In the context of camera lenses, “mm” stands for millimeters, which is a unit of measurement used to describe the focal length of a lens. The focal length is a critical specification that indicates how strongly the lens converges (or diverges) light. It effectively determines the lens’s field of view, perspective, and the magnification of the image captured.
Here are some key points about focal length (measured in mm):
Wide-Angle Lenses: Lenses with shorter focal lengths (typically between 10mm and 35mm) have a wider field of view, allowing you to capture more of the scene in the frame. They are great for landscapes, architecture, and interior photography.
Standard Lenses: Lenses with a focal length around 50mm are often referred to as standard or normal lenses. They provide a field of view that closely resembles human eyesight, making them versatile for various types of photography.
Telephoto Lenses: Lenses with longer focal lengths (generally above 70mm) allow you to zoom in on distant subjects, providing a narrower field of view. They are commonly used for wildlife, sports, and portrait photography, as they can compress the background and create a pleasing bokeh effect.
Zoom Lenses: Zoom lenses have variable focal lengths (e.g., 24-70mm, 70-200mm), allowing photographers to change the field of view without changing lenses.
Depth of Field: The focal length also influences the depth of field in an image. Longer focal lengths can produce a shallower depth of field, which helps isolate subjects from their backgrounds.
Crop Factor: When using lenses on cameras with different sensor sizes (like APS-C or Micro Four Thirds), the effective focal length changes due to the crop factor. For example, a 50mm lens on an APS-C camera with a crop factor of 1.5 will provide an effective focal length of about 75mm.
Understanding the focal length and its implications helps photographers choose the right lens for their specific needs and creative intentions.