in its most basic explanation, focal length is how much you're zoomed. the focal length most closely equivalent to that of the human eye is about 75mm. that's not to be confused with the 35mm of '35mm camera', which is referring to the film gauge. those are two different things measured with the same unit (millimeters). what the 75mm focal length means to us is that when you put the camera up to your eye, the image you see won't appear zoomed in or out but rather it will look just like it does without the camera.
alas, there's more to life than 75mm. lenses come in two main flavors - prime and variable. variable lenses allow you to change the focal length. a common kit lens with entry level dslr cameras for both nikon and canon is an 18-55mm. that means you can use the lens at any focal length between 18mm (considered wide angle) all the way up to 55mm, which will appear a little bit zoomed out compared to the naked eye. variable lenses are nice for obvious reasons, allowing you to very quickly adjust your focal length on the fly without swapping lenses. your other choice is a prime lens. an optimus prime lens has a fixed focal length. you may immediately think that to be a huge disadvantage. you may also think that you have a point-and-shoot and you can't swap lenses anyway. well, the second statement may be true, but the first is not. while primes don't afford you the luxury of quick focal length changes, they do have some advantages. there are three main advantages to primes: they are usually much sharper than variable lenses, they typically weigh less, and they often afford you better aperture for less money than a variable.
here's an example to show the difference between a shot from the same distance at 80mm and a shot at 200mm
a little background in lenses: this is where i would attempt to flaunt my minor in physics, except i honestly don't remember too much from it. i did a lot of sudokus in college. light bends in a very predictable way when it passes through concave and convex glass. those pieces of glass can be lined up in series so that each successive lens is fed light from the one that supercedes it. depending on the setup of the glass, one can concentrate or expand the light however they want within the laws of physics. remember those funny convex mirrors they used to have in the corner of department stores to make sure no one was pocketing a sweet nike t-shirt? it's kind of like that except rather than the light bouncing back like on a mirror, the light passes directly through.
camera lenses are often internally made up of multiple pieces of glass. a prime lens has all of its glass in fixed positions. a variable lens achieves different focal lengths by allowing you to move certain pieces of glass closer together and further apart from one another (usually done by turning the outside housing of the lens clockwise and counter-clockwise). the sacrifice of the allowance of motion in the glass is that the alignment is not quite as precise as the prime lens. this reduces the sharpness of the lens. the added weight of a variable lens is due to the extra glass necessary to make adjustments.
alright, that's probably more about focal length and lenses that any normal person would care to hear about. there's actually a little more about how the focal length changes the amount of 'compression' in the photo, but i think that's overkill for this mini-series. as always, if there are any questions just drop them in the comments. i'd also love to hear if this mini-series is helpful to any of you guys or if tears of boredom are streaming down your faces.
next up: flash