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Infobank

Exposure settings: Exposure compensation

Your camera might be clever, but it is not intelligent. This is especially true of the exposure metering system. All the metering does is measure the brightness of the light that is passing through the lens.

Evaluative metering, the default setting for EOS cameras, takes measurements from different parts of the scene. Based on these, the camera can often compensate for backlit or off-centre subjects, but it has no idea of what the subject is, or the conditions under which you are shooting. You might be photographing a light-toned subject in poor light, or a dark-toned subject in bright light – it is all much the same to your camera.

Most exposure meters work by assuming that scenes approximate a mid-tone grey area. This is usually taken as one that has 18% reflectance – that is, the scene reflects about one-fifth of the light falling on it.

 

Most EOS cameras offer exposure compensation, shown on a scale similar to the one above. This scale appears on the LCD panel and also in the viewfinder. The numbers indicate the amount of compensation in stops – increased exposure (+) to the right, reduced exposure (–) to the left. Under the scale is a moving index mark that shows the amount of compensation applied. When the index is at the centre no compensation is applied.

 

The camera doesn’t know that the above subject is white – it assumes that the scene is an average grey. This means that the camera sees the bright light coming through the lens from the white subject and interprets this as meaning that a lot of light is falling onto a grey scene. The camera reduces the shutter speed and/or the aperture accordingly, which means that the scene is underexposed (left). In the same way, if the scene has lots of dark tones, the camera assumes that it is a mid-tone scene in low light, and increases the shutter speed and/or the aperture. This leads to overexposure. The solution to these problems is exposure compensation. When faced with a light-toned scene, you can tell the camera to give a little more exposure than it would normally do (right). With a dark-toned scene, you can tell the camera to give a little less exposure.

Metering modes

With EOS cameras, you have a choice of up to four different metering modes - Evaluative, Spot, Centre-weighted and Partial. Your choice of metering mode will determine how you use exposure compensation.

Evaluative metering was introduced with the very first EOS camera back in 1987. It breaks the scene down into a number of zones (from 3 to 63, depending on the camera). A reading is taken from each zone and the results are analysed by the camera’s built-in computer.

If, for example, the central zones are darker than the outer zones, it is likely that the main subject is backlit. If the central zones are much brighter than the outer zones, the main subject might be in a spotlight. In both cases, the camera will bias the exposure to the central zones, giving correct exposure to the subject.

In effect, the evaluative metering is implementing its own exposure compensation. An overall reading from either scene would not give good exposure, but exposure based on the central area will improve the results.

The trouble when using exposure compensation with evaluative metering is that you don’t know if the metering has already compensated for the conditions. If it has, and you dial in even more exposure compensation, then the exposure will be wrong. Equally, if you assume that the camera has got it right, but it hasn’t, then you will also have a badly exposed picture.

The solution, as with so many things photographic, is experience. After a while you will learn to recognise the types of scene which evaluative metering handles well, and those that it does not.

When you change to a different camera, you will have to learn all over again, as the number of metering zones can change the results.

Centre-weighted metering takes a reading from all of the framed scene, but with more emphasis on the central area. It was one of the main metering modes on Canon FD cameras, which came before the EOS range. Centre-weighted metering was originally included in EOS cameras to help FD users make the transition, but seems to have become a permanent feature.

Unlike evaluative metering it makes no attempt to analyse the scene, so you can apply exposure compensation in the knowledge that the camera has not made any adjustments of its own. For this reason, centre-weighted metering is often better than evaluative when you know that some level of exposure compensation is needed.

Partial and spot metering are much easier to use with exposure compensation. You are in full control of where the camera takes a reading from, and you know that the camera is not making any further adjustments.

Unlike evaluative metering, partial and spot metering take the reading from just a small area of the scene.

In both of these modes you have a choice of techniques. You can take the reading from a mid-tone area, if there is one (for example, grass). This is convenient, because no further adjustment of the exposure is needed.

