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Lens Design

Now if we were designing lenses ourselves, we're a new lens design team, there's a lot of things that we would want to put into a lens that would make it a really good lens. First off it's got to have great resolution, right? Being sharp is the most important. Is that everything? No, not at all. We have many other things. Good contrast. You can have great resolution but not have great contrast. You do want both. You want your lens to be accurate in color of course. You don't want it to have distortion, because it could still be really sharp, but it bends the lines. We don't want that. You'd prefer to have the lens be as small as possible, and you want it lightweight as possible. But some of those first things on the list are gonna play conflict with the small and lightweight. And the simple to use and simple to build and let's add simple to repair. These are gonna be some conflicting attributes of a particular lens, and it'd be nice to have it the lowest price possible, but once again,...

that conflicts with some of the other settings on here. And so as I've mentioned before, lenses, all lenses, are a compromise of some sort. First on this list is resolution. You want your lens to be sharp, and the fact of the matter is, I would say that every lens out on the market today that sells for more than 100 dollars is capable of taking next month's cover shot of National Geographic or Sports Illustrated, or whatever your favorite magazine. They're all capable of that kind of quality sharpness. There are some that are better than others, and depending on how big you enlarge your photos and how you're using them, you may want a little bit more sharpness. But you'll pay for it in other ways. Another very important factor is flare, and this is where you're pointing your camera towards bright light. Light comes in and it bounces around, and it causes a scatter, and it causes light ending up where it shouldn't, and you might end up with these flare spots or bright lights where they shouldn't be, and that's not a good thing to have. Now the hood will help out, and we'll talk about that later on. But some lenses are better than others when it comes to the flair. Diffraction. We talked about this in a previous section about light going through a very small aperture opening. Well the amount of diffraction you're gonna get depends on the exact glass you're using, the shape and style and position of the aperture in the lens. So there's a lot of ways to manufacture a lens so that it has less diffraction problems than another design. But that may compromise some other aspect of the lens itself. A common problem on many digital cameras is chromatic aberration. And this is color ghosting. Chromatic is color, aberration is a ghosting. So as light comes through around a solid surface that has a very bright background, it sometimes can change color. So we end up as in this case, we have a bit of a red kind of halo ghosting on one side, and a blue ghosting on the other side. Now there are a lot of ways to correct for this, and it's better if the lens just doesn't have this problem to start with. Distortion is something that you might find on wide angle lenses, but can affect any lens from wide angle to telephoto. Wide angle lenses are often afflicted with barrel distortion and telephoto lenses sometimes with pin cushion distortion. This can be corrected in software, but of course, it's better just to have it right in the lens. Vignetting is a darkening of the corners, and this is gonna be very common in fast lenses, like a 50 millimeter 1.4. The corners will be a little bit darker than the inside. Now some people use this to their benefit, and it actually looks good in a photograph, 'cause they don't want the corners to be too bright, they want them to be just a little bit darker than the inside of the frame. This can be corrected in software, but brightening up those corners can cause noise in that part of the frame, so it's preferred that it didn't have it. Bokeh is the quality of the out of focus area in a photograph. So it might seem kind of strange that photographers are concerned about what's out of focus, but we're concerned about everything in the image, and we want our subject to be sharp and focused. But the background, when it's blurry, there's a lot of different flavors you might say to that out of focus look. There is good Bokeh and there is bad Bokeh, and typically the bad Bokeh will be they'll use the word jittery, it's not smooth and creamy, which is how we would describe good Bokeh. So there is slight differences with the different lenses that are available out there. And then there's the construction of the lens. How well is the lens built? Is it a plastic lens mount or a metal lens mount? How big is the focusing ring? How easy is it to use? How easy are the buttons and switches on the lens to use? Is it easy to hold, does it feel good in the hand? How many screws are in the lens mount? There are a lot of different things that go into the construction of a lens. In a very general sense, you do get what you paid for. Those higher end lenses are usually built to a higher end standard. It's not always the rule, but it's a good rule of thumb that is true in most cases. Now I don't want to spend too much time here on the technology and what goes into making a lens. There is probably a whole 'nother class that dives really deep into what's going on, but I do want to give you just kind of a surface overview of what's going on with the design and building of the different lenses. Glass is a really really funny element