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Auto Focus (AF)

Lesson 40 from: Fundamentals of Photography

John Greengo

Auto Focus (AF)

Lesson 40 from: Fundamentals of Photography

John Greengo

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Lesson Info

40. Auto Focus (AF)

Summary (Generated from Transcript)

The lesson discusses the fundamentals of autofocus in photography, focusing on the differences between SLRs and mirrorless cameras. SLRs use phase detection focusing, which is fast and accurate, while mirrorless cameras use contrast detection focusing, which is slower but more accurate. The lesson also covers different types of autofocus modes, such as single autofocus and continuous autofocus, and the importance of choosing the right focus point. The instructor also mentions the advancements in autofocus technology, such as hybrid autofocus and the Dual Pixel CMOS AF by Canon.


  1. What is the difference between SLRs and mirrorless cameras in terms of autofocus?

    SLRs use phase detection focusing, while mirrorless cameras use contrast detection focusing.

  2. How does phase detection focusing work?

    It uses a specialized mirror and sub-mirror to direct light to a focusing system in the camera, which detects vertical and horizontal lines to determine focus.

  3. How does contrast detection focusing work?

    It judges focus based on contrast levels on the image sensor itself, continuously adjusting the lens until in-focus.

  4. What is the difference between single autofocus and continuous autofocus?

    Single autofocus locks focus when the shutter button is pressed halfway, while continuous autofocus continuously adjusts focus on moving subjects.

  5. Why should one avoid using the auto auto focus mode?

    Auto auto focus can be erratic and may not accurately track moving subjects, making it less reliable than manually choosing single or continuous autofocus.

Next Lesson: Focus Points


Class Trailer

Class Introduction


Photographic Characteristics


Camera Types


Viewing System


Lens System


Shutter System


Shutter Speed Basics


Shutter Speed Effects


Camera & Lens Stabilization


Quiz: Shutter Speeds


Camera Settings Overview


Drive Mode & Buffer


Camera Settings - Details


Sensor Size: Basics


Sensor Sizes: Compared


The Sensor - Pixels


Sensor Size - ISO


Focal Length


Angle of View


Practicing Angle of View


Quiz: Focal Length


Fisheye Lens


Tilt & Shift Lens


Subject Zone


Lens Speed




Depth of Field (DOF)


Quiz: Apertures


Lens Quality


Light Meter Basics




Quiz: Histogram


Dynamic Range


Exposure Modes


Sunny 16 Rule


Exposure Bracketing


Exposure Values


Quiz: Exposure


Focusing Basics


Auto Focus (AF)


Focus Points


Focus Tracking


Focusing Q&A


Manual Focus


Digital Focus Assistance


Shutter Speeds & Depth of Field (DOF)


Quiz: Depth of Field


DOF Preview & Focusing Screens


Lens Sharpness


Camera Movement


Advanced Techniques


Quiz: Hyperfocal Distance


Auto Focus Calibration


Focus Stacking


Quiz: Focus Problems


Camera Accessories


Lens Accessories


Lens Adaptors & Cleaning




Flash & Lighting






Being a Photographer


Natural Light: Direct Sunlight


Natural Light: Indirect Sunlight


Natural Light: Mixed


Twilight: Sunrise & Sunset Light


Cloud & Color Pop: Sunrise & Sunset Light


Silhouette & Starburst: Sunrise & Sunset Light


Golden Hour: Sunrise & Sunset Light


Quiz: Lighting


Light Management


Flash Fundamentals




Built-In & Add-On Flash


Off-Camera Flash


Off-Camera Flash For Portraits


Advanced Flash Techniques


Editing Assessments & Goals


Editing Set-Up


Importing Images


Organizing Your Images


Culling Images


Categories of Development


Adjusting Exposure


Remove Distractions


Cropping Your Images


Composition Basics


Point of View


Angle of View


Subject Placement


Framing Your Shot


Foreground & Background & Scale


Rule of Odds


Bad Composition


Multi-Shot Techniques


Pixel Shift, Time Lapse, Selective Cloning & Noise Reduction


Human Vision vs The Camera


Visual Perception


Quiz: Visual Balance


Visual Drama


Elements of Design


Texture & Negative Space


Black & White & Color


The Photographic Process


Working the Shot


What Makes a Great Photograph?


