On this B&H video (made by Kelby Training) photographer Larry Becker goes back to basics to explore the the camera focal length and related topics such as zoom/prime, compression and image stabilization.
Becker starts with a simple explanation of how you can read and understand the focal length from the name of the lens – typically this is pretty simple – for example the Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6G ED II (the kit lens which many people buy with their first camera), has a focal length of 18mm which goes all the way up to 55mm. If you want to know the zoom range simply divide the larger number (55) by the smaller one (18) and in this case you will get a zoom of just over 3X – so this is a 3X zoom lens.
If your lens happen to be the AF-S NIKKOR 50mm f/1.8G on the other hand – it only has one number – 50mm which is your focal length and than you have no zoom (and you know this is what is called a prime lens).
You also need to take into consideration the lens-camera combination – depending on the size of your sensor, your angle of view (or field of view) will change – so a camera with a larger sensor will have a wider field of view with the same lens (if the lens support the sensor size of course) than when the same lens is mounted on a camera with a smaller sensor (see this video on understanding crop factor)
If you want to see how focal length effect portrait photography in more depth check out Joe Brady’s extensive video.
Becker goes on to talk briefly on the field of view of human vision, prime/zoom, wide, ultra wide and fish-eye lenses as well as telephoto lenses and compression. Finally Becker talks about image stabilization using a tripod or image stabilization.