Centre Point in AF

canon400d · February 7, 2009

As a rule I always use the centre AF point. If I take a photo of a bird or animal I will press the shutter button halfway and focus the centre point on the eyes. If I continued to keep the shutter button halfway and I wanted to recompose the animal say to the right of the image obviously the centre point would move away from the eyes. I would like to know if this would alter the focus on the eyes which I originally did. Also when is it best to use the other eight points or using all nine points at the same time.
Hope I have made myself clear.
Regards
Bob

colourbox · February 7, 2009

It completely depends on your depth of field. When you recompose, the distance to the animal subject's eye may change slightly due to your angle of rotation of the camera. If your aperture, focal length, and distance combination gives you enough depth of field, then the eye won't go out of focus and there is no problem. However, if the depth of field is very narrow and you can't set it to give you more due to the lighting conditions or equipment, then you can run into the focus error problem described in this article, and it would be better to compose first and then manually select the closest AF point instead of the middle one, as long as the camera can still lock (non-center points may not be as accurate in low light, for instance).

I don't know what you mean by "all points" except for the case in which you let the camera choose the focus point automatically, and I try not to do that because it often picks something that is not the critical focus point.

pathfinder · February 7, 2009

As mentioned Focus-Recompose has built in errors in focus, and for subjects closer than 8-12 feet these can be significant.

Unless you are more fortunate than I am, I rarely get to shoot birds at those distances, so I usually do not worry about Focus-Recompose as the distances are usually greater than 25 feet. If I was shooting in a zoo or a situation where the distance was less than 8-10 feet, I would avoid Focus-Recompose, and choose a more appropriate AF point to place on the bird's eye.

The aperture I need to use, would also play a role in my decision. I prefer to shoot at f4 or f5.6 or f8 for shooting birds, rather than f2.8 or f1.4, but if I needed those apertures I would be more meticulous about my focusing technique.

canon400d · February 7, 2009

colourbox wrote:

It completely depends on your depth of field. When you recompose, the distance to the animal subject's eye may change slightly due to your angle of rotation of the camera. If your aperture, focal length, and distance combination gives you enough depth of field, then the eye won't go out of focus and there is no problem. However, if the depth of field is very narrow and you can't set it to give you more due to the lighting conditions or equipment, then you can run into the focus error problem described in this article, and it would be better to compose first and then manually select the closest AF point instead of the middle one, as long as the camera can still lock (non-center points may not be as accurate in low light, for instance).

I don't know what you mean by "all points" except for the case in which you let the camera choose the focus point automatically, and I try not to do that because it often picks something that is not the critical focus point.

Thanks Colourbox I have found your reply and the article very interesting and a great help. When I mentioned all the AF points I was wondering if it was possible to use them all at the same time. Although from your reply I don't think so as each point has a purpose when composing the frame.
Regards
Bob

canon400d · February 7, 2009

pathfinder wrote:

As mentioned Focus-Recompose has built in errors in focus, and for subjects closer than 8-12 feet these can be significant.

Unless you are more fortunate than I am, I rarely get to shoot birds at those distances, so I usually do not worry about Focus-Recompose as the distances are usually greater than 25 feet. If I was shooting in a zoo or a situation where the distance was less than 8-10 feet, I would avoid Focus-Recompose, and choose a more appropriate AF point to place on the bird's eye.

The aperture I need to use, would also play a role in my decision. I prefer to shoot at f4 or f5.6 or f8 for shooting birds, rather than f2.8 or f1.4, but if I needed those apertures I would be more meticulous about my focusing technique.

Thanks Pathfinder for your help. In actual fact I can shoot birds on the feeders from the consrvatory a distance of about 6' and as I say I have been in the habit of positioning the centre point on the birds eye with the shutter half pressed for auto focus and then recomposed the frame. In future I will do the compose and select the appropriate point. The thing I have found is you have to be so damned quick as the bird doesn't stay in one place very long.
Regards
Bob

pathfinder · February 7, 2009

Good luck on shooting small birds, Bob.

I find them challenging also. They are so fast and their flight trajectories are so variable, they are much more difficult than larger raptors.

canon400d · February 8, 2009

pathfinder wrote:

Good luck on shooting small birds, Bob.

I find them challenging also. They are so fast and their flight trajectories are so variable, they are much more difficult than larger raptors.

Thanks Pathfinder.
Regards
Bob

Centre Point in AF

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