2.2 Aperture

Another one of those confusing numbers on lenses is aperture. In this lesson ,you will learn what aperture is and how it affects the image. You'll also learn the different between constant and variable wide aperture lenses and what this is going to look like in your shots. Aperture is a hole or opening through which light travels. In photography lenses the aperture is usually specified as an f number. And a lens typically has a set of marked f stops. That the f number can be set to. A lower f number is a larger opening. And a higher f number is a smaller opening. The larger opening lets in more light, and the smaller opening lets in less light. The photography term one F stop corresponds to a light intensity change factor of two. For example, if we set a lens to f four, we will get a certain amount of light that enters our camera. If we change the aperture to f/2.8, we will have effectively double the light we are allowing to reach the image sensor. If we change this again to f/2, we are doubling the amount of light again. Compared to when we first had the aperture set to f/, we are now letting in four times as much light. The largest f stop a lens can be set to is called it's maximum aperture. This will vary from lens to lens. Generally, more expensive lenses have a larger maximum aperture. So the most basic thing we need to know about aperture is that, it controls the amount of light. Is let into our camera. Sometimes more light is referred to as faster. For example, if we adjust our aperture from f/4 to f/2.8 you might say that we are using a faster aperture or if your lens is at maximum aperture is f/2.8 and you're not getting the correct exposure you might say. I need to get a faster lens. Fast is referring to the shutter speed. If we are maxed out on sensor gain, or ISO, we can get a good exposure with a smaller aperture by using a longer, or slower, exposure time, or shutter speed. The problem is that slower exposure times don't work for a lot of situations because they will result in motion blur. By using a faster lens that has a larger maximum aperture, we're going to let more light into the camera. And thus, we will be able to use a faster shutter speed, or exposure time. Let's look at an example. All right, in this example, we're gonna check out what difference a quote, unquote fast lens can make in terms of stopping motion, and why it's called a fast lens. So, we're looking at a little scene here. This is the back of my camera, so you can see what kind of camera settings that I'm using. And then off in the distance there you can see something moving. Now that is my automatic motorized camera slider that I built myself, Thank you very much. And the reason is because in the real world it's hard to get people and other objects to remain constant and. You know, for shooting outside the light may change and that may change our metering. So, this is a very controlled setup to see what happens and what we're gonna get in terms of the exposure here. So I'm gonna take it out of live view here. And I'm set to manual. My ISO's currently set to 3200 which is pretty high. It's not as high as it can go on this camera. But it's as high as we're going to go for this example. And the reason is because we're gonna kind of use what you might consider the highest iso on your camera. And we're just gonna pretend that 3200 is the highest acceptable iso on this camera. I'm set to f/4.5 which is. The highest that this lens can go because I have a super zoom lens on this camera. It's an 18 to 270 millimeter f/3.5 to 6.3 and it's currently set to 50 millimeters. And I'm also in evaluative metering, which is what that little icon means right there. And so when I meter this. Right now, it says that I'm at just a hair under, so it's telling you one-tenth of a second here is going to get, well maybe, maybe one-thirteenth of a second is going to be the right exposure, all right? So I am gonna take the photo and we are gonna check out what this looks like right here. I set the two second timer, and if we take a look at this photo, you are going to see that there is motion blur, because one-tenth of a second. At 50millimeters is not enough to stop the motion of that camera slider. And if this were the best ISO, if this were the highest ISOthat you could use on your camera without being ridiculously noisy, or maybe your camera just has an ISOlimit of 3200, then this would be a problem. Now, this camera can go up to ISO 12,800 but that's crazy, crazy. Noisy. So in this particular case it would be better to use a quote unquote faster lens, because it's going to give us a better maximum aperture, and therefore we're gonna be able to get a better shutter speed outta this. So why don't we change lenses, and let's check out what that looks like. All right, I've just changed lenses, and I've put on here my Nifty 50 Canon 50 millimeter prime lens. And it has a maximum aperture of f/1.8. So I can take this and I can roll it up to f/1.8. And now, when I half press the shutter button, it's going to be two stops overexposed. So now what I can do, is I can make the shutter speed a little bit faster here, and I'm going to be able to get, well it says 160 is like one-third underexposed. One fiftieth of a second looks like it's just about there. So I'm gonna take this shot, at one fiftieth of a second. And when we look at the exposure, what we're gonna see is the motion blur is a lot better. Now, it's not perfect, right, we could probably go to, maybe, one sixtieth of a second, that might help tighten it up just a hair. There is a tiny bit of motion blur but you can see that, that is gonna be a lot more usable. And so this faster lens here is gonna allow us to use a shutter speed that's