Camera lenses are an expensive investment for a photographer. A quality lens will last for years, often outliving a camera body. For this reason, most photographers make their gear selection around lenses more than any other equipment. Sigma is a company specialising in high quality lenses.
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The lens mount is the metal ring at the base of the lens that connects to the camera. It contains the electronic contacts that relay signals between the lens and the camera body.
The iris opens and closes like a blinking eye whenever a picture is taken. Exposure settings in the camera dictate how large or small the opening, or aperture, will be when the shutter button is pushed and the iris opens.
The focusing ring is turned to reposition the glass inside a lens. By moving the glass forward or backward, the focal length is changed. The focusing ring is handled when shooting with manual focus, unlike autofocus which adjusts the glass elements automatically.
The aperture ring is moved to set the "f" number of the lens. This number represents the size of the aperture created by the opening of the iris. Measured in increments called "stops," a lower "f" number translates to a larger opening and more light during exposure.
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