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I have a Canon 550D with 70-200 f4 and 50mm prime lenses. Although the camera has built-in 9 focal points, I sometimes face a strange problem. Here's a sample photo:

two people, one in focus

In reference to the above pic, you can see that both of the faces are NOT in focus. I used 70-200 f4 for this shot at max aperture. Still, I couldn't get both of them in focus. Why? I also try decreasing the aperture to attain a greater DoF but still, I couldn't get both of them into focus.

Another thing: Although the cam has 9 focal points, why can't I use all of them to focus, say, 9 points in a portrait shot? In manual mode, it has the option to select one of the 9 focal points. And in auto mode, it sometimes select more than one focal points.

This is just an example image. I've faced this problem zillions of times.

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6 Answers

up vote 12 down vote accepted

The auto focus points visible in your viewfinder approximate the key areas that the AF sensor is able to measure. The resolution that the AF sensor processes at is pretty low, and how it processes is not exactly how our eyes see focus, so what it "sees" as being in focus may or may not be entirely sharp in the final image captured by your sensor. As such, when the AF sensor detects things as being "in focus", its reading is fairly general, and not as precise as one would expect or hope much of the time.

See this for an explanation of Phase-Detection AF used in most DSLR's.

Regarding depth of field, that can be a complex topic. To start, if you use all of the available focus points with AF, and those points cover parts of a scene with significantly varying depth, then not all of them will necessarily be fully in sharp focus. With Canon DSLR's, when a set of points have "focused", they tend to blink. It is pretty rare with any scene containing a measure of depth to blink every focus point...usually only a locus around the area that is mostly in focus will. If you focus on a flat surface, such as a wall, you should be able to see all focus points blink indicating all 9 points have detected in-phase samples of the image.

If you need to have a very deep DOF, you have a couple of options. Obviously, tightening down your aperture will increase your depth of field, but within certain limits. The closer you are to the subject you wish to focus, the shallower you depth of field will be. At considerably great distances, such as you might encounter when photographing a landscape, your depth of field can be immense (but not infinite.) When simply stopping down is not enough (or stopping down softens the image too much due to diffraction), you can always opt for a different lens.

A wider angle of view (short focal length) will allow you to get extremely close to the key subject you want in focus, bringing with it a shallower depth of field. While you have a narrow depth of field, you will still be able to see the general shape and structure of the background, as wide angles tend to either 'decompress' the background behind the subject in focus, or at the very least leave it "normal" (i.e. with a 50mm lens on a 35mm sensor.) A narrower angle of view (long focal length) will allow you photograph the same scene from farther away, thereby extending the depth of field. With a narrower field of view, anything that is behind your subject will become more 'compressed' and blurred, further isolating your key subject. It is a very fine balance between depth of field, angle of view, and background compression. With the right range of focal lengths and a calculated approach, you can likely find the right focal length and distance to subject to frame your subjects as you desire while maintaining the proper depth of field.

It is important to note that there will always be a fairly thin plane within which everything is truly in perfect, sharp focus. Beyond that plane, things will begin to blur, although they may blur below the threshold where the human eye can see it. The larger you view the final image, the narrower that plane of true focus will appear to be.

Regarding the sample image you posted. First, what was the smallest aperture you used? You might be able to use a smaller one. You may also try backing up a bit to deepen your field, and crop during post processing to get the same framing. If you have already tried a smaller aperture, I would suggest trying a longer focal length. The two key subjects are relatively in a strait line from the center of the lens, so backing up and using a longer focal length at a greater distance should improve your depth of field without losing much in the way of subject framing. A longer focal length will narrow the field of view, so less of girl in the pink shirt will be captured, at a greater magnification. That will cause her to become even more blurred, possibly beyond the point of recognition (which may be desirable, as it would help isolate your forward subjects.) It may take some experimentation to find the right focal length, and a zoom lens (such as the excellent 70-200mm lens you have) that will let you vary the focal length would be most useful. I would try shooting near the 200mm end at a greater distance, with a moderate aperture of f/8-f/11, and see how things go.

Finally, if AF isn't quite getting it, you should never be afraid to switch to manual. Sometimes AF just can't lock onto your subject, especially with the limited number of AF points in a lower-end body like the 550D. If you have a tripod, Live View zoomed in 10x can be the best tool to get superb focus. Just remember to use the DOF Preview button (small button, front of the camera, just below the larger lens mount release button.)