Lesson Info

Auto Focus (AF)

Now SLRs and mirrorless cameras use different systems for focusing, and it has to do with the fact that the SLR has a mirrored system and the sensor is hidden behind the shutter and behind the mirror when it's focusing, and so they came up with something called phase detection focusing. Mirrorless cameras don't have a mirror. The shutter is open, light's coming straight into the sensor and it uses something called contrast detection, and the idea on auto focusing is it's kind of like getting directions. Where do the lens need to be? Where do I need to do this? And I'm gonna show you some ways in which this works but I think just a good analogy to think about how this works is it's kind of like getting directions from the random person on the street. Has anybody ever stopped to ask somebody where something is? Now there are two different ways that these work. The SLR system would be the random person on the street that says, "Ah, yes, I know what you're looking for." "It's four and a ha...

lf blocks north, down this street." Those are some pretty good directions, and so SLR cameras have been very good about specifically telling where it needs to go, which direction and how far it is. Now, fact of the matter is when you walk four and a half blocks down the street, you're gonna go, "Oh, this is it right here." It's gonna be really close, but it's a good estimate from where they gave you. Now the mirrorless system, that's more like getting directions from somebody who says, "Yeah, I know what you're looking for." "It's not around here." Okay, well I'll go down here, I'll ask somebody else. "Hey, do you know where this is?" "Yeah, you're pretty close." And then you go to the next person and they're like, "Yeah, dude this is it, you are here." And they are right, and so the thing with the contrast detection system is it is looking at contrast and it's just determining whether it's in focus or out of focus, and if it's out of focus it's just like, go somewhere else, move the lens. Hey, this is better, this is better, okay now we're spot on. And so SLRs are really, really fast. They're pretty accurate, and they can be adjusted and we'll talk about adjusting them. But they're really fast at getting there. The mirrorless cameras are really, really accurate, but sometimes they're a little slow on getting there. Sometimes they're a little slow on getting there. This is changing very, very quickly with the mirrorless cameras. Let's look at the SLR and the phase detection a little bit more closely. So what they've done with SLRs in order to focus, and allowing you to focus while you're just looking through the camera is they installed a special mirror in all the cameras. It's a partial mirror. It allows light through a bit of the middle of the mirror, and so not all of the light is getting bounced up to where you can see it. What happens is light goes through the mirror, it hits a sub mirror and goes down to a focusing system in the camera. And that's how your camera can focus while you just pick it up and look through the camera. If you were to look on the front it's very much like the metering system. Just goes down to the bottom and they have a specialized focusing system down there. And you'll notice that it's not measuring it from the film plane. They've all measured this so that it should be exactly the same distance as where the film is, but being off by a hundredth of a millimeter is gonna cause your focus to be off. And that can happen because we're talking about mechanical devices that have certain tolerances. So the way the system works is as light comes through it is looking very strongly for vertical and horizontal lines, and it can tell a mismatch in the lines that are coming through, and what happens is it can tell by the way the lines are offset which direction to go and how far to go, and that's why I say it's four and a half blocks this direction. It knows the distance, it knows the direction to turn the lens, and so it's very quick about making that change in the auto focus motors of the lens. And so SLRs have been very good relatively speaking at sports and action and continuous tracking types of photography. And then they have in some cases they have horizontal line sensors that are specifically looking for horizontal lines, and sometimes they can install two of these so you get horizontal and vertically. Varies from camera to camera, but once again it knows how far off the focus is and which direction to go to fix it. And so it's a very good system. It's not perfect because it's measuring the light down here and not back exactly at the sensor. It's a good, good estimate. It's gonna be fine for most people in most situations. Now just to give you an idea of how complicated this system can be, let me just show you with the Canon 5D Mark IV, which has a very sophisticated focusing system, one of the better ones out there. It has, I forget the number here, but around 61 focusing points, and these ones in the greens here are only sensitive to horizontal lines. Now you have to have a lens that has an aperture of 5.6 or faster, which is all the Canon lenses, but if you add a Canon lens with a teleconvertor, that may have a maximum aperture of F/8, in which case those don't work at all. But in any case with any of their lenses you need a horizontal line for it to grab onto it. Then it has these next group of sensors which can work on horizontal lines at 5. or vertical lines with lenses that are F/4 or faster. So you really gotta know which lenses you're using with this. Now the middle ones are the sweet ones, those are really good. Those work with all the 5.6 lenses and it's cross-type, so it's looking for vertical and horizontal lines at the same time so it does an extra good job at picking things up. But even more special than that are the Dual Cross-Types, so they're looking for horizontal, vertical, and an X pattern as well, so they're picking up on just about everything. Now this is an F/2.8 high precision sensor in here which if you have a lens that goes to 2.8, it goes through an extra step to make sure it is accurately focused. It doesn't do it with the other lenses because they don't have the shallow depth of field, so when you have those shallow