm create a wider angle of view and are called short or wide angle lenses. Extremely short lenses of 20 mm or shorter are called "fisheye" lenses for their incredibly wide angle of view.
Prime vs. Zoom
The term "prime lens" is commonly used to describe what's also referred to as a fixed lens. This means that it's only designed to use one focal length. This is opposed to a zoom lens, which can jump between focal lengths. A common zoom lens may measure 28 mm to 80 mm, giving you short, normal, and long lens options. Prime lenses are more likely to be faster lenses. A fast lens is one that is capable of a wide aperture opening -- aperture being the diaphragm that allows light into the camera in various diameter sizes. The widest aperture thus allows the quickest shutter speeds.
Lens Rings
The focus ring is located at the very front of the lens, furthest away from the body. Some lenses have a microprism or split-focusing aid that helps you bring the subject into focus. A zoom ring, if present, is usually wider and closer to the camera body. Where the lens contacts the body on very basic camera models is the aperture ring. This allows you to change apertures that aid in creating a proper exposure along with the shutter speed setting.
Depth of Field Tools
Some lenses have distance and depth of field scales, printed just beyond the aperture ring. These allow you to determine exactly what parts of your photo will be in focus given your chosen aperture. You can focus on what you want in the image, and then choose an aperture of f/4, for example. When looking to the depth of field scale, you can find the two markings of "4." Looking beyond the two 4's to the distance scale, you can see a measurement in feet of what will be in focus, for example, between three and eight feet. Everything closer or beyond those measurements will be out of focus.
Tags: aperture ring, lenses have, will focus, angle view, depth field, distance scale, into focus