Focus area

[mike21]

I like to take pictures of birds in flight, when against the sky I enable the nine focus points and when against some other background I enable just the centre spot (I use AI Servo). Sometimes I am successful in getting a sharp image but other times not. It is difficult to put the centre focussing spot on a fast moving bird and press the shutter button halway at the same time - my problem is that I do not know how much leaway I have; what is the angle covered by the centre focus spot, or indeed by each individual spot, it would help to know how accurate I need to be. I have David Busch's manual but the problem is not covered.

[jrhoffman75]

The area covered by a focus spot is close to the size of the spot. If you use Auto zFocus Point selection and start AI Servo by a half press of shutter button the camera will track the subject as it moves. It is most effective for subjects moving towards/away from the camera. 

For subjects moving across the frame you really need to pan the camera to keep a fast moving subject in the frame. 

If if you can get a sense of distance (like birds are taking off from a perch) you could prefocus and set lens to manual focus. 

And you are correct - sometimes you capture and sometimes you don't. 

[mike21]

Thanks, that explains why my failure rate is high - getting the spot on a fast moving and direction changing bird and then pressing the shutter half way must be like grouse shooting but with a lower success rate.

[jrhoffman75]

This might help a little - if you are in an area where you have frequent takeoffs you can just hold te shutter button halfway down and then when you ee a target point. That will speed up acquisition since the computer is awake.

You can see that by holding the button sown and just moving from varios targets; focus will keep changing as the target changes.

[mike21]

Thanks again - I had not thought of that, it should help.

[StanNH]

I usually have better luck using the nine point focus with AF servo for BIF pictures.  I find single point focus, which I normally use, is too fine for this type of shooting.  Make sure you're using a fast enough shutter speed as well.  I have gotten good shots at 1/800, but 1/2000 seems to be the sweet spot for most of my BIF shots.

[mike21]

Thank you again. I use a "cheap" EFS 55 - 250 Canon lens. If I had not got two images of Common Terns of near perfect quality I would have thought the lens was the problem. I have checked those two and they were taken in perfect weather f5.6, 200 ISO and 1/4000 second, others taken at 1/000 in overcast conditions are not really sharp, so it seems that luck coupled with ideal conditions and a high shutter speeds are the answer, the main ingedient being luck with the focus point. I use AV and set aperture and ISO, shutter speed depends on the light, I think I need manual setting of speed (1/2000) and aperture  plus auto ISO.