Exposure Compensation – What’s it for?

As a predominantly studio based Canon photographer, exposure compensation pretty much passed me by. I understood it would alter the way the camera metered a scene, but never quite grasped why I would use it when I could do the job manually, and in the studio that’s fine. Why bother?

It was only when I started to look closely at metering and focussing in the context of landscape photography that the fog lifted and I realised that evaluating exposure and focal points are very closely related. In order to understand why we might use exposure compensation we need to look at metering modes first.

Metering

The camera has the ability to assess the light in a scene and uses that ability in order to suggest a good exposure. In manual and all other modes that exposure is signified by the meter resting on 0 on the in viewfinder or screen based meter. -1 is underexposed by one stop, +1 is overexposed by 1 stop.

So what criteria is used to make this judgement?

18% Grey is used by cameras as the gold standard. If you visualise a grey scale running from black to white, 18% grey is the average of all the tones in any given picture at any setting registering 0 on the meter.

What this means in practice is that if you shoot a white bedsheet for example, it will come out grey even though you set the exposure to 0. Think of snow.

Similarly if you shoot a dark object, it will come out grey or overexposed. This is the result of having the camera fixate on 18% grey. In actual fact almost any scene in nature or studio will average out at 18% grey, that’s why the camera manufacturers choose it.

There are a handful of different methods the camera can use to calculate exposure, but it always relies on 18% grey as the gold standard.

Evaluative Metering (Matrix)

In evaluative metering (Matrix is the Nikon term for the same thing) the camera calculates the average light across the frame and compares that to 18% grey. In Canon cameras, the focal point is factored in to a large extent. By which I mean if your focal point is in the left third of the picture that area will be favoured in the evaluation.

This mode is the most commonly used and as a general all purpose mode it is very effective. Also works well for wide angle landscape photography.

Centre Weighted Average

Centre Weighted average metering evaluates the light favouring an area of 60%-80% (depending on camera make and model) of the centre of the frame and factors in the remaining light at the edges of the frame, so perfect for wildlife photography where the subject is in the centre of the frame.

Partial Metering

Similar to spot metering, Partial metering covers only the centre 6.2% of the frame in the viewfinder, thus providing precise metering only for the subject. When the background is brighter than the subject due to reasons like back lighting, this metering mode is particularly useful. It is useful for back-lit shots or tiny subjects.

Spot Metering

Spot metering evaluates the light in a very small area (1.5% of the frame) is perfect for circumstance where the subject is much brighter or much darker than the surroundings. Think of photographing the moon or a backlit bird silhouetted against a bright sky.

What about AF Points and Areas?

Given that we’ve said that the focus point plays a part in evaluative metering, then it follows that the location and size of the focal point is important too.

These are the AF modes available to Canon shooters.

Single Point Spot AF – The narrowest user selected sampling area. Basically a spot.

Single Point AF – Slightly wider than Single Point Spot AF

AF Point Expansion – A cluster of five AF points weighted in favour of the central point.

AF Point Expansion (Surrounding points) – A cluster of nine AF points

Zone AF – Divides the AF area into nine zones, any one of which can be selected by the user.

Large Zone AF – Divides the AF Area into three.

Automatic Selection AF – The entire AF Zone

The Point of Focus

You can use the AF Point Selection button on the back of the camera to display the focus points in the viewfinder. Use the wheel on the back to move it to a different point.

Focus Modes

Focus Modes for Canon cameras include

One Shot AF

One-shot AF mode suits most subjects that stay in one place while you take a photograph. The focus is locked with the first pressure on the shutter button or back button if you have swapped focus out to back button focusing.

AI Servo AF

AI Servo AF mode is designed for fast-moving subjects. The camera calculates where the subject will be at the moment the shutter fires and focuses the lens accordingly.

AI Focus

AI Focus AF mode (available in some cameras, for example Canon 5Ds) switches between One-shot AF and AI Servo AF according to the movement of the subject. The camera makes the decision.

Exposure

Metering allows the camera to calculate the ‘perfect’ exposure based on the 18% target. This becomes somewhat more abstract once you move away from evaluative metering because the size of the area used to extract the light measurement is proportionately less.

The photographer should use this to his or her advantage in using the metering mode most likely to produce a good result in the scene. Some thought should go into the choice as there are some “gotchas”. The main one is where very dark and very light areas make up the subject. Think of a rugby match with white and black jerseys, or a white and black horse. Using spot metering might produce an overexposed white in the case your spot is in a dark area or an underexposed black where the spot is in a light area.

Of course we can compensate manually for this, but why would we when the camera can do it for us?

Exposure Compensation

It’s important to note that exposure compensation only works in modes other than manual. What it does is take the reading from the scene and adjust the exposure to compensate for anomalies like the ones listed in the previous paragraph or wider issues like snow or nighttime photography.

So if I’m photographing a bird with a black head and blue plumage on a wire against a bright sky, spot metering will produce very different results depending on whether I focus on the head or the plumage. In cases like this I can use Exposure Compensation to force the exposure up or down.

The way in which exposure compensation operates is different for different shooting modes.

In Aperture Priority the exposure compensation will alter the shutter speed.

In Shutter Priority exposure compensation will alter the Aperture.

In Program mode exposure compensation will alter the shutter speed

There is also the question of ISO..

Auto ISO

If you are shooting landscapes in Aperture Priority it makes a great deal of sense to choose Auto ISO – this is because you’ll want to be certain of the depth of field and also the shutter speed. Simply set these two and shoot. If you do get into underexposed territory, noise reduction software like PureRAW is very good these days.