However, if there is no mid-tone in the frame, you can take a reading from a light or a dark area and then apply compensation. For example, if you take a reading from a white area that still retains some detail, +1.5 or +2 stops compensation will give similar results to a reading from a mid-tone area with no compensation. Similarly, a partial or spot reading from a dark area that still retains detail, together with –1.5 or –2 stops compensation, will also give results similar to a reading from a mid-tone area.

If the area used for the reading is not in the centre of the frame, set the exposure compensation first, then move the camera so that the area is in the centre. Now partially depress the shutter button to take a reading, and use the exposure lock button to hold the reading as you recompose the image. Finally, press the shutter button fully to take a picture.

 

You could use partial or spot metering to be sure of good exposure for this eagle. Either meter from the white features and apply +1.5 stops exposure compensation, or meter from the dark feathers and apply –1.5 stops compensation. Each method will give similar results.

Correct exposure

How do you know if your images are over or underexposed? Basically, they look wrong! But this can be a subjective matter.

If you are photographing something light or white and it comes out looking grey or murky, you’ve underexposed the picture. Similarly, if you are photographing something dark or black and the image looks grey, you’ve overexposed the subject.

There is no such thing as a correct exposure. There are good exposures and bad exposures, but ultimately it is down to how you want the image to look. Some people deliberately ‘underexpose’ and image by 1/3 stop to make the colours in the image appear more saturated.

 

Which is the correct exposure? With exposure compensation you can experiment with slightly different exposures to find the one that gives you the result you want.

Setting your camera

There are differences in the way exposure compensation is set on EOS cameras.

EOS-1D, 1D Mark II, 1D Mark II N, 1D Mark III, 1D Mark IV, 1Ds, 1Ds Mark II, 1Ds Mark III, 10D, 20D, 30D, 40D, 50D, 5D, 5D Mark II, D30, D60, EOS 7D

  • Half press the shutter button to activate the camera and take a meter reading.
  • Check that the Quick Control Dial is switched on.
  • Turn the Quick Control Dial to set compensation – right for positive, left for negative.

EOS 300D, 350D, 400D, 450D, 1000D

  • Half press the shutter button to activate the camera and take a meter reading.
  • Press and hold the +/- Av button.
  • Turn the Electronic Input Dial in the desired direction – right for positive, left for negative.

Once you have set exposure compensation, it will stay set until you reset it back to zero again, even if you turn the camera off and back on again. This can catch you out if you forget, so when you’ve finished taking a series of pictures, remember to reset the compensation to zero to avoid ruining pictures you take later.

Most of the EOS film cameras have increments of 1/2 stop. This means that you can set values such as +0.5 stop, or -1.5 stops, in addition to integer (whole number) values, such as +2 and -1.

Some cameras allow you to choose between 1/2 stop and 1/3 stop exposure compensation increments. This is done via a custom function, usually C.Fn.6. Use 1/3 stop increments if you have the choice, as it is more precise, but 1/2 stop increments are perfectly adequate.

C.Fn.6 can be a bit confusing as it works differently on different cameras. On the EOS-1 film camera, you have the option of 6-0 or 6-1 where 6-0 is 1/3 stop increments and 6-1 is 1 stop increments. On the EOS-1N, 1V and 3 there are three options: 6-0, 6-1 and 6-2. Again, 6-0 allows 1/3 stop increments but 6-1 sets 1 stop increments for setting shutter speed and aperture and 1/3 stop increments for exposure compensation. 6-2 turns everything into 1/2 stop increments.

By the time digital cameras arrived, Canon had rationalised the settings. The EOS D30 and D60 have two options – 1/2 stops or 1/3 stops, but Canon moved it to Custom Function 4-0 and 4-1, where the default setting of 4-0 is 1/3 stops.

By the time the EOS 10D arrived, the setting of 1/2 or 1/3 stops for exposure compensation had returned to its original location of C.Fn. 6-0 and 6-1. It has remained that way on every EOS digital camera since, with the exception of the EOS 300D which does not give the option, being permanently set at 1/3 stop increments.

