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Focal Length: Overview

We've gone through the camera, we checked out the sensor, now we're gonna attach the lens, we're putting all the pieces together as we go forward in this fundamentals of photography. Alright, so what we're gonna be doing first is talking about focal length, then we're gonna go through the other aspects of the lenses as we need to. So, focal length is the main reason that you're gonna choose one lens over the other. It's the angle of view. So that's the most important characteristic in a lens, is what are you actually going to see with that lens. The second most important characteristics is its light gathering ability, how does it do in low light, what is the aperture on it, and with the combination of the angle of view and the light gathering ability, that's gonna give us a particular depth of field for that lens. And so some people are choosing a lens on depth of field, but what they're really doing is kind of looking at the first two categories. Now, there's a lot of other reasons wh...

y you might choose a lens. They're usually not the first item on the list, but they're something that you might wanna check off and make sure that you're aware of, and there's lots of things to do with lens quality, and we'll go through these individually a little bit later in this section. There's the whole physical size, the weight of the lens, how it feels in the hands, size of the focusing ring, the zooming ring, does it have a tripod collar, all sorts of things that you're gonna want to know about your particular lens and how it compares with another lens that you might want to choose. And so we're gonna go through and talk about pretty much all of these things. So here is probably the most common lens you're gonna find out in the market today, is the 18 to 55 lens. And that is the focal length of that lens, it's usually listed right on the front or right on the top of it, another very common lens would be a 50 millimeter lens, so this is talking about the focal length which will result in the angle of view as you'll see in a moment here. The other important number is the maximum aperture. And this is always a little bit confusing to newcomers to photography, I know I've been asked by some people who are brand new to photography, I have a one point 1.4 lens, and it's no no no, that's a colon, and we forget about that one colon number, it's the number after the one colon. So I will talk more about that as we talk about the aperture of the lens. So we're gonna concentrate right now just on the focal length of the lens, which is the distance from the nodal point to the image plane. It's kind of, you know, in very general terms this is not exact right here, but it's basically the length of the lens. Now because it's being mounted on a camera and the lens doesn't go all the way back to the image sensor, there is a little bit of discrepancy here, but in general it's the length of the lens, which is why a 50 millimeter lens is a relatively small lens, and a millimeter lens is a relatively large lens. Now we can't really go very far in the discussion of focal length without jumping back and talking about image sensors, because that's how we determine the actual angle of view is by figuring out what focal length, angle of view, and adding up the sensor size on top of it. So the two most common sensor sizes for most of the kind of serious photographers out there, are the full frame sensor, based on the 35 millimeter format, and the crop sensor of 1.5 to 1.6. And so we need to be able to understand the differences between these, and how they interact with the lenses themselves. So let's take a lens, doesn't really matter what lens, just a lens, and as light comes through that lens, it is projecting a circle of light on top of the image sensor itself. And so that's what our image is going to be in this case. Now if we take a copy of that lens, the exact same lens, and we now project that image on top of a smaller size sensor, the light coming through the lens does not change, it does exactly what it did before, but this time we have a smaller sensor capturing a smaller image area, thus we have a different angle of view even though the focal length of the lens is exactly the same. And so I can't determine the angle of view with just one factor, I need to know what lens and what sensor in order to figure out what the photo is going to look like. And so focal length is not the same thing as angle of view, they're very closely related, but they are not the same. Now there are some lenses that are designed exclusively for the smaller format sensors on the market. And there's a lot of different smaller sensor formats, and there's a lot of lenses designed for these smaller sensors, and if you did try to mount them up on a full frame camera I want to show you what would happen. Now, these smaller size sensors have lenses with special names, like for instance, Nikon calls their lenses DX lenses, Canon calls theirs EF-S, and there's a variety of names that the manufacturers use to designate that these are special lenses designed for these smaller sensors. If you could mount that on a full frame camera, what would happen is the light would go through the lens, and it would do the same thing it did on the other sensor. The problem is, is that the sensor is too big, and you would end up with what's called vignetting, or a darkening of the corners, and that just doesn't look real good in most photographs, I know some people add that in as an artistic add-on, but that isn't the way you want to shoot on a regular basis. And so be aware of what lenses are available for what sensor in your camera. So let's take a 50 millimeter lens, let's