The 35mm film camera format has been with us since 1924 when the German Leica was introduced. The image size (format size) measures 24mm height by 36mm length. Now digital cameras are replacing film cameras. Most were built to house a digital imaging chip that has the same format size. These are called full frame cameras. As technology marches on it has become possible to shrink the digital image sensor size. The newer compact digital cameras sport an image sensor that measures 16mm height by 24mm length.
The crop factor, sometimes called the magnification factor is derived by dividing the diagonal measures. The full frame diagonal is about 45mm. The compact digital frame diagonal is 30mm. Thus 45 ÷ 30 = 1.5. This is the crop factor. What does the crop factor tells us? OK 1/1.5 = 0.66, in other words, the compact digital is 66% of the size of the full frame.
If we mount a lens with a focal length about equal to the diagonal measure, the angle-of-view delivered is 45⁰ with the camera held horizontal. In other words a 45mm lens mounted on a 35mm format is said to deliver a “normal” view. Same is true if a 30mm is mounted on a compact digital. Special note: While a 45mm delivers a “normal” view on a 35mm camera, lens makers have traditionally rounder this value up to 50mm. Thus a 50mm lens is considered normal for the full frame.
The full frame became so popular that most photographers became super familiar with how it performed with most lenses. Thus it became the gold standard and this is why some think we need a crop factor. The truth is, if you have never used a 35mm full frame, the most often quoted crop factor is useless.
The fact is, the focal length engraved on the lens remains factual regardless. Better you become familiar with how lenses preform on your camera, regardless of its chip size. As the future unfolds, image sensors will shrink and you will have even more difficulty trying to us a crop factor.