Focusing and Autofocus with Nikon® Lenses
Okay, so we've got our lands held design. We got our elements in place. It's time to figure out how to make this lens focus. All right, this is beginning lens making here that we're doing so let's. Just make an easy lands. The easiest lens to make is one where all the lens elements moved together and focus the light under the sensor that's the easiest system have now, from a photographer standpoint, I don't like this thing poking out the front of the lens. That means I can't use a filter on it, or it might cause some other sort of problems changing the weight. Remember, on some older lenses, they were so big and heavy with weight, the centre of balance would change. Now we're gonna put this in a little housing. We're gonna put that on a track. And when we auto focus, the other focus system will run that back and forth. And so that's what's happening in some of our most basic fifty millimeter, one point, eight lenses on lenses of that design and sew. All the elements are gonna move in t...
hose now, if we wanted to find design a little bit more advanced. Lance there's, a lot of other things we can do, we can do something r f rear focusing, and this was a nice innovation back, probably in the seventies and eighties where what they decided this behind wherever the aperture is, they were going to design it. So just a few elements in the back move, the less movement, the less weight changed, the faster we can get auto focus to work in the camera so we're focussing is better than having the entire unit move, and then I f if inter focusing, which means basically elements somewhere in the middle of the lens air moving, which is kind of nice because they're not exposed to elements light, they say elements, I'm thinking like rain and dust moving back and forth, so its elements in the middle of the lens moving back and forth and I could I can't imagine how difficult it is designed the lands and say, can we make that focus in the middle rather than at the back or the front? Because that changes the whole set of optics that you're using in there, then there's times when they need to have multiple groups moving at the same time. Now this is where it gets really complicated. We have two groups moving in opposite directions in order to maintain focus as we're zooming this lance and so it's amazing that this works. And so if you do this right, you're going to reduce the size of the lens, which is always nice to have, and you're gonna get a lens that has fewer aberrations and just better image quality. Now, another little issue to deal with is that lenses are designed for optimum image quality at a distance, and that distance is usually infinity that's her lenses were designed to be their best, and when you focus up closer, sometimes the focusing isn't as good. And so they have designed a system that has a floating lens element it's not particularly designed for focusing the lens, but it is designed for compensating for ares that happened when you focus up close. And so this little floating lens element will move independently of all the other focusing elements in order to maintain the best image quality possible. Nikon calls this close range correction, so you'll see a crc when it has that particular feature, and this is going to improve the image quality. The cue when you are focusing up close and it's going to help out a lot when it comes to thie aberration fluctuations, which is those chromatic aberrations that we've been talking about, also on spherical aberrations, so that's, where things are not focused in the middle properly, and it also can reduce flare as well as distortion and so one of the things we're going to see is we're going to see this a lot on lenses that focus up very close micro lenses and so if we look at this list, we have some white angles and a lot of their micro lenses that have this built into the last something else to be aware of and it's not something that has talked about from nikon is the rotation and lens extension of a lands we knew focus the lens when you zoom lens does it extend? Does it expose itself more and that's something that I want to know kind of from a durability standpoint, I want to know if my filters air going to rotate on the front of its if I have a polarizer that's going to change things doesn't make things a little mechanically more vulnerable. And so uh whether resistance to dust and water that's also going to play an impact this first how much stuff is moving in and out now there's a large collection of lenses that have all internal movements, which is what I would prefer everything inside the limbs on other ones due to the complexity of the design or their desire to make an inexpensive lens they've had to go with an external movement in order to make it happen and so a couple of lenses with different types of external movements this one on the left that could be protected with the filter because it's all within where the filter is but the one on the right is a really wide angle lens and you're not going to be able to protect that from a filter but it's just necessary due to the design of the lamps this next group here you can see lens elements moving back and forth where we could suck in dust water or out of the other sorts of problems on the right hand side you'll notice those threads are spinning as we rotate in and out, which means if we put a polarizer aura filter on there it's going to shit change orientation as we focus our lands back and forth which is less desirable but it's a lot easier to make in the construction process and this is where you get what you pay for a type all right you pay a little bit more you're getting extra features sometimes they're small and you don't notice them at first and so here's a list of lenses that have all internal movements and so yeah I kind of like these lenses now I'm not gonna pick a lens and say oh, these are the only lenses I'm gonna pick from because there's some great lenses that have external movements that you want for other reasons but just be aware of what your lenses d'oh some of your lenses will have a limit switch on the focusing and this enables you to either reduce or expand the search area of where your camera's gonna auto focus so that you can achieve the fastest best auto focus the situation you are in let's say you're at the end of a field if you want you could put it in full which allows you at least on this lens to focus from three point six meters to infinity that's a good general per place to put the lens but if you're photographing a sporting event, chances are you're not going to be photographing people really close to you they're probably going to be twenty feet eight meters or so beyond and so if you don't have ask your camera toe look in this close up area you could reduce the search time for focusing by fifty percent make it much much faster if you're only looking in the right area for where to focus on the subject you will also see this on macro lenses and macro lenses have a very big focusing throw from close