Focal Length
Focal Length is how we see the world through our camera's lens.
It is our angle-of-view, expressed in millimeters (mm).
Housed inside a lens are clear (glass or plastic) optical elements. The shape, thickness and arrangement of these individual elements determines the Focal Length of that particular lens, giving us everything from a wide-angle to a telephoto perspective.
A lens can be fixed at a particular Focal Length (referred to as a prime lens) or it can cover different Focal Lengths (zoom lenses). Without a doubt, prime lens are superior precisely because the elements don't move back and forth like they do in a zoom lens.
Now, an optical zoom lens is not at all like the digital zoom feature on a smartphone. Digital zoom just magnifies what is being photographed by the camera's standard Focal Length. That's why there is a visible blocky pixelation when using digital zoom. To avoid that, either move closer to your subject if at all possible OR shoot where you are and then crop later on - using a duplicate image, of course. NEVER manipulate your original images.
I photographed these next examples several years ago, using 2 different cameras and multiple lenses in order to cover the entire focal range seen here.
Watch the tombstone in the center and notice how the perspective changes from one photograph to the next depending on the Focal Length...
300mm
Normal human viewing perspective has always been considered to be roughly 50mm.
Although some would argue that technically with digital cameras it's really between 38mm and 43mm, I'm staying with 50mm.
Anything lower than 50mm (wide-angle and super-wide angle) pushes the subject away from us, thus opening up our field of vision to include more scenery. Lenses can get so wide in fact, that it distorts the viewing perspective and gives a circular "fisheye" look to the world.
Going over 50mm brings objects closer to us (telephoto, super-telephoto and even telescopes).
Unfortunately, capturing light does have limitations and the downside is that as we increase telephoto and super-telephoto ranges, the light passing through the lens becomes diminished. This often is remedied by increasing sensitivity (usually via ISO).
One other thing, telephoto lenses require the use of a tripod or some other stable platform when taking a picture.
Tripods are necessary for super-telephoto lenses, long exposures (waterfalls) and night photography. Even in bright daylight, using a tripod will always garner a sharper image than one that was hand-held. That's because unlike us humans, a tripod doesn't move (with the exception of wind, gravity and our clumsiness).
Unfortunately, we either may not have a tripod with us or can't use one at that location. In such cases, rest the camera on anything stable you can find and use the self-timer.
If hand-holding the camera is the only option, here's an old rule-of-thumb: The Shutter-Speed value should be no lower than the Focal Length you are shooting at.
Example: If my Focal Length is out to 200mm, my Shutter-Speed value should be no lower than 1/200 of a second. If it's 80mm, then 1/80 is a suggested minimal Shutter-Speed. (Of course, if you just drank a double-espresso, all bets are off.)
Why the rule? Well, the physical human act of holding a camera, factoring in our movements and breathing, can be such that we ourselves are introducing blur into the equation.
I personally use everything from a tripod, to a monopod, to a small, heavy-duty bean bag to support my camera and lens. Sometimes I will have no choice but to shoot hand-held and that's fine, but if at all possible, I'm using a tripod.