If a crop sensor is used, it will look like the focal length is much longer. Photographers that need long telephoto lenses can benefit from this. But instead of using a crop sensor, it is also possible to crop a full frame sensor to have the same field of view. Let’s take a look at the differences.
It isn’t really that difficult. A small sensor will record only a part of the projected image of the lens you are using. A bigger sensor will record a larger part of that same projected image. A full frame sensor with the dimensions of 24 x 36 mm will have a larger area compared to a 1.5x crop sensor that measures 23 x 15 mm. But when the recorded image is viewed at the same size on a screen, the image of the crop sensor will result in a magnification of 1.5 times that of the full frame sensor.
This is why a lens on a camera with a sensor that is smaller than a full frame sensor will act as if it has a longer focal length. A 24mm the lens will act like a 36mm lens, a 50mm lens will become a 75mm lens, and a 400mm lens will imitate a 600mm lens. This is when the sensor has a 1.5x crop. If a micro 4/3 sensor is used, with a crop factor of 2x, the focal lengths will be 50mm, 100mm, and 800mm compared to its full frame cousin.