Autofocus on DSLR and Mirrorless Cameras
Auto focus. Let's talk about focusing the lenses so one of the things you want to look at in the sl ours is the number of focusing points. Now you can't just simplify it as one number, just like in megapixels. It's more complicated than just choosing this number versus nat number, but higher in cameras have more focusing points, then the cheaper cameras and, in general, all things being equal. Mohr is better than less. Now. The way these cameras work is what's with no one is a phase detection autofocus system, and what happens is that light travels in the camera. It goes to the mirror and the mirrors kind of different type. Amir it's, a partially silvered mirror, which means it let's light through the center portion of the mirror, and it does this so that light can hit a secondary mirror, which bounces like down to your auto focusing system. This is how your camera focuses when you hold it up to your eye and you take a picture and you focus and take a picture is it needs to see exactly...
what you were pointed out. And so this is how it does it in your camera. And so your camera has very specific areas that it can focus at and along with number of picture number. Points that you can focus at what you also want to be generally aware of and there isn't a rating for this but you can clearly see it with your eyes is the area of coverage how much of the frame is being covered by the auto focus points now this is going to be a different importance to different types of photographers sports photographers are the ones that are demanding that one a lot of focus points covering a large area because you could have aa lot of focus points in a very concentrated area and depending on the type of shooting that may not be good or it may be better, it depends on the situation in general mohr area more more points of coverage are generally better when you have a group of point you will have the option to select a single point any one of the ones you want you can select all the points if you want and you can also select on many cameras a group of points on some cameras you even have more options different size groups and these will go by different names and each of these is handy for a different type of reason. In general I use single point when I have a single subject that I want to be very precise about getting focus on I'd like to use group point when I'm photographing action I need a little bit bigger target and if my camera doesn't have a group point, then I would go toe all point for shooting action and hopefully not going to overwhelm you here, but we're going to we're going to really nerd out here in the next section. These things can get very complicated if you really want to get in and comparing the autofocus system, so I'm going to show you the autofocus system for the cannon five d mark three in the way that it works in the camera are some horizontal line sensors, which means they're looking for horizontal lines and as a horizontal line gets passed into the auto focus censor, it crosses this beam splitter and the camera comptel how to focus the lives to correct for focusing but it can only do so on horizontal alliance, and some of the sensors are good at horizontal lines. Other sensors are good at vertical lines they sense when it's broken and how to focus the lens to fix it. So some of these air good at horizontal, some of them are good on vertical. Ideally, I prefer ones that are cross type sensitive, which means they can look for horizontal and vertical lines and the cannon has a whole bunch in the middle that do these cross type wants and then they take it a step further and they have a dual cross all right, so they khun look it almost any type of line and figure out how to focus the lands and so the five points in the middle are the absolute best you may have noticed that these air different colors well there's another level to this you need an f five six lens to make these blue focus points active you need an f four lands to make these magenta ones active and those red dual cross ones you need to have enough to point eight lands which is they're hiring more expensive lenses to make those really work out and so this is one of the more complicated focusing systems out there and in general the focusing points are better in the middle and they taylor off to the edge now these systems have been getting better and better my first autofocus camera had one focusing point and it was good on ly on vertical lines and that was it and so they've been making great strides but there is a limitation to how far they can go with this and we're going to see some interesting advantages of the mirror lis camera in just a moment one of the disadvantages here is that these points are pre selected you can't change to something that doesn't exist out on the edge of the frame and these are very important for people who do a lot of action so if you do focus on dog agility we've got people who do dog agility dogs are jumping through hoops and walking on rails and jumping around if you do that, you want a lot of focusing point because those dogs move very, very quickly. So look for the number of focusing points, how big of coverage area is it covering most of the frame where they kind of concentrated in the middle of the frame? And then you may want to dive in and looking a little bit? What exactly are the specifications of the type of focusing system that's on there? As I say, this is going to be most important for people who are shooting lots of action, and you might want to see if they're compatible with lenses that you have there's another aspect of what exposure levels can you get down to? There are some cameras now that will focused down in very, very low light levels because they have a very good, sensitive auto focus system in their cameras. Now the marylise cameras use a completely different system on hand. They don't have a mere they don't have that con phase detection autofocus that the sl ours have what this house is, it has light coming straight innings straight to the image sensor, and the camera has to just look at