It's a valid question... I used to Google search the same thing back before I went to college. Lenses are essential to filmmaking and photography, they're they eye of the camera. When starting out, you want the most bang for your buck.
I own a 17mm-85mm lens for my Canon C100, I could only afford one lens at the time so I wanted to cover all of my bases. I wanted a wide lens for filming primitive Slacker videos (which was a stylistic choice and still is), but I wanted a more narrow lens for close ups and portraits. Sounds great right? Well, let's talk about the different parameters you should look at when selecting a lens. We've got a lot to talk about, if you'd rather watch videos on the subject, we've linked some trusted sources for you to watch through near the bottom of this post.
the focal length of a lens is defined in millimeters from the point of convergence to the sensor/film. People like to say that it has to do with the diameter of the front element, or that it's the measurement of length between the rear element to the sensor. Those are wrong.
Lenses are complex as hell. They can use one singular piece of glass (called an element) to manipulate the light, or they can use a lot, like the Canon 70-200mm f/4 which has 20 elements and 15 groups. There isn't anything wrong with either option. They will give off different looks but that's all subjective and you really shouldn't worry too much about that right now. The differences aren't that apparent.
There are two kinds of lenses, prime lenses with a fixed focal length, or zoom lenses, with a variable focal length. Going back to my 17-85mm zoom lens, it sounds great until you understand some limitations of zoom lenses. Cheap zoom lenses are the worst. Don't even bother. They don't look good when zoomed all the way in. They lose sharpness, which makes everything look slightly out of focus. Cinema zoom lenses like these $45,000 Canon 30-300mm and 14-60mm don't have that problem. Don't get too excited though, they still suffer from a slightly less problematic shortcoming that we'll talk about in a bit.
I don't want you using the zoom function on your lenses until you understand composition. People are too quick to use the zoom function of their to "fix" a shot rather than just physically moving the camera. If you are shooting portraits and you're too close, don't zoom out, back up. focal lengths change perspective. Think of it like you're driving up to a house. You can see the roof of the house from far away. As you get closer, you start to lose sight of the roof until you're right there in the driveway and can no longer see it. Same principal. Zooming in or out rather than moving forward and back can do different things. In the animation of the tree, you notice how the perspective changes. Once you start to analyze the motion, you really start to understand perspective. If we were to crop the part of the animation where the camera appears the furthest away from the tree, you'll find that it matches the part where the person zoomed all the way in.
I know the cropped photo is a pixelated mess, but bear with me. These are the same photo. note the mountains in the background, the rocks in the foreground. Everything matches. That's how perspective is affected. Theoretically, if you had a camera with a high enough resolution, you could shoot the shot on the right with a 17mm lens and crop in to get the final photo. That's an option as well, but don't forget about the limitations of your camera's resolution. Don't plan on cropping unless you know that your camera's resolution can handle it without pixelating your final image.
Now if we're talking about human faces, those are affected as well. Humans don't look flattering in wide focal lengths. It almost forms a caricature look by exaggerating features. Noses look bigger, foreheads pop more. It doesn't look great. To contrast, zooming in too far will make your subject look like Gumby, just a flat mass. Again, this is subjective, but I prefer to take my portraits using focal lengths between 50mm and 135mm.
Landscapes, skate videos, and real estate listings with shots of rooms look great with wide lenses. They're meant to highlight the big picture. See as much as you can. But again, it's all subjective, it depends on how much depth you want, which we'll talk about later.
The aperture is like the pupil of the lens. Just like your eye, the bigger the opening of your aperture, the more light will enter your camera and hit your sensor. We're in the process making a huge video on exposing photos and videos correctly, but for now, lets just talk about what it means in correlation to choosing a lens. Lenses are very straight forward with their names. A 50mm F/1.7 lens is a 50 millimeter lens with a maximum aperture of f/1.7. The lower the number, the larger the hole. Think of it like piercing or gun ammo size. Anyways, bigger apertures let in more light, they are also contribute to "depth" or "depth of field." Lenses with a large maximum aperture (lower number) are considered "fast" and are more expensive. Letting in more light combined with more depth is considered a good thing. Obviously that means lenses with a smaller maximum aperture (higher number) are "slower."
I like depth of field. I like bokeh (when light sources make balls of light when they're out of focus in the background of a photo). I shoot my portraits with a lot of depth of field.
Some zoom lenses have two aperture numbers. In the case of my 17-85mm Canon lens, mine reads f/4-5.6. That second number is another set back to zoom lenses. Even those $45k lenses we looked at earlier have this problem. The first number is the maximum aperture when you're "zoomed out" or as wide as your lens will let you get. The second number is the maximum aperture size when you're "zoomed in."