to work with, because it's not always as perfect as it may seem. One of the problems when you are projecting light through glass is that light from the different rays, red green and blue, don't always land exactly where you want them to. And so they have something called low dispersion glass which will make sure those light rays end up where they are supposed to. But this glass is more difficult to manufacture and more difficult to deal with, which makes the whole lens more expensive and harder to build. And so you'll find that most all lenses these days are using a lot of low dispersion glass in them in order for you to get the sharpest image possible and the best color possible. Fluorite lenses, which is a rare earth element, is very challenging to work with. But it solves a lot of the problems of these light rays not ending up in the same spot. One of the things that I look at here on this list of you know some of the aspects of the fluorite lens is that it takes four times longer to grind those lenses. And so they have special grinding machines for making each element in the lens. And if you have to leave that lens on the grinding machine for four times as long, that's gonna cause the price of the lens to go up. And so when you buy a higher quality lens, this is part of the price that you pay, and this is what you get, is that you're getting better quality glass in your lens. Now most lenses in most cameras are gonna be of a spherical element design, which means it's a nice smooth curve. But sometimes it's the actual curve that is causing the light rays to not project into the exact area that they want them to. It's not just the glass, it's the shape of the glass. And so recently they've found that aspherical elements will sometimes fix this problem, and it's something that's often used on some wide angle lenses and some compact zoom lenses. And so it's a very challenging way to make lenses, but it's just another tool that the designers are using to get the best quality optics. But you'll often see lenses referred to as having an aspherical element, or maybe it has three aspherical elements, which means it's getting to have a very complicated lens design. There's only a few lenses like this out on the market, but they're very fascinating. Some Fresnel lenses. One of the concepts that anybody who designs lenses says is okay well we have a lens, what if we just designed everything smaller and closer together and tried to do the exact same thing in a smaller sized package? Well the light rays don't end up where they're supposed to. But they have found a secret little way to get around this, and what they've done is they've used this diffractive structure to get the light rays to go where they want them to go so that they can produce a smaller sized lens. It wasn't very sharp so they had to add on another layer that would correct for the problems that they had, but still kept the benefit of being able to get the light rays where they needed to to make a smaller sized lens. So Cannon was one of the first companies to do this. They figured out what a 400 millimeter F4 lens would look like, and they decided to design it with what they called diffractive optics, which is a Fresnel lens. And so what they were able to do is they were able to reduce the size of a 400 millimeter F4 by a significant portion. So they have a 400 DO lens that is available that is 27% smaller and 31% lighter than a standard 400 millimeter F4 lens. And they've done so with their current version at an extremely good optical quality. The first version had some problems with the optical quality but this latest version is really amazing at what it does. One of the things you'll see talked about with lenses and filters is the coating on the lenses. And so beyond the glass and the shape of the glass is light bouncing off of the glass. And this is one of the problems. Any time light passes through glass, it tends to want to reflect about four to 10% of the light. And then it can refract kind of on the back end of it another four to 10% of the light. And some of that light can hit the backside of the glass and back onto the sensor which causes color ghosting, flare, a lack of sharpness, because it's got a lack of contrast to it. And so you want as much of that light going straight through the lens as possible. And it's important if you want to have the right color. Now even a relatively small lens might eat up a lot of light just going through all that glass surface. It's hard to get light to go through that many elements of glass. You'd only end up with 61% if you had an uncoated lens. If you had a really big lens, you might only get 15% of the light entering if you didn't have all this coating on the lens. And so there's many different lenses, any particular lens, lens elements. And most all of them are gonna be coated with multiple layers on them in order to get the maximum amount of light. And so once they've built their lens elements, they've got them in the right shape, they've ground them down, they're all doing what they're supposed to be doing, then they're going to layer them with a multi coating on 'em which allows the maximum amount of light to get through that. Now recently we've been seeing a new type of coating called a fluorine coating, and this has nothing to do with image quality. This is practicality. This makes the lens easy to clean, easy to get dust and water specks off of. I know the current lenses have been much much easier than some of the older lenses at cleaning a fingerprint off of. And so any lens that has a fluorine coating is just gonna make life very very easy to deal with.