 So in our last video we talked a little bit about how lenses work, and in this one we're going to talk about what makes a lens a good lens. This is kind of an important distinction if you're going to be putting a little bit of money into a camera, you might want to make sure that you have a good lens. Now we're going to do the example here on an SLR, so all of you who've been watching the videos know what an SLR is, but many of these principles, except for maybe one or two, account for pretty much any kind of camera that you have, right down to a cell phone and all the way up to a really fancy camera. So we're going to start out with the aperture, because for me at least that's a very important part of it. You want to have an aperture that's going to allow you to photograph in the sort of situations that you want. So you want an aperture that is going to be faster than I would say about four, maybe F4. You want an aperture between F1.4 and 4. So you don't want to, and that will be written here on the front of your lens on the barrel somewhere in here around the edge of the glass, and on almost any kind of camera you find, and if it's not on there it'll be in the technical details. And you want to make sure that the most open aperture is going to be somewhere between F1.4 and 4. Once you get past 4, it's going to be a little bit too slow to photograph in dark situations, so you want to make sure of that. You also want to take a really good look at the glass. You want to maybe even take some looks at reviews that other people have done about the camera or about the lens, and make sure that it's going to be the kind of glass that you want. Sometimes you'll get like an algae or a fungus inside of the lens, and you want to look through and make sure that you don't see any of that. You also want to make sure that you don't see any cuts or grooves or anything like that in the glass, any divots, like little holes. You want to make sure that it is clear when you look through, and that everything looks nice, and as if it's really just never been used in the best case scenario, especially if you're buying used equipment, you want to be careful of that, and obviously new equipment that's a little bit less of a concern. You also want to check the construction of the lens, so you want to take a look at how it's built. Construction is really important because lenses are, there's something that are expanding, retracting, they're being bounced around all the time, and so you want to make sure that it's stable, that it's made out of good material, and you also want to make sure that it is well sealed. This is one thing that I have made some mistakes on in the past in buying lenses. You often see here, this lens is going to be zooming in and out, and in these little spaces here between where the lens is kind of being given its space to do its expanding and contracting, you'll sometimes see a little mini barrel expand outside of the main barrel right here, and in that little space between air will get up inside, and it'll shoot in, and you'll drag dust inside of the lens. Now, I'm not saying this lens will necessarily do that, but sometimes it happens. Read your reviews, make sure everything is good on that. You want to look inside and make sure there's no dust inside of the lens if it's a used lens, and this doesn't apply as much to, especially to cell phone cameras and to smaller cameras, not quite as much as with SLRs, but some compact lenses zoom very far, especially ones that have what's called a long travel, so when the lens expands, it's much longer than it is before. It's sucking a lot of air in and out, in and out all the time, and with that comes dust, and eventually it can get inside and get on your lens, so you want to make sure that you're finding a lens that's pretty well sealed. You also want to make sure that there is no chromatic aberration. I pronounced this incorrectly in the last video, so let me take this chance to say it's chromatic aberration, and I'm so used to saying aberration that I can't get it out of my vocabulary. When the lens is kind of being pushed to its limits, it will do this thing where you're looking up at something, and a really common example is you'll be looking at branches of a tree, and you'll see here your branch, and then you'll kind of get this little fog behind it of sort of like a shadow of another color that'll be sort of there, and you can see it, and you know that it doesn't really exist, you know it's not really there, it's often sort of like a purple, or maybe a red, or something like that, and it's just basically an optical problem that comes up when light is coming off of one object, and it's traveling through your lens like maybe right here, and traveling onto your film, and say the red is coming in and it's coming into exactly at the same time as a yellow, but when they go through the lens, the red sort of splits off a little bit in one direction, and the yellow splits off a little bit in another direction, so it's a really subtle difference, but if you can see here, they're basically landing on the sensor at a different time, and in a different place actually, and this will cause this chromatic aberration on your image, and so you want to make sure that your lens doesn't do that, and you can research that by looking up reviews online, or you can also try taking some sample shots with the camera, and then go take a look at them, and you want to try that at many different focal ranges. A lot of times a lens will be fine at its widest, so when you're shooting at like 28 millimeters or something like that, it'll look totally fine, but then you'll zoom in to maybe you'll zoom in to 70, and that is where you'll first start getting this chromatic aberration, so you want to make sure that that is not happening. Other than that, basically, I mean you're pretty good, you want to make sure that if you're looking at point and shoot cameras, I would definitely take a look at the size of the glass. A big piece of glass is not always better, but often does lead to a bigger aperture sometimes, so you want to make sure, I mean you're really most worried about the aperture, but as a way of finding that, you can always look for the cameras that have the bigger lenses on front, and finally for those people who have SLRs, you want to make sure here, take a look at the mount to your camera, and you want to make sure that is the correct mount, because there are a lot of, especially in this digital age, a lot of complicated camera mounts out there, and you want to make sure that the lens that you're buying is going to work with your camera and have all the functions that you want. Sometimes you'll be able to actually put the lens on the camera, but you won't have autofocus or you won't have some features that are important to you, so you want to make sure before buying anything like that that you're getting the right one. All right, so that is a little bit about lens quality, that kind of gives you a little bit of an introduction to what a good lens is, and what a bad one is. We're going to move on from here into some more technical photography stuff that's going to introduce you to the basic technical things that you need to know to make good photographs, and also we're going to do a little section about how to care for your camera and lenses.