So the next section here is dealing with lens speed; the maximum aperture of the lens. We were dealing with the angle of view and now we're starting to talk about how much light the lens lets in and so this is a very important factor. It's one of the most important things I want to know about in a lens I'm choosing to buy or use and so that is the maximum aperture that we're going to be looking at. So this is the opening that all the light gets to come in through the lens. And so how big of an aperture do you have on your lens? And this is the maximum aperture and these come in a variety of different numbers; they'll go down to one point four, some will go beyond it and five point six is the maximum aperture on one of these giant super telephoto lenses. And this is going to be listed right on the front of the lens and it's listed in kind of an unusual fashion; that kind of confuses some people and it's a one colon one point four and I know that I've heard some people that kind of didn'...
t know what they were talking about and they say "I have a one point one point four lens". I go "No, you don't, you have a one colon one point four lens". And what that means is that it's a fraction. And this is how you write fractions when you don't have two lines and you have a typewriter and you just have one line of information; this is how you write a fraction. And what it means is the focal length of the lens over one point four. Now, the focal length of this lens is 50 millimeters and so it's a math problem; 50 millimeters divided by one point four gives us 35 millimeters. And 35 millimeters is the opening of the lens and so you can... I know this sounds like a lot of fun. You get to go home, and run home, (laughter from someone in audience) and divide all your lenses by this and you will see what the opening is of any of the lenses that you have. That's the maximum opening. So, a 50 millimeter lens has a maximum opening of 35 millimeters. Now with Canon they also have a 50 millimeter one point eight lens that has a 28 millimeter opening and they also have a 50 millimeter one point two lens that has a 42 millimeter opening. Now I will let it be up to you to guess which one cost more money (audience laughs). Alright! But, yes, bigger lenses are going, you're going to spend more on it but it allows you to shoot under lower light levels. Now if you want to run this math with other lenses, the 24 one four only needs a 17 millimeter opening because it's a very short focal length lens at 24. A 300 two eight is going to be a really big lens and it needs a really big opening even though it's only two point eight. So you can run this math to figure out how big of an opening does it need to let in that amount of light. And so the maximum aperture is the lenses' maximum opening and it's going to vary from lens to lens. And there are lenses we consider fast and lenses that we consider slow. A lens that is one point four is considered a fast lens. It's considered fast for a couple of reasons. It acquires light very quickly so it can very quickly acquire the light you need which ends up being fast shutter speeds. And so we use fast for a couple of different reasons. And then there are slower lenses which typically have maybe a larger zoom range or their longer telephoto and it's more difficult to get all that light in there. Photographers love fast lenses because they give us options of shooting under very low light conditions. We'll talk more about that. So the maximum aperture on zoom lenses. The most common lens in photography these days is the 18 to 55 three five to five point six lens and so this is what is known as a variable maximum aperture. The maximum opening will adjust as you zoom the lens from 18 to 55. Same thing on a 70 to 300; it adjust as you zoom the lens back and forth. There is then a fixed maximum aperture. For instance, a 24 to 120 F four; 24, 35, 50, 120; F four is the maximum aperture in all those cases. These are more convenient to use in my mind because if you set you camera up manually and you decide "I want this shutter speed, this aperture, this ISL, and I'm going to shoot a photograph; and now I would like to adjust the zoom lens a little bit". You don't need to make any further adjustments on your camera. If you do that with the variable maximum aperture, you may need to make an adjustment depending on where you've set your aperture on it. And so these are more money; they are more expensive to make but they do tend to be a little bit more preferred by people who do a lot of manual exposure. There's a lot of record holders and top of the field as far as the fastest lenses in any particular category. And so there's a lot of one point four lenses that are the fastest of any particular focal length. At around 50, 51 point two is going to be a very fast lens. The 85 one point two is a very fast portrait lens and you saw earlier I did some examples with the equivalent 56 one point two for crop frame and a 42 point five one point two. They all have the same fast aperture but you do get a different depth of field because you have a different size sensor recording those images. And so when you get up to the telephotos, sometimes F two point eight is as fast a lens as they make and when you get all the way up to 800, the fastest lens they make is a five point six. And there are a number of ways of going even beyond it. Leica makes a zero point nine five and why do they make it as a zero point nine five when they just don't make it a one or just round it up to one. Bragging rights! Under one point zero but then there are some other after market manufacturers that can make some really fast things and there are some people who are like "Oh, fast lens; that's kind of cool; I gotta do more of this; more, more, more". And then they start going to all sorts of extremes and what you'll find is in the optical industry, beyond photography, there's x-ray's in medical and other things they have designed lenses for that are even faster than point eight five. It's just that they're not very good optically at developing what we would call normal photographs. They can create images and so there's some weird aftermarket things out there but for the most part one point four is going to be a pretty fast lens for most people in most situations. With zoom lenses, they're more complicated to make so they tend to be slow in their aperture and so a two point eight lens is kind of the gold standard for most zoom lenses. If you have a zoom lens with a two point eight aperture, that's a lot of light for a zoom lens. And so 24 to 70, 70 to 200; these are mainstays of professional photographers. There are some longer telephoto lenses that will go up to F four. Sigma's a manufacturer that has really come on strong and made some unusual products. Yes, they do have a 200 to 500 two eight. It should probably come supplied with its own car it's so heavy. They do have, very interesting, they've introduced recently a few zoom lenses. The 24 to 35 F two is designed for full frame lenses and it's one of the only, maybe the only, zoom lens faster than two point eight for full frame cameras. But for the crop frame, they do have some one point eight zoom lenses and if you run the equivalent aperture math, this 50 to 100 is the equivalent in angle of view and in depth of field to a 70 to 200 two point eight lens. And so if you like that shallow depth of field, you will get that same effect with a 50 to 100 with the added benefit of being able to shoot at one point eight for really low light situations. So there are some interesting options out there. Now the faster these lenses are the bigger and heavier they tend to be. Now, you might be wondering if you're new to photography, "You sound pretty passionate about these really fast lenses. Why do you hold these in such esteem"? There's a lot of benefits of having a faster lens. They just give you more options for solving problems. You can shoot under lower light levels; you can shoot with faster shutter speeds; and so it opens up new possibilities. The brighter view finder is for those of you with SLR's because when you look through a lens on an SLR, Nikon or Cannon, Pentax, you're seeing your lens at maximum aperture. And the bigger that opening is, the brighter and easier it is for you to see your subject in there. In many cases that's going to mean you're going to be able to focus faster because there's more light coming in for your camera to focus. And so I know that using a two point eight lens or faster when you're shooting sports is a big advantage. The lens, the camera, just seems to do better with two point eight lenses than they do with four or five, six lenses because there's more light coming in. So more light coming in is a good thing but it's expensive and it can be heavy so it's a compromise like everything else in photography.
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