focal point

A camera lens’ focal point is the point at which the light reflected from an object at a distance converges into one point when passed through the lens elements. If the light captured converges before or after the camera sensor or film, it will appear blurry, and the range within objects appear in focus is called the depth of field. Focus can be adjusted, usually by moving the lens closer to and further away from the camera sensor or film, and doing so allows for different distances in the captured image to appear in focus.

The focal point/range of a lens can be moved closer to the front of the lens system with extension tubes, which intentionally puts the lens’ convergence point in front of what the camera is designed for. This allows for any lens to be used for macrophotography. Similarly, teleconverters are lenses that attach to other lenses, using their own additional elements to enlarge the resulting image, which also decreases lens speed due to the loss of light around the magnified frame.

The magnification effect teleconverters introduce is actually the same concept behind crop sensors: the light around the point of magnification is too far away from the sensor to hit it, which makes the resulting image appear “zoomed in” or magnified by the same amount as the camera’s crop factor.