depth of field lenses it actually understands that and takes it an extra step to make sure that it's really in focus the way that it's supposed to be. Every camera on the market that I know of seems to have a slightly different tweak on their own system and so what I'm talking about in this phase detection focusing for the most part is Nikon, Canon and Pentax. The more money you spend typically, the more points you get, the more cross-type points you get, the better it is in low light. So it varies from camera to camera but this is just an example of the types of things you'll find out there. Looking at the mirrorless camera, they use a contrast detection system. They don't have the mirror, they don't have the sub-mirror, they don't have this auto focus system in there. They take the light that's coming straight to the image sensor and they're judging whether it is in focus or out of focus by the contrast levels, and so it's doing it right on the sensor itself. And if it's out of focus it says, "Turn the lens," and they turn the lens, and then it eventually goes, "Oh wait man, this is in focus, stop right here." And when it does that it is 100% accurate, it is just dead-on perfect, and so if you want perfect focus you would use this system because it's never off on this, it always understands that. Now there are some situations that'll fool it and it has a hard time working, but when it gets it, it's got it. And so it's a very good system. It tends to be a little bit slower on the system. Now what they've introduced in the last few years is Hybrid auto focus systems that has a bit of the best of both worlds. And what they've done is they've taken some of these phase detection AF sensors and I don't know how they did this, I just know that they did this, and they have made them really small and put them right in between the pixels. Now what I imagine they had to do is they had to take out a few of the pixels and put in the sensors, and then they had to put in a special little algorithm because they're not recording data in those spots so that it would kind of clone over and mask those problems. Now I haven't seen anybody that has discovered image artifacts because of embedded auto focus sensors, and so they seem to have done a very good job at it. But there are some companies that do so in a limited area. I know some of the first Fuji cameras kind of had a block in the middle where we've got the special Hybrid auto focus area, and if you want to shoot action you want to keep things in this box, and then the next generation had a bigger box, and so it varies from one mirrorless camera to the next as to how good it is at embedding these and spreading them out over a large area. Now as we go forward in technology we're gonna see this improve and get better and better and there's gonna be new things that come along, and one other thing that's quite good on the market right now is something by Canon called Dual Pixel CMOS AF, and they've used this on their mirrorless cameras as well as in their SLR cameras. When the mirror's up and it's just the image on the sensor and it's that contrast detection, what they've done is they taken their pixels and they have broken them in two so that one is left looking, one is right looking for auto focus information. So the pixels are actually working like phase detection sensors, and so now they don't need to pull out any pixels, it's all just built right onto the sensor, it's built into the way it works. And one of the nice things about this is that it covers a much larger area than the focusing points do on your camera, so you can focus way up in the corner, not just in this little area in the middle. And so this has been something that Canon has done that's really good and it works in the video and works in their stills. Now it's still compared to their phase detection, horribly slow, in my mind. I would never want to shoot sports, go to the Olympics and use this, no, no, no, no. But it is just much better than contrast, and so it's somewhere in between the two, and so maybe this is gonna continue to get faster so that we can shoot really fast action with it, but it's something that's very good, and so of the mirrorless cameras right now I would say that Sony is probably the fastest, their A/9. I've tried it and it does focus tracking at 20 frames a second, and that arguably might be the best focusing system on the market. I use that to photograph the Blue Angels airshow here. Which I've lived in Seattle all my life, and I have probably rode my bike down to the I-90 bridge to photograph the airshow probably 20 times over the last 30 years, and the experience I had this last year, shooting with an A/ was head and shoulders above ever other experience I've had and I've shot it with Nikon and Canon and other cameras, and 200 to 400s and all sorts of things, because that particular camera could track and being the mirrorless camera that it was it didn't have any blackout, and so I was able to see the planes the whole time. There was no blackout at all, and so it was a very good system. Nikon and Canon have super highly developed sports systems, and so if you look at any sort of sporting event, look at the Olympics and all the photographers on the side you're gonna see a sea of white lenses from Canon, you're gonna see a bunch of black Nikon lenses. Sony doesn't have anything there. They don't have any big lenses for their mirrorless system. They're gonna bring one out later this year I think but they don't have enough lenses there. And so for tracking action Canon and Nikon have been the mainstays, they are the leaders. Sony is starting to peck at them but Olympus and Fuji and Panasonic are bringing on some very good tracking systems including face and subject tracking that is starting to leapfrog some of the things Canon and Nikon can do, and so there's a mish-mash of best items out there. There isn't just one that is the best, but there's a lot of different good options out there. All right. Enough of the focusing basics, let's get into the auto focusing system itself. Now there are two primary ways that we can focus on our camera. The first is with what's called single auto focus, and this is gonna be for most basic situations where you want to focus