The Canon 7D has 3 focus modes:

  • One-Shot AF: 1 time
  • AI Servo AF: continuous as long as you keep your finger on the button
  • AI Focus AF: Starts as a One-Shot AF, then switch to AI Servo AF if the Canon 7D senses movement

Then there’s the selection of the area to focus on:

  • Single point
  • Zone
  • Auto-select 19 points
  • Spot AF
  • AF point expansion

and finally:

  • Live

That’s 49 different combinations with each of the focus points, 19 single points, 9 zones, 1 auto-select, 1 spot and 19 expansions, for the One-Shot AF and another 49 different combinations for the AI Servo AF. So for Birds In Flight, you just need to select…

Duh!! Wrong! There are many more factors to take in consideration:

  • The subject’s size and contrast
  • Your distance to the subject
  • The direction and intensity of the light or lack of
  • The focal length
  • The depth of field

  • Wide angle lens at f/8 with the subject at 15 meters/45 feet. Who cares! It’s will always be in focus, unless it’s an aircraft carrier.

  • Tele in the savanna, making photos of lions and elephants at 50 meters/150 feet. It’s bright so far away. The depth of field will take care of everything.
  • Tele in the house making photos of a shy cat. 2 or 3 meters/6 to 10 feet at f/2.8. Now it’s more difficult. Does the cat move around or just ignores you?

The lens I use the most is my 70-200 zoom, fairly wide open. My personal preferences are:

  • 60% of the time I use single point auto focus
  • 30% of the time I use zone auto focus

Depth of field:

  • 1/3rd in front of the actual focus point
  • 2/3rd behind the actual focus point, roughly, as the focal length gets longer it becomes closer to ½

Other factors

  • The speed of the subject
  • The speed of the shutter
  • How well the camera is held/supported: hand held/vibration control or monopod/tripod
  • The quality of the light: the more contrasted the light/subject the better the autofocus.
  • The size of the print or the photo on the screen
  • If somebody tells you that spot auto focus or expansion point… is best. It’s BS.
  • There are far too many factors involved. That’s why Canon gave us such flexibility. That’s the beauty of the Canon 7D.
  • The most important factors are the distance to subject, the focal length and the depth of field.
  • Many different AF point selection will give you the same result.
  • The motor in the lens for focusing is not continuous, but it’s a step motor.