Exposure compensation

When set at Aperture priority mode the camera almost always gives the correct exposure. We are able the set the aperture of our choice based on the desired depth of field and the camera sets the corresponding shutter speed, right for the correct exposure.

Now what is the correct exposure ? The camera may interpret the correct exposure over a particular aperture and shutter speed combination but you may want your photographs to look brighter or darker than how they appear otherwise. You may want to do so for artistic reasons or you are already aware that what you are shooting is not an average situation and there is a potential for the camera to set the wrong exposure. The subject may include too much of black, white or it may be against the light or have the light source itself included in the frame.

In such situations, simply rotate the exposure compensation dial in the right direction. For correction, if the photograph due to the cameras metering is expected to turn dark, rotate it towards the plus side and if it is expected to turn out too bright rotate it towards the negative side. Plus means, more light will be added and negative means, less light would be added.

Sometimes the built-in light meter in your camera can misinterpret a scene and give you a poor exposure. A good way of correcting this to get a good exposure is to use exposure compensation. Most cameras offer this feature, which often appears as a +/– button or menu choice.

Exposure compensation enables you to modify the exposure up or down from the metered reading by a specified amount. By doing this, you can continue shooting using the modified meter reading settings and get good exposures. For example, a +1 exposure

compensation increases the exposure by one f-stop or the shutter speed equivalent, and –1 reduces it the same; a +1/2 setting increases the exposure a half step, and –1/2 reduces it by the same amount.

Exposure compensation can be particularly useful if your scene is overall very bright, such as on a beach or in the snow, or very dark, such as when a lot of shadow fills the image area. In those cases, the meter misinterprets the exposure. Make an exposure compensation adjustment and see if your histogram has improved. 

Canon EOS 5D Mark III 127 mm 1/80 at f/4.5 Not fired, compulsory mode +1 EV Aperture priority Aperture priority ISO 125 Pattern

Canon EOS 5D Mark III 127 mm 1/80 at f/4.5 Not fired, compulsory mode +1 EV Aperture priority ISO 125 Pattern

Canon EOS 5D Mark III 85 mm 1/500 at f/3.5 Not fired, compulsory mode +1 EV Aperture priority ISO 200 Pattern

Canon EOS 5D Mark III 85 mm 1/125 at f/5.6 Not fired, compulsory mode +1 EV
Aperture priority ISO 200 Pattern
You may underexpose the image slightly to get more deeper colors, something which can be said to be more true with film especially the color transparency as there was almost no control later on. The marginal deeper colours which one may get with the under exposure can be easily achieved in the camera raw convertor, so it may not be a great idea to make the efforts while shooting especially in quick decision situations.
Exposure compensation towards under exposure may also be applied in situation when a flash is added for the main subject. one may want to underexpose the background for deeper colours and more depth. The under exposure on the subject may be compensated by the over exposure on the flash. So the flash will throw in more light, and the main subject would not be under exposed.

The following pictures have been shot against the light in the train compartment which is relatively having less light. Since the flash and reflector were not used, exposure compensation was the only possibility. Although evaluative metering was used which takes in to account various parts of the image still the subject is under exposed due to brighter background.

shot with canon eos 5d - Lens EF24-105mm f/4L IS USM - on Aperture priority mode with Evaluative metering, Auto white balance without Flash, at ISO 200

Tv( Shutter Speed ) 1/100  Av( Aperture Value ) 4.0  Exposure Compensation 0  Focal Length 24.0 mm

Although backlit, since the face is filling up most of the area it is not underexposed.
Tv( Shutter Speed ) 1/25 Av( Aperture Value ) 4.0 Exposure Compensation 0 Focal Length 70.0 mm

Tv( Shutter Speed ) 1/50 Av( Aperture Value ) 4.0 Exposure Compensation 0 Focal Length 24.0 mm

Tv( Shutter Speed ) 1/100  Av( Aperture Value ) 4.0 Exposure Compensation 0 Focal Length 24.0 mm

Tv( Shutter Speed ) 1/30 Av( Aperture Value ) 4.0 Exposure Compensation +1
Focal Length 24.0 mm

