How Auto Focus in DSLR works?


Active Member

I have never shot auto races, but in principle, I agree with you. In continuous shooting mode (tracking), I don't see why I would need to refocus.
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Leh'd and how!
Another great guide Henry!
I changed the AF button to "AE-L/AF-L" after reading the other guide and it has turned around how I used to focus! Its much more convenient now! Thanks :)

Looking forward for more such tuts on metering and bracketing also!


Active Member
Thanks sir for such a wonderful article.
Though i still have a little doubt.

I came across this article over the net - Achieving Better Focus and Sharpness in Your Images . In the second point "Understand your view finder" it also explains how to AF points work and how to use them but i am able to completely relate what you've posted here and what the article is saying about using AF points.
Kindly take a look at the article and, if you can, please post it here in a little easier language :p

Thanks :)


I am saying the same thing what he is saying. However, this guy is using quoting Canon and I am using Nikon, there could be few difference in auto focusing system though 90% it is same.
Can you ask specifically what point you did not understand from there or my article?


Active Member
I am saying the same thing what he is saying. However, this guy is using quoting Canon and I am using Nikon, there could be few difference in auto focusing system though 90% it is same.
Can you ask specifically what point you did not understand from there or my article?
Thanks sir for reply.
I, again, carefully read both the articles word to word and now its clear to me :)
In the first place, i think i didn't read it very carefully and thus skipped few imp things and hence was unable to relate all the things.

Thanks again. :)


never ever give up
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.

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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.

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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.

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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.

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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.

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

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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!

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

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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.

Note:Some of the illustration is taken from net.
All this, can be/ has to be done in P mode aswell or only in M mode.
Same way does one can tweak metering in P mode too?