Focal Length
Focal Length is our viewing perspective, expressed in millimeters (mm).
Housed inside a lens are a series of clear (glass or plastic) elements, and the type and arrangement of these individual elements determines the Focal Length of that particular lens. Everything from wide-angle to telephoto.
A lens can be fixed at a particular Focal Length (referred to as a prime lens) or it can cover a range of Focal Lengths (zoom lenses).
Without any doubt, prime lens are superior precisely because the elements don't move back and forth, like they would in a zoom lens.
(An optical zoom lens is not at all like the digital zoom feature on mobile device. Digital zoom is just magnifying what would be photographed at a lesser Focal Length, that's why there is a blocky pixelation when using digital zoom. To avoid that, move closer to your subject if at all possible.)
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 your perspective changes from one photograph to the next...
18mm
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 still going 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).
The downside is that as we move out to the telephoto and super-telephoto range, the light passing through the lens becomes more and more diminished. This is remedied by a bump up in sensitivity (usually via ISO) and also the use of a tripod.
A tripod provides a stable platform for a camera to rest on when taking a picture.
Tripods are necessary for super-telephoto lenses, long exposures and most kinds of night photography. Even in bright daylight, the use of a tripod will always garner a sharper image than one that was hand-held. That's because unlike us, a tripod doesn't move (with the exception of wind and clumsiness).
Unfortunately, we either may not have a tripod with us or we are unable to use one at that particular 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.
Why? 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 our photograph.
Make sure the camera is well-supported by your hands and lock your arms into position.
When you exhale and are relaxed, that's when you take the photograph.
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.