So there are like 3 things that makes a lens unique from other lenses: maximum (and sometimes minimum, if it matters) aperture, focal length (and if it's variable, ie a "zoom lens"), and random extra features.
Aperture refers to how wide the iris of the lens can become. It is displayed as an f-stop number. the lower the aperture, the wider it is, so f/1.4 is wider than f/3.5. The number refers to some mathematical calculation I don't understand.
Maximum aperture grants you two important qualities, the extremeness of out-of-focus elements and the 'speed' of the lens, or rather the wider your aperture, the faster shutter speed you can use. A wide aperture lets you use less light than you would need normally, which basically means less unwanted blur from camera/subject motion, and lets you take good photos without a flash, which is always a plus when you don't want that to distract people.
The wider your aperture, the narrower your focal depth becomes, and so with a very wide aperture you could have the eyes of a portrait be in focus yet have the tip of their nose and their ears be blurred, which is pretty cool. Wide apertures are what let you take a picture of someone and have everything else in the room be blurry, enhancing the attention given to your subject in the photo's composition.
Next comes focal length. It's displayed as a number in milimeters. The smaller the number, the wider a shot you can take. When looking through the lens of a camera, consider the corners of the picture. Imagine straight lines coming from the camera sensor, projecting to the corners of the picture that the camera is framing. From above, this will look like a triangle, with the flat side at one end being the physical area the camera is observing, and the corner toward you as the camera itself.
A focal length of 18mm would let you take the picture of an entire room, where the focal length of something higher like 100mm would be zoomed in so that you could only look at one object at a time in the room. The longer the focal length, the less objects are distorted based on physical proximity. This is important to consider when taking pictures of people with a wider lens, since whatever body part is closer will seem freakishly large, unless the person is further from the lens.
Zoom lenses can work within a range of focal lengths, usually noted by something like "18mm-100mm". Prime lenses are lenses which can only work at one focal length.
Because of the simplified structure, prime lenses can have much wider apertures than zoom lenses. So from the inflexibility of a prime lens, you are afforded the ability to take better pictures at that specific focal length.
As a general rule, the more a lens can do, the more it will cost. So you could feasibly buy a lens that does 70-200mm at f/2.0, but it will cost you out your ass.
Lastly there are little extra features like Auto-Focus (and AF-S magic with Nikkor), Vibration Reduction and weird glass properties that reduce light from smearing around.
So when I go out today I can either use my 50mm f/1.8 which would be cool for taking pictures of stuff around 5-20 feet away or my 18-105mm f/3.5-5.6 which will still be able to take decent photos in day-light and be better for taking pictures of stuff I can't walk up to.
it's almost noon. wakeup, b.
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