2 February 2009

Exposure metering lock – what the manual doesn’t tell you

Only the evaluative metering pattern locks the exposure when focus is achieved
Canon EOS cameras have a choice of ambient light metering patterns available when you shoot in the creative zone modes; P, Tv, Av, M, A-DEP. Most cameras feature Evaluative, Center-weighted, Partial and Spot metering choices. One of the hidden gems in the Canon engineers’ wisdom is that there are two different approaches to automatic exposure lock that depend on the metering pattern employed. If you use the focus lock and recompose technique in your photography you need to read this.

center-weighted metering patternWith evaluative metering and one-shot AF the camera will lock the exposure reading at the same time as the AF locks. You can prove this yourself, put your camera in one-shot AF and evaluative metering pattern. Then half press the shutter and once the focus is found keep your finger on the shutter and recompose your scene with a significant change of brightness, point the camera skyward for example. You should see that the shutter speed and aperture don’t change. To prove this we set up a test scene and pointed the centre AF point at the 7 in 70-200 on the lens and took a picture, then we repeated the shot and once focus lock was acheived moved the camera to put the centre AF point over the black strap on the edge of the camera. Surprise - or not - both shots have the same exposure settings.

IMG_0931  IMG_0932

Both shots, 1/30s at f/5.6 - autofocus lock while framing with the 70 on lens at the centre AF point, exposure locks with the focus.

spot metering patternIf you are using Spot, Partial or Center-weighted metering, then the camera doesn’t lock the exposure until the shutter is pressed to take the picture. This means if you use focus lock and recompose then once the AF is complete and locked the metering isn’t. If your recomposed shot is significantly different in brightness then you will have the exposure for the recomposed scene not the focus locked scene. This can be a significant difference with spot or partial metering. Imagine you thought you took a spot meter reading from a persons face, focussed and locked the focus on the eyes then recomposed to put the person off centre for composition, except the person is in front of a black background. You just got over exposed skin tones in your picture.

Back to the test scene and partial metering, though spot, partial and center-weighted all exhibit the same behaviour.

IMG_0929  IMG_0930

left image: focus lock on the lens, exposure 1/40s at f/5.6
right image: focus lock on the lens, recompose on the strap, exposure 1/13s at f/5.6

Exposure lockSo how to overcome this? The answer is right under your thumb actually, the button marked with the * symbol is the exposure lock. You may need to make use of this when you use Spot, Partial or Center-weighted metering.

Got an EOS-1D series camera? On these models you can choose to have the spot meter reading point linked to the active AF point. If you use this AF point linked spot metering with automatic AF point selection then you may have less problems since the spot meter is most likely on the main subject of your pictures.

This post all applies to one-shot AF, if you use AI servo AF then there’s no automatic focus lock in any metering mode. Also in Manual exposure mode there's also no exposure lock at any time. Check the function availability table towards the back of your camera instruction book.

In another article we’ll cover custom functions for ‘back button focus’ which also has a bearing on when and how the AF and AE settings are locked; don’t you just love interdependencies.

If you follow our twitter feed you may be expecting flash posts, this is laying the ground for those.


1 comment:

Anonymous said...

This i absolutely right! I have recently started using a lot more of the creative settings and metering modes other that evaluative. Now I understand why my exposures are sometimes off in theses modes. I tend to do a lot of recomposing, so i will now remember to use the exposure lock button. Thanks!