exposure - How do I "focus and recompose" using spot metering?

I usually use the "[auto]focus and recompose" method for taking candid photographs. In situations where there is a high degree of contrast, I would like to use spot metering to correct expose for skin-tones. However, when I do this using the focus and recompose method, exposure changes during recomposition. This does not happen using the "evaluative metering" mode.

I know I can use exposure lock to help in situation, but I am not sure how. What exact sequence of button presses on a Canon 550D do I need to use to get the result I am looking for in aperature priority mode?

Answer

There are two basic ways to accomplish this, and it all boils down to being able to separately control autofocus and your meter. You can meter first and then autofocus, or you can autofocus first, then meter. I've listed a few different methods, but ovens vary; adjust cooking time accordingly.

Meter, Focus, Recompose, Shoot

Method one: AE-lock on half shutter, AF-lock button.

Camera settings:

• Make sure your camera locks exposure when you half-press the shutter release.


• Set autofocus to continuous servo.


• Your AE/AF lock button should be set to AF-lock only.

When shooting:

1. Place your meter point where you want to meter. Hold the shutter release halfway down.


2. Place your autofocus point over your subject.


3. Wait until autofocus locks on.


4. Press and hold the AF-lock button.


5. Recompose


6. Fully depress the shutter release.

Method two: AE-lock button, single servo AF.

Camera settings:

• AE/AF lock button should be set to AE-lock only.


• Set autofocus to single focus (One Shot in Canon nomenclature).

When shooting:

1. Place your meter point where you want to meter.


2. Press and hold the AE-lock button. (On my camera, this will start up the meter if it has turned off after a delay)


3. Place your AF point over your subject.



4. Half-press and hold the shutter release.


5. Recompose


6. Shoot.

Focus, Meter, Recompose, Shoot

Method one: AF-On, meter on half-press.

Camera settings:

• AE/AF lock button should be set to AF-On (Or whatever Canon calls this. Basically, autofocus only happens when you're holding this button.)


• Make sure your camera locks exposure when you half-press the shutter release.



• 
  

  Set autofocus to single focus (One Shot in Canon nomenclature).

  
  When shooting:
  
  
  
  1. Put your focus point over your subject.
  
  
  2. Press the AF-On button until AF locks, and continue to hold the button. (You might be able to let go of the AF-On button if your camera can be to shutter priority, as opposed to focus priority.)
  
  
  3. Put your meter point over where you want to meter.
  
  
  4. Half-press and hold the shutter release.
  
  
  5. Recompose
  
  
  6. Shoot.
  
  
  
  
  

Method two: Focus on half press, AE-lock.

Camera settings:

• AE/AF lock button should be set to AE-lock only.


• Set autofocus to single focus (One Shot in Canon nomenclature).


• Make sure your camera does not lock exposure when you half-press the shutter release.

When shooting:

1. Put your focus point over your subject.


2. Half-press the shutter to focus. Hold the shutter release halfway.


3. Put your meter point over your metering target.


4. Press and hold the AE lock button.


5. Recompose.


6. Shoot.