on something and it's not moving and you're not moving, and you just want that distance figured out. Now, if the subject gets closer to you, you've gotta refocus because it's focused for that one distance where that subject is, or if you get something if something changes, you have to be very careful about that. The next type is called continuous auto focus, and this is gonna be for tracking subjects moving around, forward and back from you, and the cameras can do a very good job at this, so we'll be talking about this focusing mode. The other part of this that we'll get into, and this is just kind of a preview slide right here, is the focusing points. All cameras will have different areas that you can select in which to focus. I remember my first auto focus point. One right in the middle, my camera just had one point and that was it. And wow, the new cameras has three, and now we have a hundred, or 151 or something in some cases, and we'll talk about where you choose to focus in the focus point section. And then the third thing is that you have different buttons on your cameras for controlling the auto focus. There's locking buttons, there's back button focusing, and there's a lot of different ways that you can use your camera to control when the camera focuses, when it doesn't focus, and that can be very helpful for different types of people doing different types of work, just makes that whole process easier, and so we'll get into the button aspect as well. All right, the focus mode is determining how your camera focuses. The first option is single auto focus and this goes by a variety of names depending on what manufacturer you are with. Canon calls it Single Servo, most everyone else calls it AF-S or some derivative of that, which means the camera will focus on a subject and when you press halfway down it then locks that focus and it won't adjust until you raise your finger back up and press it back down again. So looking at this in practice, and for the moment we're just gonna use a single focusing point in the center for simplicity. You press halfway down on the shutter release, the camera knows where to focus and it locks in on that contrast and that detail, it focuses, the focus is then locked on the camera. This is where it is set, you want the picture you press down the remainder of the way. So if your subject is in the middle of the frame this is gonna work fantastic for you for most everything you're gonna shoot a photograph of. If by chance you want to get a little artistic and you want to get your subject out of the middle of the frame, when you press halfway down now there is no detail, there is no contrast, there is no lines for your camera to focus on and it's not gonna be able to focus, and in fact if you press all the way down it will not even allow you to take a picture. You are not allowed to take a picture on your own camera. I don't like that. It's for safety protocol. Most cameras are put into a mode called focus priority, and most cameras you don't have any control of this at all. There's a few higher end cameras that you can turn this on and off if you want, and this means the priority is that your camera, your picture is in focus before you take a picture. And so if anyone has ever tried to take a picture and the camera won't fire, it just won't fire, and that's because your camera's focusing box didn't have anything that it could focus on. It could be because it was low contrast, it could be that it was too close to the camera or there was something else obscuring it, it just wasn't catching on something. And I have found that to fix that you just need to be more aware of what focusing point and where it's at. And if you do that it's gonna focus fine and then you'll be able to take a photo, but this is why you can't take a focus with an auto focus camera is that it can't focus and it's preventing you from doing that. So if you do want to have your subjects off to the side it's pretty simple. You need to use a focus lock technique. You press halfway down, the camera focuses and it locks it in. It is set there. You can now move the camera anywhere you want as long as you keep your finger pressed halfway down, and then press all the way down to take the photo. And this is a really good way of quickly focusing on a subject and then no, let's put it off to the side because I don't want it in the middle of the frame. And so for this part of the class we're just using the center focusing point. Move your camera over, press halfway down, it's gonna get focus, it usually turns on a little light. You press down the remainder of the way and you get yourself a shot. And so a lot of times, and we'll talk more about this in composition. I know there's somebody out there, "Why would you want to take a picture of something" "that wasn't right in the middle of the frame?" We'll get to that part in the composition section later on. So, focus on your subject, move it over to the side. This is really important if you're gonna take a picture of your two best friends, because they're gonna be standing beside each other and if you point it right in the middle it's gonna focus on the background between their heads. And so it's important in a lot of different situations. So that is single auto focus. The next focus is generally called continuous auto focus, and this is where the camera will track subjects that are moving towards you or moving away from you. Now, this is usually called AFC by most manufacturers. Canon calls it AI Servo, which I think is a terrible name, but they do it. So AFC and continuous by a variety of other manufacturers. Now what's happening here is that when you press halfway down the camera's gonna look for proper focus, and then what's gonna happen is as that subject moves around it's gonna continually check so it's always checking, "Where is the subject," "where is the subject, where is the subject," and it's constantly adjusting for it. So as the subject comes towards me and I press all the way down, and the other feature I have turned on here is I have the motor drive turned on so I can get lots of pictures because I never know what the best moment is going to be. And in this case what's actually going on is it's not just continuously focusing on the