Name: 自動對焦如何工作 - Computerphile (How Autofocus Works - Computerphile)
Uploaded: 2021-01-14T10:15:51.000Z
Duration: 18 min 24 s
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未來進行式

So today we're gonna be looking at different ways of doing auto focus with a camera.

程度副詞

So when you're taking a photo with any kind of camera, really?

情態助動詞 B1

So a lot of phones or definitely things like salons like this, you might see an auto focus mechanism.

So here we've got M f for manual focus and then Forte focus.

So if we got it set on auto focus, essentially the camera.

And these days the computer inside the camera decides whether a picture is is blurry or no, and corrects that by moving the lens.

So we're gonna look at a few of the methods.

So the first kinds of focus assist, I suppose you could call it.

So back in the sort of run about the 19 thirties, there was something called range finder.

These are cameras that have a sort of extra separate mechanism on the side of them, which was the range finder mechanism.

Now, this was all done with optics, and the way it works is you basically see a viewpoint of what you're looking at, and then you get a kind of like a ghost image that shifted to one side.

Normally, what you have to do is change a dial on the range finder mechanism to bring the two images kind of lying on top of each other.

So these days it's all kind of joined into the imaging system.

So, generally speaking, there's two different ways you can do auto focus.

There's active methods, and there's passive methods, So an active method will fire something out from the camera on DIT will use that to work out how far away something is.

So some of the most some of the earliest mechanisms for doing active, orto focused used something a bit like sonar, so it would send out a ping from the camera, and it would basically time how long it took to hit something and bounce back, just like the way so no works.

And it would use that to work out a distance that you could then use to focus it and seeking you sound.

You can also use light so some active systems will fire out light from the camera.

What we're actually gonna concentrate on today is the passive approach is how do We actually focus a camera when we're not sending out light or sound, so we're just using on the light that's coming into the lens to do the focusing.

One of the most popular passive mechanisms of auto focus is something called phase detection.

So you might be aware of face detection systems on your camera.

You might have different auto focus points, different areas of an image that you can choose to focus on.

Let's have a look at how face detection works in each one of those regions with a phase detection system.

If this is our lens coming into the camera, what we do is we essentially measure how light behaves at different points on the lens.

So if we have a ray coming from here and we sort of follow it through, so I'm not gonna draw the complicated optics here, your light rays that go through the lens fall upon an auto focus sensor.

So this is really essentially a sort of one D strip of photo died so set of pixels.

Essentially, it's like a little image sensor that the light hits here.

Now the trick behind face detection is you measure to parts of light so you would actually have a second sensor.

So this second light ray here actually hits us a different sensor.

Imagine behind the scenes that this is separated vier optics, but just to kind of simplify things.

So what we get hitting this set of pixels, if you like, is we get to see one of the image features in the image.

So if we've got a very simple image, what we might have here is a little peek.

So perhaps the edge of something on one of those detection points on, because in this case, the images well focused, these two curves will overlay if you imagine our two images hitting our auto focus sensor and they're perfectly aligned in the case where the images and in focus what happens is these beans go through the pixels like this, and they actually focus just behind the pixels.

So what that means is, if we draw our curves, we have one peek that's kind of up here like this, and we have another peek that's a little bit below like this because we're not in focused.

So the nice thing about face detection is that what you do is you measure the officer of these two picks on the distance between them tells you how out of focus they are.

So let's just draw the last case where so this one is kind of focus past the censor, and the other way that you could be focused is in front of the sensor.

So we have light coming up here, perhaps doing something like that.

So when it hits the auto focus senses, they're going to be offset again.

So the two sensors will give a reading off one kind of curve up here on one curve down here, drawing very well, Okay.

And then we get another distance out what these sensors do.

So remember, in reality, there's probably two of these inside the system that the light's hitting.

This is a very simple image that we're making here.

The two curves might be quite complicated, you know, they might be different features that we see They're not just gonna be a straight Pete.

Most of the time, it be some kind of pattern of light that's hitting the senses.

And so the job off the phase detection mechanism is to work out how to move one of these curves so that it lines up with the other one.

So mathematically you can do something called cross correlation there, which is a way of essentially looking at how to best match two signals that are offset from each other on what that gives you is a distance.

And it's that distance that phase detection uses to drive the lens.

So the nice thing about phase detection is that once it's calculated, this distance is very fast to focus because no only doesn't know it's our focus, but it knows by how much.

So once it's calculated this difference, it can say to the mechanism driving the focus lens.

But we also know whether we're focused too far away or focus too close, because if you notice here, the red peak here is above the green peak.

When we focus behind and hear, the green peak is above the red beet.

When we're focused in front by knowing which way to shift these patterns, it knows how far to meet the lens and which direction they face.

Detection tends to be one of the quickest ways to focus the camera.

Most systems use this kind of people do most is so a lot of systems will use both to say a lot of salons will use both.

And the reason is when you're focusing through the viewfinder, it tends to use face detection because it's using the optics of the the lens system to steal a bit of the light and pass it to these pairs of auto focus senses.

So you get a one pair for each water focus region, but you can only do that when you're looking through the viewfinder.

If you open the life, you so that changes how the optics in the camera works.

And so then it will tend to use a process called contrast detection, which will look at now.

That's hitting the imaging sensor, so we're not using the optics in here to divert light around to the auto focus senses.

This is just using the sensor that is essentially used to capture the final image.

What we're gonna be doing is reading off some values of those pixels that make up your image on dhe.

One of the things one of the properties about focus is that the contrast of the image so sort of, the differences between the bribe, its and the dark bits get more extreme.

So when you have nice, crisp focus, you get nice, clear differences between black areas and white areas.

So what that means is if we have a way of calculating those differences.

So how kind of sharp are edges are on our corners are and how different are regions of light and dark are so we can measure our contrast.

We can kind of work out how it in focus we are.

So if we just work through how we would do that, using a really simple example, we could look at some other kind of watches that happen on the way.

Andi think about why it's quite Slater do this as well.

So we've got a photograph here, which I've just turned black and white because it just makes the processing of it simpler.

So we're just using a tool here called Image J, which allows us to descend pretty simple scripting just to get a pixel values and to blur the images as well.

You can just download this, and what's going on here is we can get the values of the pixels on DDE in orderto work out how much contrast we have in the image.

Probably the very simplest thing we can do is just look at pairs of pixels and calculate the difference between them in terms of brightness.

So if we just go through an image, a pair of pixels at a time, and calculate the difference between them when we kind of maximize that the total of all those differences, then we're in pretty good focus because we got the most contrast we can have.

So that's what this simple example here will dig on.

This line here is just calculating the difference between them, So I'm just calculating differences in the ex direction.

So in the row along here, sometimes, and this is true with face detection as well your calculations off contrast or phase, you can either be sort of in the ex direction along the rows or along the columns, or you can get sort of cross sensors that do both in phase detection.

In the contrast here, we're just going to do a neighboring ex pixels.

Okay, so we could calculate all the neighboring.

But just to keep it simple, I'm doing this.

We could use probably a better measure of contrast.

So something like a sub l operator or something else that's good for detecting edges.

But just as a very simple example, Let's just measure the difference between neighboring pixels and see how that changes as we go out of focus, which I'll simulate by blowing the image.

So if I run this, it will move over the image it takes a little while because I'm moving across all the pixels, we get a number here, which is essentially the total of a ll.

Those differences between the pairs of pixels.

So it doesn't really matter absolutely what the number is.

What we're gonna do is try and find the peak okay of these these values.

So, actually, we're starting off in focus here, and we've got a value of about five million.