much, much better for this particular situation. If our limit was iso thirty-two hundred, then the other lens would not be usable inside without a flash. With a flash. We would be able to add more light to our scene, and we'd be able to use a faster shutter speed without a flash, right? If we were just shooting in available light, this is the best we would be able to get. Now f one pointing is not the fastest 50 millimeter that you can get. You can get a f/1.4. You can get an f/1.2 lenses, 50 millimeter lenses that are f/1.2 are above $1,000. And you can get a manual focus only lens that is F .95, I believe, which is even faster. That's gonna definitely be fast enough to stop the motion, at least this motion here in this example. But you can see how this very inexpensive $100 Prime here gives us a nice bump. It's over two full stops better, from f/4 to f/2.8 is one stop. F 2.8 to f 2 is two full stops, and this is 1.8, that's like two and I think a third. Maybe, so it's giving us two and a third stops better, plus we started at F 4.5, so it's probably two and two-thirds stops better. We can really get something that's a lot nicer with a faster lens. A faster lens makes a huge difference when you're shooting inside, or anytime that the light is low. Usually, that's gonna be when you're inside, but. I suppose it could be when your outside in the moonlight you know, and that would make a difference there as well. But primarily you'll find the biggest benefit for fast lenses inside when your not using a flash. What this means is that when the light is low, as in when your shooting inside you want a lens with a larger maximum aperture. As noise levels on cameras gets better as time goes on. This will become less of an issue. For now, getting a lens with a fast aperture will result in cleaner, sharper images in low light. So, what's considered fast? For a zoom lens, it's f/.8 and for a prime lens, it's f/2.8 and above. Like f/2, 1.8, 1.4, 1.2, and even 0.9. We'll talk more about zooms and primes later in this course. Aperture has a few side effects that we need to talk about. Such as depth of field and diffraction. Depth of field, sometimes abbreviated as DOF. Is the distance between the nearest and the farthest objects in a scene that appear acceptably sharp in an image. Another way to think about this is the area in front and behind the point of focus that appears to be in focus. Depth of field is a rather complex topic. And if you want more information, you can check out a fantastic course that I put together called Mastering Depth of Field and Exposure. Right here on Tuts Plus. For now, we're just going to think about depth of field as the part of our scene that is sharply in focus. Aperture controls depth of field and it works like this. A larger aperture, or lower number, has a shorter depth of field and a smaller aperture, or larger number, has a longer depth of field. This is important because this is an area that you will want to control. If you are outside and you need a lot of depth to filled, you're gonna use a smaller aperture like f/11, f/16, f/20 maybe even f/22. If you are shooting a portrait, you might want to use a shorter depth to fill to isolate your subject. Maybe something like f/4 or f/2.8. Most all lenses will be able to give you a small enough aperture to satisfy your long depth of field needs. So, that's not really an issue. If you want super short depth of field, you're going to need a faster lens. And this is what can separate a lot of lenses. Some zoom lenses have a constant wide aperture. This means that no matter where you are in the lens zoom range, the aperture remains the same. This is contrasted by lens that have a variable wide aperture. These lens usually start at a wider aperture on the short end of their zoom range. For example this Tamron 18 to 270 millimeter lens is f/3.5 to 6.3 as we go from 18 millimeters to 270 millimetres the aperture gets smaller this means that at the long end this lens is letting in a lot less light. An aperture of f6.3 isn't nearly as wide as f2.8. But if you're shooting outside in nice, bright sunlight, it would work just fine. The thing to remember is that, with variable aperture lenses, you have less control over the depth of field and light levels, as you use higher focal lengths. The other effect of aperture that I mentioned earlier is diffraction. Again, without getting super technical, this is what you need to know. Diffraction is the bending of light rays at smaller apertures. This causes the image to become noticeably less sharp. This is sort of an odd thing, because usually lenses become more sharp as you use smaller apertures. But diffraction is not an on-off switch. It's a gradual effect. Diffraction is also limited by sensor size and not lenses. On a full frame sensor, you would be able to use apertures down to f/22 without seeing diffraction start degrade the image. On an APSC and Microforth thirds cameras, the limit is around f/11, which is two full stops higher than full frame. This doesn't that you can't shoot on f/22 on the smaller sensors. You can, but it will be a softer image. If you wanted to reduce the light coming into the lens on a crop sensor camera, without losing detail, you could use the smallest aperture before you start to see diffraction, and then pop on an ND filter to cut a few more stops. Again, the main thing to understand about aperture is that it controls the amount of light and conversely the depth of field. As you move throughout this course, you're going to see examples of fast primes and zoom lenses with variable apertures. For now, you're ready to move on to the next lesson where you will learn about what a good lens is by checking out what a bad lens looks like.