subject. It's predicting where that subject is going to be so it looks at the movement. So it's tracking me, it's looking at my movement here, and here, and it's estimating that when I get here at this time this is where I'm gonna be. And when I get here I'm gonna be here, and so it's estimating this. And this might not be perfect if you're photographing a soccer player that's running forward, and then stopping, and then running forward and then stopping because they're changing speeds. And so consistent speeds it does a much better job. Well, I should say not a much better job, it does a better job, but with inconsistent speeds it's quite possible that somebody's stopping faster than the camera's predicting and they're gonna be a little out of focus. But that's just the nature of the phase detection system in most of the SLRs that are doing this. So this is continuous, it works out quite well, but you do have to have your focusing points on your subject for being able to track this sort of action. Now when it's in the continuous focusing mode you may notice if you do a lot of this photography, you're gonna get some out of focus pictures and it's partly because there's a lot of random movement that I just talked about, but your camera is inherently in something called release priority. Which means the priority when taking a picture is that you can release the shutter. It's not for focusing. And there is a strange dynamic going on. I mean imagine your camera is a business. It's got two priorities. Number one, focus accurately. Number two, focus fast. Now those two things are gonna be in competition because you could imagine one group is going, "Hey, we're not in focus yet," and the other team says, "We need this picture now." "Well let us get this a little more right," "No we need the picture now." And so the cameras have to make some pretty quick decisions like, "Is this good?" "It's good enough, go to the next one." And so what's gonna happen I have found is that the percentages of pictures that you are gonna get goes up with the amount of money you have spent on your camera and lens. The lower end cameras don't have as good auto focusing system, and they've gotten better over time, and they've gotten where you would get 20 or 305 accurate to maybe where you're getting 50 or 60%, but when you get to the pros, they're getting it right like 95% of the time. And it depends on how complicated the movement is as well. A football player with referees and players crossing in between can be really, really hard to track. A car racing down the street that's got a lot of detail, that's got no interference, it's going at a steady speed that's gonna be pretty easy and you might get a 100% on a regular basis on those sorts of things. So the release priority can be turned off on some cameras so you're only getting in focus pictures. But you might be missing really important moments, and sports photographers wanted their cameras to be in this mode because they have found that the cameras can be a little too fussy about taking pictures that are only in focus. What would happen is they would have a great moment and it's misfocused by two inches, but at the reproduction size that they use no one's gonna notice it, it's just kind of a fussy detail, and so most cameras, all cameras that I know of will be in release priority. So you are gonna get some out of focus pictures in challenging situations. But when you do have good contrast on your subjects, subject moving at a relatively even speed, the cameras do a really good job. And if fact even some that are erratic movement can do a really, really good job. And so it's just a matter of getting your camera set up. So when I go to a sporting event this is usually the first thing I change in my camera. Even before I change my camera to manual or put the shutter speed any place, turn the motor drive on, is I want continuous focusing. Because that is just the most important thing, otherwise everything's gonna be out of focus. So we have single and we have continuous, and that's it, with the exception of one more thing, but those are the only two ways that a camera can focus. Now many cameras do offer a third alternative way which is Auto auto focus, and this is where the camera looks for movement and makes a decision for you. So we're letting the camera make a choice instead of us. And this is a disaster. This is horrible. And if you want to know how bad this is, when you buy the top of the line Canon, or the top of the line Nikon camera, it's not even optioned. It was so weird when I was doing one of my early fast start classes on cameras and I was working with one of the higher end Nikons and I went to the previous Nikon class, was more the intermediate level and I was kind of copying and pasting over a lot of the things that it did, I'm like, "Where this feature?" How can the top of the line camera not have a feature that a lower end camera has? And it's because they know professionals don't want this, and it's because it's a little bit erratic. It doesn't know intrinsically what's going on the way you do. When you go to a sporting event, let's call it a dog agility event, that's kind of fun to shoot. You know these dogs are gonna be running around. Your camera has no idea if you're shooting portraits or action shots, but for us it's a real easy call. Are you shooting motion, or is it still? Is it running around or is it just sitting there? And that's why we want to choose either single or continuous. When you throw your cameras into the green autobox mode, this is where it puts the auto focus. And if it starts to move, it may track it, I'll give it that. It may, or it may not. A basketball player, do they move or not? Well sometimes they're moving quite quickly, but then other times they kind of stop when they're guarding somebody, and then they start running again. Same thing with soccer players. Your camera might decide, "Oh, this person's still, we need to do a portrait." And so it doesn't do the continuous focusing and so I would say never ever, ever use auto auto focus. Auto focus is fine, but auto auto focus is not good. So single or continuous. And for instance Fuji they don't even do that sort of thing, they don't even give you an option on that one.

