How Auto Focus in DSLR works?

hensil

Guru
Auto Focus in SLR

Autofocus (AF) or call it power-focus because a small computer is used to run the motor to focus the lens. Focus is the moving of the lens in or out until the sharpest image is formed on the film or sensor.
There are two types of autofocus systems: active and passive. DSLR cameras with interchangeable lenses use the passive system.
Active autofocus on cameras uses an infrared signal, and is great for subjects within 20 feet (6 m). For example, the camera uses a system that reflects an infrared pulse of light off the subject and looks at the intensity of the reflected light to judge the distance. Infrared is active because the autofocus system is always sending out invisible infrared light energy in pulses when in focus mode. This system has a problem in candle lights(infra red) which confuse the camera, black subject absorbs infra red.

Passive autofocus, determines the distance to the subject by computer analysis of the image itself.
A typical autofocus sensor is a (CCD) that provides input to algorithms that compute the contrast of the actual picture elements. The CCD is typically a single strip of 100 or 200 pixels. Light from the scene hits this strip and the microprocessor looks at the values from each pixel.

001-In Focus Scene.jpg


002-Out of Focus.jpg


For good Autofocus, the image needs to have enough light and some detail in it that provides contrast. If you try to take a picture of a blank wall or a large object of uniform color like the sky, the camera cannot compare adjacent pixels so it cannot focus.

002a-in-out focus copy.jpg


The cameras focusing screen has several focusing points, some are cross hair and some vertical and some horizontal. These strips are made of pixels to detect the contrast in a scene. The information is then sent to onboard micro processor which calculates the distance and rotates the focusing motor. Obviously the cross hair which has more pixels on both axis performs better.

I take D70 auto focus for illustration. D70 has only five focusing points, the center is the cross type and two vertical and two horizontal. You can assign only one focusing point (Single Area) or all five focusing points (Dynamic Area). In (Single Area) mode the focus lock is quicker because the micro processor has to calculate for only one point. Whereas in (Dynamic Area) the focusing is bit slower because the micro processors power is divided into five.


006b.jpg


Focus and Recompose - Single Area (One Point)
In default mode, the center point is active. You can change the point from the joy stick but only the center point is cross hair. this mode works well for static or very slow moving object.

010-Single.jpg


1. Compose the picture so that the subject is either in the left third or the right third of the picture. (This makes for pleasing pictures.) You will come back to this position.

007-Compose scene.jpg


2. Move the camera right or left so the square brackets in the center of the viewfinder are over the actual subject

008-Focus scene.jpg


3. Press and hold the shutter button halfway down so the camera focuses on the subject. Keep your finger on the button.
4. Slowly move your camera back to where you composed the picture in step 1. Press (squeeze) the shutter button all the way down. It may take some practice to do it right, but the results will be great!

009-Recompose scene.jpg


Sports and Action - Dynamic Area (All Five Focus Points)
Use this mode while tracking a moving subject and use little wider DOF.

011-Dynamic.jpg


As you keep the shutter button depressed all the five focus points will be active and will look for contrast in subject. If the center point catches the bird the focus is locked, when the bird moves out of center point the other outer points pick the subject and keeps it in focus.

Henry
Note:Some of the illustration is taken from net.
 
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hensil

Guru
Again a wonderful tutorial, thanks Henry.

So you recommend focus and recompose method?

I am little confused on this and earlier asked the question in a different thread. Here is what @arindamdas said about it:

http://www.bcmtouring.com/forum/photography-f17/do-you-use-af-button-focusing-t41841/#post578562
Thanks.
Here is my set-up copied from the other thread

My Focus Set-up

I use auto focus using the dedicated AF button. I have removed focusing from the shutter release button. This includes the shutter set to Release Priority.

This involves three set-ups.

1. Using AF button to focus. If your camera doesn't have this, you can program the AF-E/AF-L button to AF from the menu.

2. By default the Shutter Release button performs two task. First on light depress it will focus and second with hard press, release the shutter. You need to set-up from the menu to perform only the shutter release and remove the focus option.

