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Old Nov 7, 2006, 8:07 PM   #1
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if i used just the center focus point instead of all 7 how would it affect the focus on the picture? is there any time you would want to do this. thanks for any advice
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Old Nov 7, 2006, 8:26 PM   #2
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i use centre focus almost 99% of the time. Based on camera orientation and the subject i want to frame i may select the required focus points.

In the new generation Xt cameras the centre point is suppose to focus better for lens's faster than f2.8(including)

When centre point focus is selected the object/subject which is in focus at the centre will be sharper while the rest will appear to be hazy based on the lens ur using.

Usually the 7 point focus is useful while tracking moving objects, AI Servo etc. In case of AI Servo, the camera tries to keep the subject in focus using any of the seven focus points whne the subject is moviing.

Hope this helps

Vj
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Old Nov 8, 2006, 2:54 AM   #3
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http://visual-vacations.com/Photogra...pose_sucks.htm
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Old Nov 10, 2006, 12:00 PM   #4
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Ok, so to get this straight, and based on my understanding when i read the article (link), & do please correct me if am wrong coz i really need help in this area:

1) Focus the subject in view finder.

2) Decide which part of the image should be the sharpest.

3) Based on your decision in number 2), select the desired auto focus point.

4) shoot the subject.

& the more light there is, the more focus area you can get, correct? because if there isn't enought light, you're gonna have to widen the aperture, and here it would be difficult to focus.?
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Old Nov 10, 2006, 12:21 PM   #5
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photo_q8 wrote:
Quote:
Ok, so to get this straight, and based on my understanding when i read the article (link), & do please correct me if am wrong coz i really need help in this area:

1) Focus the subject in view finder.

2) Decide which part of the image should be the sharpest.

3) Based on your decision in number 2), select the desired auto focus point.

4) shoot the subject.

& the more light there is, the more focus area you can get, correct? because if there isn't enought light, you're gonna have to widen the aperture, and here it would be difficult to focus.?
You have to select focus point before you can do (1)

I use center focus point most of the time as even when using 1.4xTC on my 400mm lens, I am too far from my subjects so I end up cropping. But if I am closer, I use the focus point which falls on the bird eye.

Center point on 20d/30d/XTi is extra sensitive at f2.8 or higher so if you have fast glass and light is low, I would use that.
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Old Nov 10, 2006, 1:50 PM   #6
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photo_q8 wrote:
Quote:
Ok, so to get this straight, and based on my understanding when i read the article (link), & do please correct me if am wrong coz i really need help in this area:

1) Focus the subject in view finder.

2) Decide which part of the image should be the sharpest.

3) Based on your decision in number 2), select the desired auto focus point.

4) shoot the subject.

& the more light there is, the more focus area you can get, correct? because if there isn't enought light, you're gonna have to widen the aperture, and here it would be difficult to focus.?
For #1 above, it should read: 'Frame' the subject (not focus the subject).

And, yes, the above is true IF you have the luxury and time to change focus points. You'll likely have to do SOME recomposition because quite honestly your subject may not be on ANY of the focus points. So you might have to focus using, say, the far left focus point and then slighly recompose - the angle you're changig your framing is still much smaller than if you used center focus point.

But, depending on your subject matter, changing focus points isn't always possible. This is especially true with subjects that are moving - sports and wildlife photography being 2 areas that come to mind. When I'm shooting soccer, say - there's no way I'm going to cycle through focus points to select the 'best one'. It's just not practical. In those instances though, I'm also not going to 'focus and recompose'. I'm going to shoot with center point only and adjust my framing by cropping the photo after the fact. For sports shooting, that's the difference between theory and practical application. Same is often true of moving wildlife.

Finally, let's be clear - the affect of focus and recompose on image sharpness is directly related to the DOF of your image. Shooting portraits with wide apertures will typically have very limited DOF - maybe only a couple inches. Focus and recompose can drastically cut down on the sharpness of the result. Shooting something with an 8 foot DOF, a focus-recompose is not likely to cause any problems.

So, bottom line - selecting the right focus point and never recomposing will always yield the best results. But that's not always practical - the key, as with anything else, is to understand what options you have (select single point, use all points, use center-only and keep image centered and fix framing in PP, focus and recompose) and decide which one is going to allow you to accomplish your goal. As with just about anything else there is no 'one way is always right' rule. You have to consider these other variables (DOF, subject movement, time required to change focus points vs. missing the shot).

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Old Nov 10, 2006, 5:07 PM   #7
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Exactly so.

You need to know what your DOF is.

At longer distances, or with smaller apertures, focus-recompose is fine, because your subject will still be comfortably inside the acceptable zone of sharpness.

But sometimes, particularly with wide apertures and close subjects it will lead you astray.

Having said that however, the centre AF point is a high-precision point if you have a lens with a 2.8 or better maximum aperture. So under the correct conditions it can be a good idea to use the centre point only.

As a matter of preference I actually like a square format for many of my portraits, this in fact means I can use the centre AF point with a fast lens and crop out the sides, so focus-recompose problems are minimised.

You just need to experiment and learn the limitations of your equipment.
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Old Nov 10, 2006, 6:15 PM   #8
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I have my 20D set to center focus point 100% of the time. I also have custom function 4 set to 1 which lets meuse the AE lock button (*)on the back of the camera for auto focus. Pressing the shutter button half attains AE lock but does not try to refocus. So you can lock your focus with the back button,and not have to hold the shutter button while you try to recompose. This also works really great for sports, you can lock your focus where you know the action is going to take place and you're ready to shoot when it happens. It does take a little getting use to, but once you do it works very well.
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Old Nov 11, 2006, 1:45 AM   #9
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Well, i finally got it and tested it.

I always thought that if you dont see it in the view finder then there's something wrong with your focusing skills. Didn't know that Increasing the F number would get you the unfocused focused.

I always use my cell phone for testing. Never got it all into focus. Today i set the aperture at 18 and shutter at 4. the result is really satisfying to me.
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Old Nov 11, 2006, 6:08 AM   #10
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I generally use the centre focus-point and push the button half-way to focus and then reframe. The only time I think I ever use any other focus point is when I'm using a tripod and reframing isn't easy.
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