Let’s start by imagining your lens is a projector – in fact it is exactly that: it projects an image of the scene onto your camera’s sensor. It bundles the light from the scene into the lens, then sends it to the sensor: there is a point in the path of light where the image appears to come from – like the light fanning out of a film or transparency projector. This point goes by various technical names such as rear vertex or rear principal point, but the key thing is that we measure focal length from that point.
Now, we set up the lens to focus on a very distant subject (said to be “at infinity” because the light rays are for all purposes parallel to each other) and measure from the point from which the image appears to be projected in a straight line to the plane in which the sharp image lies – typically, that’s a shade below the physical surface of the sensor.
The point from which the image is projected can lie within the lens – we’d expect that. But this definition reveals the odd thing that the point from which the image is projected can also lie behind the lens and even in front. Look at a typical modern telephoto lens: its marked focal length is 300mm, but its physical length could be as short as 120mm or less. Similarly, you’re sure to have noticed that a wide-angle lens or focal length setting of 24mm is way longer than 24mm. Even the space between the rear of the lens to the sensor is greater than 24mm.
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