that image and decide whether it's in focus or not, and one of the things that you'll have here is you'll have a box and this is a box that you can adjust in size, you could make it smaller, you could make it larger, and one of the neat things about marylise cameras is you can choose whatever size box you want, and if you want to focus way over here, you can focus way over there. If you want to focus over on the left side, you just move it around to wherever you want. You can choose the entire frame to focus on and so it's much more versatile in where you can focus. The problem is, is how the camera focuses and the way the camera focuses as light is coming in on the image sensor, the camera is looking at how contrast e oven image it isthe if it is not a contrast image. So this image is not contrast there's, not a lot of sharp lines. What it does is it tells the lands this ain't good. Do something move, you're not in the right spot, it doesn't know where you should I don't know where you should go, but just move and then it slowly finds its way to the right spot. Now the cameras are getting faster and faster, but in general they are slower. On acquiring their focus and they tend to kind of wobble in this place where as an auto focus sensor they immediately no go here and it will be in focus so they're very quick in that regard some manufacturers have been able to combine the two systems together and they take small auto focus detectors and they bed them within the pixels on the sensors themselves you know obviously if they put too many that's going to degrade the picture quality because they're having to move pixels out of the way and you will see some things called hybrid systems that use a bit of both technology and they help out focusing in many, many situation so let's kind of summarize thes two different systems we have the phase detection which is on the sl ours and the contrast detection on the mirror list cameras the face detection is faster clearly it is definitely better at tracking action and when we get over to the contrast detection system we have unlimited focusing areas it's going to work the same in the viewfinder is on the lcd and it's going to be a little bit more accurate and it does have face detection the cameras also have a facial recognition search system that you can have the camera turned on I'm not a big fan of that because sometimes it gets confused as to which face you want but it is possible to do that so I prefer working with the slr I think it's a better focusing system but the mere list system is more than good enough for a lot of my photography I don't have a problem going now if you learn the system you can work with it and it works really well now we're not done with focusing this was simply deciding where we focus cameras will also have how we focus now this does not vary from camera to camera very much there are basically two ways to focus the first way is in single auto focus and this is perfect when you're going to focus on a subject that is not a movie and so it's gonna happen and let's just take an example we want to photograph the guy up in the day tree and what we need to do is we need to get the active focusing point which in this case is just the area in the middle and we're going to reposition our camera on our subject there we go we're going to press halfway down on our shutter release and now our camera is focused in while our finger is halfway down we're focused locked all right so the camera will not change focus so long as we don't take our finger off the shed a release we now recompose the photograph and we take the picture and this is the way that we can take pictures of subjects that are outside area of where our camera wants to focus this is a system that we've been using since the early days of auto focus and it works very well. The other type of focusing is a continuous focusing system. This is for a subject that is moving towards us or a uefa or away from us. And so in a situation like this, how often have the motor drive turned on so I could take several pictures in a row? I will press halfway down on the shutter release, and when the subject gets close enough, I'll start shooting pictures, and if the camera has a good auto focus tracking system, it will be able to track the subject's moving and keep it in focus every step of the way this was shot with an slr camera. This would be very hard to shoot on a marylise camera. They're not nearly as good at tracking action, they're getting better all the time. Each generation is better than last, but it's still it's going to be several olympics before we have all the olympic photographers shooting with marylise cameras because the sl ours rule this world just dominate it right now, but it really depends on how much you need that as to whether that's an important issue, and so in summary, we have different areas that we can focus at, whether it's mere or a sl ours for focusing modes, all the cameras are going to have single all the cameras are going to have continuous and most all the cameras are going to have a third option, which he's called auto auto focus, where it decides for you whether to two single or continuous. I'm not a big fan of that because it's a little bit unpredictable, but these air not going very, very much between either the muralists or the sl ours is just at the sl. Ours are much better at continuous autofocus e so when it comes to the focus modes and the areas, some things to think about using the single point for the most accurate focusing, for instance, I want to focus on your eye, not your nose and there's, a subtle difference in where they are in the frame and you could be very precise when you have a small focusing are you like to use a group of points when I'm photographing action it's really hard to keep a single point on a subject that's moving around a lot. We have our single and our continues that's. One of the first things that I changed when I go to a sporting event is changing it to continuous single is for stationary subjects, the continuous is for a moving subject moving towards you and moving away from you.