Class Materials

Bonus Materials with Purchase

Fundamentals of Photography Class Outline
Learning Projects Workbook
Camera Keynote PDF
Sensor Keynote PDF
Lens Keynote PDF
Exposure Keynote PDF
Focus Keynote PDF
Gadgets Keynote PDF
Lighting Keynote PDF
Editing Keynote PDF
Composition Keynote PDF
Photographic Vision Keynote PDF

Ratings and Reviews

a Creativelive Student

Love love all John Greengo classes! Wish to have had him decades ago with this info, but no internet then!! John is the greatest photography teacher I have seen out there, and I watch a lot of Creative Live classes and folks on YouTube too. John is so detailed and there are a ton of ah ha moments for me and I know lots of others. I think I own 4 John Greengo classes so far and want to add this one and Travel Photography!! I just drop everything to watch John on Creative Live. I wish sometime soon he would teach a Lightroom class and his knowledge on photography post editing.!!! That would probably take a LOT OF TIME but I know John would explain it soooooo good, like he does all his Photography classes!! Thank you Creative Live for having such a wonderful instructor with John Greengo!! Make more classes John, for just love them and soak it up! There is soooo much to learn and sometimes just so overwhelming. Is there anyway you might do a Motivation class!!?? Like do this button for this day, and try this technique for a week, or post this subject for this week, etc. Motivation and inspiration, and playing around with what you teach, needed so much and would be so fun.!! Just saying??? Awaiting gadgets class now, while waiting for lunch break to be over. All the filters and gadgets, oh my. Thank you thank you for all you teach John, You are truly a wonderful wonderful instructor and I would highly recommend folks listening and buying your classes.


I don't think that adjectives like beautiful, fantastic or excellent can describe the course and classes with John Greengo well enough. I've just bought my first camera and I am a total amateur but I fell in love with photography while watching the classes with John. It is fun, clear, understandable, entertaining, informative and and and. He is not only a fabulous photographer but a great teacher as well. Easy to follow, clear explanations and fantastic visuals. The only disadvantage I can list here that he is sooooo good that keeps me from going out to shoot as I am just glued to the screen. :-) Don't miss it and well worth the money invested! Thank you John!


Dear John, thanks for this outstanding classes. You are not only a great photographer and instructor, but your classes are pleasant, they are not boring, with a good sense of humor, they go straight to the point and have a good time listening to you. Please, keep teaching what you like most, and I will continue to look for your classes. And thanks for using a plain English, that it's important for people who has another language as native language. Thanks again, Juan

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