put it on a full frame camera, we get a 40 degree angle of view. We put that same lens on a crop frame camera of 1.5, we get a 27 degree angle of view, and we put it on one of those micro four thirds, and we get a 20 degree angle of view. So all three of these cameras are using the exact same focal length, the same lens, but they're getting different angles of view from them. The normal lens, traditionally in 35 millimeter photography, has been the 50 millimeter lens. But, with the crop frame sensors, it's something closer to a 35 millimeter lens, and for the micro four thirds group, it's gonna be something closer to a 25 millimeter lens. And so all of these, they're different lenses, they're different sensors, but they're gonna result in virtually identical angles of view. So what you see through the viewfinder, the types of photos that you're gonna take, are very similar when it comes to the angle of view. Now, one of the things that is very unfair in photography is that not that many people, percentage wise, own full frame cameras. I don't know what the number are, but I would just hazard a guess that maybe 20, 15, 25%, something in that range own full frame cameras. But that is the standard that everyone talks about. When you take a photography class here at CreativeLive, the instructor is probably gonna be talking in terms of 35 millimeter full frame sensors. When you read an article on the blog about some sort of photography aspect, and you should use this lens to do such and such, you hear a couple of photographers talking, they're probably talking about the 35 millimeter equivalent, it's just the standard in the industry because it was incredibly popular for about 50 years. And the most popular sensor out there, well, if we want to be honest it's probably the sensor in the iPhone, okay? And so this is the standard, it's been kind of the standard just because it's popular among fairly serious photographers. And so this is what we call equivalent focal lengths. And we have to be very careful, we have to tiptoe folks, because there are equivalence police watching us right now, and the equivalence police is waiting, they're just ready to pounce if I say the wrong word. And so these are not the same, okay, they're equivalent, they have the same angle of view, but they do have some different properties, and we're gonna talk about those and show those in there. So on the wider end of the spectrum, if you wanted a reasonably good wide angle lens, you're gonna need a 28 for a full frame, around a 17 on that crop frame, and a 14 if you go down to the micro four thirds. All of these are gonna have an equivalent or very similar angle of view. If we want to go to the telephoto end, we can use a 300, a 200, and a 150. And they're radically different focal lengths in many ways, but they're all gonna give us about the same angle of view. And so just be aware of what sensor you have, and what lenses you're gonna need for the types of things that you wanna do. In general, we can break the lenses into two different categories, we have prime lenses, which are single focal length lenses, 50's the normal, hit 35 we're at a wide angle, once you hit 20 we start calling 'em ultra wide angle, 'cause they're so wide at that point. Anything above 50 is gonna be telephoto, so 85 is a nice little short telephoto, once we hit 40 we start using the terms super telephoto. Now these same ranges are going to be available when we get into the zoom lenses as well. And so it just depends on what is available from that particular manufacturer, but we can go just as wide in zoom and for the most part just as long as well, depending on the manufacturer that we go to. So if you're thinking, well what's the difference between a zoom and a prime, what should I get, what's better, that's a common question, everyone has that question. Should I have a zoom, or should I have a prime? Well, the zoom lenses are really versatile because they have all the different focal lengths in them. It is smaller than a collection of lenses, so for instance I have a 24 to 70 pictured here. If you were to buy a 24, a 28, a 35, a 50, and a 70, that's gonna be a big collection of lenses. And obviously less lens changing, so if something's happening quickly, just zoom it, you're there. So it's a very very convenient lens, and I, for the most part, would recommend starting with a zoom lens, unless you know that you're doing something very very particular. If you said, I have a home based studio and I'm doing head shots of people in my studio, okay, I can recommend a specific focal length for something like that. But if you said, well, I'm traveling to the Grand Canyon and I wanna take a lot of different photos with my family. Well, that's probably a great example of using a zoom lens, 'cause you're gonna have a wide variety of things to photograph. The prime lenses are really nice, and this is what a lot of photographers kind of aspire to, as these are a little bit sharper, they're faster, which means they let in more light, which means you can use them in a wider variety of situations especially low light situations, and in many cases they're smaller. For instance, I have pictured here a 50 millimeter 1.4 lens, and look at the size of it compared to this 24 to 70, which also includes 50. And so this little lens right here, which costs a lot less than the other one, lets in a lot more light, is probably sharper, but it just does one thing. And so what I usually recommend for most people is start off with a couple of zooms, figure out what you do and what's really important to you, and then maybe get yourself a prime that fits that need. And if you find something else you're doing a lot, get a prime that fits that particular need.