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Old May 31, 2005, 8:33 AM   #1
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:sad:

I have a Canon 70-200 f/2.8 which works very well by itself. However, when I add the 2x extender, the autofocus seems to be out of whack. When shooting people at about 200 feet, I use the single point focus, but people and objects about 10 feet behind the subject are in focus; the subject is soft. I'm using a Canon Rebel, and manual focus at that distance is difficult, to say the least. Any thoughts?
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Old May 31, 2005, 9:53 AM   #2
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How about the lighting/contrast - they both affect AF performance (Try in sunlight)

-> Remember your lens is now two stop darker, i.e. much less data for the camera to work with!
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Old May 31, 2005, 10:00 AM   #3
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Ironically, I was able to try both with the same results. I'm shooting softball games from behind the outfield fence. When trying to shoot the batters, the backstop and people behind it are in focus, but the batter is not, even though I used the single point autofocus to zero in on the batter. I shot one day in bright sunlight, and the next day in overcast conditions. Both sessions yielded the same problem. I tried backing off slightly with manual focus, but usually a slight turn brings the focal point too far forward. I realize that stopping down would help, but I need fast shutter speeds to stop the action. Even in bright sunlight, shutter priority at 1/850 sets the aperture at 11 or larger. I've only been doing this for about a year (just got the new lens a month ago), so I realize I may be doing something stupid.
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Old May 31, 2005, 10:08 AM   #4
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I would usually try for the batters foot or the ground where he's standing on to help the camera out... and hold the shutter 1/2 pressed to lock focus while you recompose.

Same thing with small birds, the AF sensor area can be bigger than the subject making it hard for the camera to decide which AF planes to focus on, unlike your 'smart' eye
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Old May 31, 2005, 2:22 PM   #5
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Also, the DOF is so narrow that you leaning forward a bit after the camera focussed can make a difference.

Of course, you could have a back-focus problem with the 2x TC.

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Old May 31, 2005, 2:27 PM   #6
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Okay, I'm still what most would call a "newbie". Can you explain "backfocus", or is there a website where I can read about it? Another thing is that I am at maximum zoom, if that makes a difference. I can use the same lens from the same position and shoot closer objects, and the problem does not occur, or it is minimized.

Thanks
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Old May 31, 2005, 2:47 PM   #7
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I doubt it's a generic problem with any TC...

The AF sensor is in the camera -> the camera 'see' through all this just like your eye would on the subjects(s) and will compensate for all this stuff - Very much like the E-TTL metering with an ND filter in front of the lens (with flash or no flash)!

Remember folks use extension tubes all the time to modify a lens focusing parameter or stack various diopter filters (myopic glass) in front of their lenses without affecting the autofocusing any :?

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Old Jun 1, 2005, 12:52 AM   #8
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tmilner wrote:
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Okay, I'm still what most would call a "newbie". Can you explain "backfocus", or is there a website where I can read about it? Another thing is that I am at maximum zoom, if that makes a difference. I can use the same lens from the same position and shoot closer objects, and the problem does not occur, or it is minimized.

Thanks
Hi

The Depth of Field (DOF) at 200 feet using the 2x extender at wide open aperture (F5.6) at maximum zoom is about 2 feet. That is the range around the focus point that will be considered sharp in the image. Anything outside that will be blurry. See http://www.photozone.de/3Technology/demos/depth.htm for more info and a calculator. Anyways, that is sufficient DOF that my comment about leaning forward or back doesn't apply. You cannot cover 2 feet by just leaning I think :-)

Backfocus means that the camera plus lens combination think they focussed at a certain point, but in fact the real focus is behind that point. Front-focus is the opposite effect. But since you say the problem goes away when you focus closer objects, my guess is there's no backfocus problem. Also, closer objects (at the same zoom and aperture) mean a narrower DOF, thus if you had such a problem it would be more pronounced....

That makes me think of possibly three things:

1. The AF sensor is actually larger than the square in your viewfinder. That could mean the AF sensor can pick up, and focus, on objects behind what you are trying to focus on. This can happen if the square in the viewfinder is on the edge of the object you want to focus on. See this picture (which I kindly borrowed from NHL, thanks!)

2. You accidently touched the manual focus ring (I've done that by holding the lens in the palm of my hand and my finger just touched it and changed it a little)

3. You focussed by pressing the shutter half way, then zoomed in.

Anyways, some suggestions to look at. Hope you'll figure it out!

Barthold

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Old Jun 1, 2005, 8:02 AM   #9
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Thank you very much. The larger sensor area makes sense; it could very well be reading the fence behind the batter. I appreciate the info.
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Old Jun 1, 2005, 10:53 AM   #10
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tmilner wrote:
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Thank you very much. The larger sensor area makes sense; it could very well be reading the fence behind the batter. I appreciate the info.
You can use Canon's fileviewer utility to load the image and figure out where it focussed. It will super-impose the focus points on top of the image. That way you can get an idea if the AF sensor might have picked something up behind the object. Of course this doesn't work if you focussed first, then re-composed.

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