up to distant work and so for general purpose yeah you can leave it in full so it can focus close up and far away but if you know that you're only going to be focusing on things further away you can reduce the focusing time increased the focusing speed now the speed of the lenses still focusing at the same speed but you're reducing the area that it's searching just to where you know your subject is going to be and so some lenses will have that focus which and that's what it's for the focusing scale we talked a little bit about this before some lenses have it someone's is don't one of the features to be aware of when you're looking at the lens doesn't have the skill because these air very similarly priced lenses but to get that extra bonus feature and it's really nice for manual focus photography and various areas where did the lens focus? You look down on the lens and actually see it when you are focusing on your lens kind of the default position is infinity as you focus closer lens elements move away from the film plane and when you focus back to infinity they moved back closer to it to the body now in this one of the things you'll notice up here is the infinity scale is that you can often go past infinity lenses are designed to focus beyond infinity not that there's anything to focus on beyond infinity but you wouldn't want your lens to focus right at infinity because if you were to go up into the mountains where it was really colder high elevation or hot it may change the heat and cold expansion on your lens and you may not be able to focus all the way to infinity so that they let you go a little beyond just in case the parameters changed ever so slightly on your lands. This is important for anyone who does nighttime photography and they go outside, they go well, I want to take a picture of the stars so I'm gonna take my lands and go all the way over to infinity and point my camera and take a picture and they because they went all the way to the end probably went a little past infinity. And so sometime when you do have something really far away to look at, look at where that is when you are focused on infinity because it might be a little to the left, it might be a little to the right. It might depend on the temperature something else to take a look for in the distancing scale is this little tiny dot here and this is for infrared. If you were teo, use an infrared type filter on your camera. Infrared light focuses at a different distance than does the light that we see with our eyes. So the visible light and infrared light focus in different places and it's usually just a very tiny little subtle thought that doesn't say anything about it. But that's what it's doing a note about your cameras in an slr camera there is a focusing screen that we used to have interchangeable, but for the most part it's built into all cameras with little chance of interchanging but that focusing screen has changed over the years from manual focus to auto focus. When we made the change from auto focus to manual focus they found out that because of the autofocus system that was in the camera they had to make a change in the focusing screen and they had to make a decision and they had to decide do you want a bright viewfinder or do you want an accurate view finder and this is a really good debate brightness over accuracy and what one out might surprise yu brightness dead and the reason brightness went out it's because they said you know what would really be nice to have a three five two five six lands because we could sell a lot of these lenses because they're small they're lightweight in their cheap and people can see through him if they're bright enough and the downside is for those of you who buy really expensive fast lenses fifty eight one for eighty five one a one o five to as examples the view that you get in the viewfinder is not one hundred percent accurate you're focusing in the right area but you are seeing the depth of field of a two point eight lands and so when you shoot with an f one point eight it's gonna look shallower when you look at the final photograph then when you look through the viewfinder and because of this it makes manual focusing a very fast aperture lens challenging to do with any great accuracy and so for those who do have the faster lenses I often recommend using auto focus even though I like to manually focus but in some of those cases where your shooting really shallow depth of field your camera will probably do a better job than you will all right let's bring in auto focus now we have a manual focus lens here they still sell a few of these you still find a lot of money use market so this is going to be manual focus on every camera there is no way to make a lens autofocus at any reasonable price and so there will be limited exposure compatibility on newer cameras depending on the exact era of the land next up was nineteen eighty six the introduction of the autofocus system this was the driveshaft system we've talked about here it needs a camera with an autofocus driveshaft notably not on the d three thousand siri's or five thousand siri's cameras next up was the a f s kind of through the fbi and there's well there's only few lenses that this has the silent wave motor which will also be abbreviated s w m that you will see on your lenses sometimes in the letters following the description of the lens and so this is a very fast quiet focusing motor but it is limited with some of the earlier auto focus cameras especially before nineteen ninety so you gotta be careful a little bit with which cameras you're using it with there is a switch on the side of all the modern lenses all the f s lenses that allow you to manually focus or auto focus and it's a pretty simple switch when it's just a temp the key thing to know about is in auto focus don't grab the lens and try to turn it it's gonna feel kind of stiff and if you force it you will be able to turn it but you are not doing any good to the lance it's damaging the motors the higher end lenses will have an m am switch manual focus and then auto focus with manual override so there's essentially a clutch system built into the drive that allows you to manually grab the lands and just turn it whenever you want no damage done to the lens at all and then the third option is only going to be seen on some of their violent very highest in lenses and option for manual focus an option for auto focus with the manual override and then a subtle slight different version of it a slash am auto focus with manual override it requires a little more torque and so on these big lenses this has an a it has the street three position switch here on the side of it and that is because a photographer who's using this might just want to grab this and focus whenever they want. But they might mostly wanted to take care of it. And they, if they just bumped this, they don't want it to start to focus. And so it needs a concerted turn before you start manually focusing. And so you're only going to find that on some of the bigger lenses like this.