For those who are looking at a variable maximum aperture zoom lens, hear me out. In terms of exposure, a variable maximum aperture lens could be an issue. You'll have to change your camera settings to compensate for the full stop of light you lose dropping down to a f/5.6 from a f/4 just because you zoomed in. In terms of depth of field, it could be an issue as well.
I'm also not saying variable max-aperture lenses are bad, I'm just letting you know what to look for. I own plenty of them.
There is such a thing as a constant maximum zoom or fixed aperture zoom lenses, don't have the same problem. As long as it's sharp, you won't have any real drawbacks.
The relationship between Focal Length and Aperture
The glass itself has an effect on depth as well. The longer the focal length, the more depth. There's a reason that GoPros, with their wide field of view have everything in focus. Wide lenses don't have as much depth of field as longer lenses do. a 135mm lens should, no matter what aperture setting you use, still have some depth of field.
I'm not saying that wider lenses don't have any depth, some do if you have a large enough aperture, but for the most part, it is very minimal.
When I film Slacker videos in my home office/studio space, I use a wider lens and I use bright lights to expose me more so that I can use a smaller aperture which darkens my background, while also keeping me in focus. I move a lot, lean forward and back, if I didn't use a smaller aperture opening, I would go in and out of focus constantly. I use a wide lens for two reasons. I like the look, I'm a former sk8er boi, and because I don't have a lot of room to work with. I usually use a 28mm lens.
Wide angle lenses are defined by having a focal length shorter than 50mm. 49mm lenses are considered wide, as well as 38mm, 28mm, 16mm. They have a wider field of view than normal lenses.
A normal lens is defined as having the same field of view as the human eye. 50mm is the focal length that most people agree shows images from the same perspective as your eyes.
A lens that has a focal length longer than 50mm. They have a more narrow/magnified field of view than normal lenses. They also have more depth of field.
This is basically a long lens that is actually shorter than it's listed focal length. Its essentially a compact long lens. Long lenses are sometimes called telephoto lenses, which is incorrect. I mean, they're the same, but they're not. All telephoto lenses are long lenses, all long lenses are not telephoto lenses.
With a macro lens, you can focus very very close to the camera, these lenses are made for taking photos of very small things... bugs, leaves, toy cars. They still come in different focal lengths, which means they still have different fields of view. They can be wide like the new 24mm probe lens, or they can be longer like my 70-205 macro lens.
Tilt shift lenses physically tilt and shift in front of the camera, hence the name. They are used to either fix perspective, or to make things look like miniatures. This is a highly specialized lens.
This is another very specialized lens that allows for a wider aspect ratio than traditional 16:9. What it does is it essentially compresses the image horizontally, so that everything looks super skinny. When played back, it has to be "desqueezed" or "stretched" either digitally or projected through an anamorphic projector lens to undo the horizontal compression. The end result is a wider image aspect ratio than the film or sensor could create without the anamorphic lens.
Not to be confused with a wide lens, fisheye lenses are very wide but they come with distortion. Although fisheye lenses are wide angle lenses, not all wide angle lenses are fisheye lenses. They're so wide that they distort straight lines in an image, making them curve. This is the kind of distortion you see on action cameras, such as GoPros.
Vintage vs Modern
I shoot a lot of things on vintage lenses. In fact, I buy them cheap off of ebay, take them apart, restore them, and sell them. I keep the best ones. Mirrorless cameras adapt vintage lenses to them very easily, other cameras can only adapt certain vintage lenses to your camera. We'll cover vintage lenses in a different post at a later time. for this, you'll have to do some of your own research to find out if you can adapt vintage lenses and vintage lens mounts to your camera. If you can, you'll have to watch out for sensor cropping.
To make it easy, full frame sensors have no crop factor. 35mm APS-C sensors have a crop factor of 1.5x (Nikon) or 1.6x (Canon). Micro 4/3 sensors have a crop factor of 2x. This means that a 35mm Full Frame lens adapted to a Micro 4/3 sensor would turn that lens into a 70mm lens. Very similar to the principal we brought up earlier with the tree animation.
Vintage lenses are also fully manual most of the time. They have a physical aperture ring that you use to select your aperture. There is no auto focus. There is no image stabilization. As a professional, you shouldn't use auto settings at all, but image stabilization really helps when no tripod is available. I use it on my Sony A7S II which uses the sensor to stabilize the image rather than the lens.
This next video explains lenses very well, BUT he calls longer lenses telephoto lenses. Remember that all telephoto lenses are long lenses, but not all long lenses are telephoto. See above for more.
So what lens should I get?
Honestly, it's up to you.
If you're really indecisive and can't figure it out, comment below what you want to photograph. Tell me about what you want to do with your camera, what you want to film with it, shoot with it, whatever. Tell me about the camera you want, and I'll suggest a lens for you.
Other than that, I hope you learned something!