20D AI Servo AF Mode behavior question

Proving my lack of BIF skills I took some shots of an egret and vulture in flight. 20D was in AI Servo AF mode and function 4 set to zero. Here is description of several consecutive shots.
1. Shot with center focus point in red on egret.
2. Shot with all but two focus points in red several on egret
3. Center focus point in red not on vulture but close
4. Last shot no focus points in red center one on vulture
Shots 1 & 2 were about 1 second apart on the egret and shots 3 & 4 were about 2 seconds apart on the vulture.
The 20D manual is totally unclear as to wheather a shot can be taken in AI Servo AF mode if not in focus. I accidentally mashed the trigger when the camera was aimed at a wall well within the minimum focus distance of the 400mm f/5.6 lens. Can anyone explain this behavior? Thanks.

Hi,

While I'm not sure I can answer everything, I can confirm that the 20D when set in AI Servo mode will allow you to take a photo when there is no clear focus. (This shows as no focus points lit when viewing the photo in the Canon image software eg. Zoombrowser Ex)

I would also recomend that you try centre focus point only for birds in flight, try to keep the shutter speed up around 1/1000 sec when possible (you can use f/5.6 with the 400mm f/5.6 to keep the shutter speed up) and practice manually focusing on the approaching bird whilst placing the center focus point on the birds head (or neck) before engaging the AI Servo auto focus.
Also unless you have birds flying very close set the focus range on the lens to 8.5 metres.

I know this as I practice BIF regularly with the very same camera/ lens combination!

Quote:

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Originally Posted by Julian Mole

Hi,

While I'm not sure I can answer everything, I can confirm that the 20D when set in AI Servo mode will allow you to take a photo when there is no clear focus. (This shows as no focus points lit when viewing the photo in the Canon image software eg. Zoombrowser Ex)

I would also recomend that you try centre focus point only for birds in flight, try to keep the shutter speed up around 1/1000 sec when possible (you can use f/5.6 with the 400mm f/5.6 to keep the shutter speed up) and practice manually focusing on the approaching bird whilst placing the center focus point on the birds head (or neck) before engaging the AI Servo auto focus.
Also unless you have birds flying very close set the focus range on the lens to 8.5 metres.

I know this as I practice BIF regularly with the very same camera/ lens combination!

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Thanks for the reply. On the one shot where 7 of 9 AF aimpoints were selected I had the center aimpoint selected. The next shots had center aimpoint only. The only thing I can think of is that I accidentally engaged all aimpoints. But would this just stay that way? I have never used this when shooting in AV priority mode.

Hi,

I'm not sure, however occasionally I have noticed some weird things when reviewing the photos (in Zoombroser Ex) with focus points lit up that weren't actually active at the time time of taking the photo (I almost always use centre point only for flight), so I just put this down to a glich in the capture data. However you could be right that you accidentally had all focus points active.