Here are some great articles on the subject of focus and sharpness:

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+1: The king is back! –  Alan Nov 9 '10 at 7:40
    
It's always good to be back. ;-) Although I can't complain about where I've been...being out using my camera and taking pictures is always the best. :D –  jrista Nov 9 '10 at 7:43
    
+1: For descrption. @Rish I recommend You also read this kenrockwell.com/tech/unsharp.htm –  Rafal Ziolkowski Nov 9 '10 at 11:16
    
@Rafal Thank you for the link. Really helped :) –  Rish Nov 9 '10 at 12:36
1  
@Rafal: Thanks for the Ken Rockwell link. I should have remembered that one. I'll work it into the answer. –  jrista Nov 9 '10 at 17:10
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You've got some good answers already about how focus works but here's the answer to your focus-points question.

The camera may have any number of focus-points (9, 11, ... 51, it does not matter) but focus is always at ONE focus distance (the distance in front of the sensor at which things are in perfect focus).

When you are using one focus point (and you choose which one), the camera measures the distance at THAT point and sets the focus to the measured distance.

When the camera automatically chooses the focus point, it measures the distance at all points. It then decides which one has priority (depending on the camera, it could be the closest to the camera or the closest to the center) and sets the focus-distance based on that point. THEN it lights up ALL the points which are in focus, which always includes the chosen point but can include other too if they just happen to be in focus as well.

And to confirm what's already been said: Using the widest aperture you got shallow depth-of-field so not many points end up in focus. Use a smaller aperture and you'll get more.

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So there are two issues at play.

First, the image isn't properly in focus. DoF has nothing to do with this. It's hard to tell, but I suspect the man's hair is where the focal point is.

Second, as you mentioned, the DoF is too shallow, so the woman is blurred.

The 70-200 is a telephoto lens, and as such, unless you are shooting an subjects that are far off in the distance, you will still have too shallow of a depth of field to have subjects that are placed that far apart (just based on her knee, I'd say at least 2 feet behind the man).

With regards to your question about AF points, it's a little unclear what you are asking. Even in manual shooting mode, you can have all "AF Points" active, and the camera will attempt to determine which AF point should be used (I think this is what you are asking about).

If you are planning to continue to use AF mode, I would stop using any AF point besides the center point. The center AF point is a higher sensitive cross-type point (@f2.8 so not as good on your 70-200F4), and will give you better accuracy. You can use Focus-Lock/Recompose to frame your subjects as you need, without having to rely on the other AF points.

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If you really do want to focus at all 9 AF points, maybe try using the A-DEP mode on the 550D?

A-DEP stands for "automatic depth of field" - it works out the closest and furthest focus point, picks a focus point midway between them, and then attempts to change the aperture so that the depth of field is sufficient for all 9 AF points to be in focus.

Personally, I usually find that I don't really want all the AF points in focus - normally, at least one point is over some random thing in the background - so I can't say I use A-DEP much myself, but it's there if you want it.

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A-DEP tries to get as many points as possible in focus, not all, but definitely the way to go. –  Rowland Shaw Nov 9 '10 at 12:48
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  • When all AF points are enabled the camera tries to take a reading from all points. Some of these points won't yield a reading if they don't correspond to an area of contrast in the image (such as a line or edge). Out of all the points that produce a distance reading the camera picks the closest one and tells the lens to focus at that distance.

  • The depth of field extends both in front of and behind the plane of focus, thus for this type of shot you want to focus somewhere between the subjects. You tend to get more DOF behind than in front (which is why the camera picks the closest point) though it's about even for close distances e.g. less than 10m.

  • You can tilt the plane of focus using a special lens, you can shoot two images with different focus and blend them, and you can stop right down. None of these solutions are ideal, the first costs you money, the second time and the third light.

  • I would recommend you change the composition, place the subjects at the same distance (side by side) or include a background element in the picture so you can back off or use a wider lens and make it look intentional. At the end of the day this type of shot is very hard to get with a DSLR due to the shallow DOF that a large sensor provides.

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I have limited understanding on some of these aspects.

I thought that with focal points you can only use one point to focus - i.e. that is why it is called the focal point. For the camera to be able to focus on multiple areas it would need to be a technical masterpiece.

If you are not happy with the autofocus - have you tried manually focusing? do you get shots you like when you manually focus?

At F4 I would expect the main subject to be in focus and those behind to be out of focus, if I wanted to have both of them I would be changing the App to 6.3 or greater

I hope someone who knows more can come along and help you more

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