EOS 5D Mark II and movie exposure

How many readers have the EOS 5D Mark II?
Of you, how many have tried the movie modes and have figured out the way to accurate exposure?

You essentially have two controls over exposure on the camera. Firstly, there is the AE Lock button. Just like in still shooting, the AE lock button will hold the exposure at whatever value is currently set when you press the button. The one thing to remember is that you don't need to keep the button pressed. Just press it once and leave it alone. The exposure then won't change unless you press the button again... or use the second exposure control: The quick command dial.

The quick command dial acts like exposure compensation in still shooting. Essentially it makes the scene brighter or darker and that is about as much information as it gives you.

Clearly there are a couple of different ways of getting the right exposure then - you can either point at a dark area, let the camera open up the exposure, use AE lock and then use the command dial to make it darker again, or you can start pointing at a light area, use AE lock and then open up the exposure. Both will work, but it is worth experimenting as you only have +/- 2stops of compensation available so you need to make sure it's within the range of adjustment.

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Back button focus

Back button focus is one of the functions on EOS digital SLR cameras that gives a new layer of control over the picture taking process. In simple terms it places the shutter release, exposure lock and focus lock or activation on up to three separate buttons. If you have only ever used the 'out of the box' configuration with the AF activated with the half press on the shutter and the AE lock occuring with the focus lock in evaluative metering and this is not suiting you then it may be something to experiment with - though it's an acquired skill.

Probably one of the best articles on the topic is actually on the Canon USA website in their digital learning center. The article highlights five often quoted advantages of back button focus;

1. Easier to lock focus
2. Easier timing of shots
3. Less risk of focus errors with moving subjects
4. Easier over-riding of AF with full-time manual focus
5. Easier macro and close up focussing

With the latest EOS cameras from the mid-range up providing separate buttons for AF-ON and AE lock '*' then there's also a function to switch round the operation of the two buttons. Great if you plan on using back button focus on an EOS 450D and an EOS 50D, as the EOS 450D doesn't have the separate AF-ON button and then the placement of the buttons may be out of sync with the two cameras.

Some photographers find that the use of back button focus eliminates the need to switch between AI Servo and One-shot AF modes. They simply use AI Servo all the time and switch the AF off when they have the right focus for their picture.

You might also find the previous article on blabpictures about AE lock and the metering pattern valuable further reading.

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

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	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.

With 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.

  

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.

If 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.

  

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

So 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.

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