3. There is another set-up in the menu that says Focus priority and Release Priority. By default it is Focus Priority. This setting prevents the shutter to click until the picture is in focus. You may have experienced this at some time. It helps in preventing out of focus pictures. I have left it on Release Priority, that means the shutter will click regards less of picture is focused or not. I have a valid reason for this set-up and will explain some other time.

Other than this set-up I use AF-C for action shooting with Single point or Dynamic Mode depending on how fast is the action and AF-S with Single point for everything else.

I now its bit difficult to comprehend for a new person to DSLR. Then to get the best out of DSLR you have to understand this.

Now to answer your question; I do actually opposite to what arindamdas said.
When shooting sports where the action is quite fast you do not have enough time to focus and recompose. Before you click the subject has moved from its initial position. The focusing has to be AF-C, i.e continuous and the shutter button should be used to only click the shutter.
I think this discussion will make things clear. Please ask question if not clear.
Henry
 

arindamdas

Active Member
I think there's confusion here. What I said in my post is, I use my shutter to both focus and meter when shooting wildlife/sports. I am not sure where I said or implied that I use focus and recompose for these. Henry?
 

asheshb

Perfection, is it worth?
Thanks Henry for the explanation. What I have understood so far about Focus and Recompose(F&R) is:

1. For action, F&R has no context.
2. Landscape usually F&R would not play much role since the distance would be too large to matter.
3. Portrait: This is where F&R can play a role at wide aperture for close ranger subject.

#3 What is best? F&R using AF-C or choosing the whatever focus point falling on the subject.

I will do the experiments but it is always good to hear from the masters.
 

hensil

Guru
I think there's confusion here. What I said in my post is, I use my shutter to both focus and meter when shooting wildlife/sports. I am not sure where I said or implied that I use focus and recompose for these. Henry?
Sorry, I misunderstood you. So what you meant is you meter and focus using the shutter button but use relevant focusing points using the joy stick?
This also works nicely.
Henry
 

arindamdas

Active Member
That is exactly what I meant, Henry. No worries. Bari bari 'alochona' mein choti choti misunderstanding hote hain (Small misunderstandings do happen in big discussions) :)

And ashesb, I am no master, but I do have an opinion. I would try to compose first, then focus using the closest focus point. It would be good to hear from those who regularly shoot portraits.
 
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hensil

Guru
Thanks Henry for the explanation. What I have understood so far about Focus and Recompose(F&R) is:

1. For action, F&R has no context.
2. Landscape usually F&R would not play much role since the distance would be too large to matter.
3. Portrait: This is where F&R can play a role at wide aperture for close ranger subject.

#3 What is best? F&R using AF-C or choosing the whatever focus point falling on the subject.

I will do the experiments but it is always good to hear from the masters.
Yes, you have understood very well. Also, to make thing more clear

1. Correct. When you focus and then recompose the subject has already moved.
2. Correct if you shoot at wider dof or at infinity, but F&R will work for landscape if the dof is narrow.
3. Correct. While shooting portrait if the dof is wide enough to cover the whole face the F&R will work. If the dof is very narrow the F&R will not work because the little movement of the camera to recompose will off set the focus. Here using the appropriate focusing point is the best option.

Henry

---------- Post added at 12:44 PM ---------- Previous post was at 12:33 PM ----------

That is exactly what I meant, Henry. No worries. Bari bari 'alochona' mein choti choti misunderstanding hote hain (Small misunderstandings do happen in big discussions) :)

And ashesb, I am no master, but I do have an opinion. I would try to compose first, then focus using the closest focus point. It would be good to hear from those who regularly shoot portraits.
Thanks. This kind of discussion helps us to learn.
As you mention, using shutter for focusing works fine for shooting portraits. But for fast action sports like auto race it does not work for me when shooting multiple Continues frames. This is because after every frame the shutter button will try to refocus and slow me down or get some frames that are out of focus in that sequence. This is the precise reason I use AF button for focusing.
Do you agree?

Henry

Henry
 
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