Once you have made your choice from the custom function menu, it is unlikely you will want to change it again.

How much compensation?

There is no foolproof guide to the amount of exposure compensation you need. It will vary with every scene you photograph, and many shots will not need any compensation. As a rough guide, try +2 to +3 stops for scenes with lots of snow or sand, and +1 if there is a lot of water behind your subject. Minus exposure compensation is needed less often, and around –1 will handle most dark backgrounds.

‘If it’s light, go right’ is a handy way to remember which direction you need to go on the exposure compensation scale. If the scene is lighter than a mid-tone, you need to move in the direction of ‘plus’ compensation to improve the exposure.

Is three stops enough?

Most early EOS digital cameras offer ±2 stops of exposure compensation. Some of the professional models extended this to ±3 stops. A few of the very first film models offered ±5 stops, but Canon soon realised with film cameras, this was too much for most situations. The advent of HDR imaging has changed this though. To enable freedom in exposure for HDR images, the EOS 7D, EOS 60D, EOS-1D X and EOS 5D Mark III feature +/-5 stops of exposure compensation.

If you find that the amount of exposure compensation available is not enough for the scene you are trying to record, there is a solution. Take the meter reading with no exposure compensation, and then set the indicated exposure values (shutter speed and aperture) manually on the camera in M mode. You can then apply as many stops of compensation as you need by altering the shutter speed or aperture.

Camera Amount 1/2 or 1/3 stops
EOS 60D ±5 stops User selectable
EOS 5D Mark III ±5 stops User selectable
EOS-1D X ±5 stops User selectable
EOS-1D ±3 stops User selectable
EOS-1D Mark II ±3 stops User selectable
EOS-1D Mark II N ±3 stops User selectable
EOS-1D Mark III ±3 stops User selectable
EOS-1Ds ±3 stops User selectable
EOS-1Ds Mark II ±3 stops User selectable
EOS-1Ds Mark III ±3 stops User selectable
EOS 5D/5D Mark II ±2 stops User selectable
EOS 7D ±5 stops User selectable
EOS 10D ±2 stops User selectable
EOS 20D ±2 stops User selectable
EOS 30D ±2 stops User selectable
EOS 40D ±2 stops User selectable
EOS 300D ±2 stops 1/3
EOS 350D ±2 stops User selectable
EOS 400D ±2 stops User selectable
EOS 1000D ±2 stops User selectable
EOS 450D ±2 stops User selectable
EOS D30 ±2 stops User selectable
EOS D60 ±2 stops User selectable

Using histograms

Every image, be it shot on a film or digital camera, can be broken down into a histogram. A histogram is a graph that shows you the frequency and distribution of tones in your image. The horizontal axis is the tonal value, and the vertical axis is the frequency – the taller a peak, the more pixels there are at that particular tone.

 

To create a histogram, your digital EOS categorises every pixel value into one of 256 groups, where 0 is black, 255 is white, and all the other numbers represent shades of grey. The vertical axis indicates the relative number of pixels in each group. The impression is of a solid area – in fact, what you are seeing is 256 upright lines crammed so close together that they appear to be touching each other. If there are a lot of tall lines up against the right of the graph, the image is probably overexposed. If there are many tall lines against the left of the graph, the image is probably underexposed. When looking at a histogram, it is not possible to say what an ideal shape should be as it is totally dependent on the scene being photographed. High key, or bright images, will have a lot more tones in the right hand end of the scale than darker images that will have the majority of tones at the left hand side.

 

These histograms are almost identical, except the lower one shows the exposure compensation applied to the image. The scene is the same, but the extra exposure applied to the lower image has moved the histogram further to the right hand side.

 

The tone in this image is broken up into three sections. The white head is shown by the peak to the right of the histogram, the dark body feathers by the large peak to the left, and the green background by the two smaller peaks just left of the centre.