Canon Digital Rebel XT Autofocus

aphelion

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Sorry if this question has been asked elsewhere; my search wasn't very fruitful.

In a nutshell, I'm having trouble understanding the nuances of autofocus between the creative modes on my camera and the fully automatic modes when I shoot with my Digital Rebel XT and my 24-70mm f/2.8L lens.

Let's say that I'm focusing on a subject in a fully automatic mode, and the camera shows that it's using the center focusing point in the focusing grid. I take the picture, and it comes out perfectly focused at--oh, let's say--1/60 second and f/3.5.

If I set the camera to fully manual and get the shutter and aperture to match the automated shot, set my AF point to be the center one, and hit the exact same target, there's a very good chance that the AF will fail (attempt to focus until it gives a failure focus error), or the picture will come out soft in comparison to the automatic one. This is the case even if I remove hand shake via a tripod and remote control.

I HATE using the automated zones, but it drives me crazy that I get soft shots when I use this lens wide open in the creative zones.

I know that this has got to be some newbie mistake that I'm making. Is metering having an effect that I'm not aware of? Are the lens electronics used differently by the camera depending on the mode, so perhaps a fault in the lens is causing problems?

Any help would be greatly appreciated!
 

Big Mike

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Welcome to the forum.

I'm with (Ghastly) Krueger, as far as I know...the shooting mode has no affect on the auto focus.
 
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aphelion

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Welcome to the forum.

I'm with (Ghastly) Krueger, as far as I know...the shooting mode has no affect on the auto focus.

That's why this has me so baffled, as I thought the same would be true. Obviously, if the DOF in creative was too shallow compared to the DOF resulting from the automatic settings, that would make sense. But, as far as I can tell all settings are the same.

I'll try and take some test photos later to demonstrate the point.

Perhaps there's a workaround. If I set the AF focal point to use all the points in the grid (there are 7), should that have any effect? I still think the shots come up soft when I do this, but would a " try focusing on everything" approach work better than a "try focusing on one point" approach?
 

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