Tv( Shutter Speed ) 1/15  Av( Aperture Value ) 4.0 Exposure Compensation +1 Focal Length 40.0 mm

Tv( Shutter Speed ) 1/20 Av( Aperture Value ) 4.0 Exposure Compensation +1 Focal Length 40.0 mm

Tv( Shutter Speed ) 1/25  Av( Aperture Value ) 4.0 Exposure Compensation +1  Focal Length 24.0 mm

Tv( Shutter Speed ) 1/40  Av( Aperture Value ) 4.0 Exposure Compensation +1 Focal Length 24.0 mm

Besides this being a snow covered scene, the exposure is same as suggested by the Camera. One may debate that the snow is white and not grey but then if this is made a little brighter the detail would be lost. Moreover, this is going with the feeling of cold as it was at that time. Canon EOS 5D Mark II  / 24 mm 1/125 at f/8 Aperture priority ISO 200 0 EV 

Canon EOS 5D 200 mm 1/125 at f/4 0 EV Aperture priority ISO 200

Since the indoors is dark and the outside is bright, the camera is reading the exposure from outside as much more light is entering from outside. However, the photographs would not be the same, of exposure was compensated to have more detail of the two unknown persons.
Canon EOS 5D Mark III  70 mm 1/200 at f/11 Not fired, compulsory mode 0 EV Auto Aperture priority ISO 160 Pattern

Although the light is coming from the right, towards the back of the subject but it is not rendered dark as the light is warping around due to reflections around. In the photograph above, the two people were inside so there was very less light reflected back. In this case, on the summit floor of Eiffel tower in Paris, there is still enough ambient or available light that the subject is almost well exposed, as much as it should be for an against the light shot.  
Canon EOS 5D Mark III 70 mm 1/125 at f/8 0 EV Auto Aperture priority  ISO 100  Pattern

Shot on a Boat ride at Alleppey, Kerala, The boats man would have been dark anyways dues to the bright sky. So i decided to under expose the image by one stop to get a deeper sky instead of trying to get the detail of the person and the boat.
Canon EOS 5D Mark III 17 mm 1/100 at f/5 -1 EV Aperture priority ISO 400 Pattern

Canon EOS 5D Mark III  24 mm 1/320 at f/4 0 EV Aperture priority ISO 500 Pattern

Assessing the degree of compensation

Photographers calculate the degree of compensation from MIE in a variety of different ways. The method chosen is often dictated by whether speed or accuracy is required.

Digital histogram - Most DSLR cameras and high-end prosumer digicams allow the user to view a ‘histogram’ of the exposure immediately after capture and/or indication of highlight clipping (overexposure). In the case of many of the prosumer cameras the histogram can be viewed live with the preview before capture takes place. This is now the most popular method for assessing whether exposure compensation is required when capturing images with digital cameras. It is worth noting however that DSLR cameras shooting in the RAW format are capable of capturing a broader dynamic range than the histogram may indicate (see the chapters Exposure’ and ‘Camera RAW’). Although this is a reliable method for assessing appropriate exposure compensation it does not replace some of the traditional methods where compensation must be immediate and reasonably accurate.

Bracketing - The photographer can estimate the necessary compensation by bracketing the exposures. To bracket the exposure the photographer must expose several frames, varying the exposure in 1/3 or 2/3 stop increments either side of the MIE.

18% Gray card - Photographers can use a midtone of known value from which to take a reflected light meter reading. A midtone of 18% reflectance is known as a ‘gray card’. The gray card must be at the same distance from the light source as the subject. Care must be taken to ensure the shadow of neither the photographer nor the light meter is cast on the gray card when taking the reading. When capturing in JPEG make sure the indicated exposure is suitable for an SBR not exceeding 32:1. If highlight or shadow detail is required the exposure must be adjusted accordingly. When capturing RAW files the indicated exposure is suitable for an SBR of approximately 128:1 or greater.

Caucasian skin - A commonly used midtone is Caucasian skin. A reflected reading of Caucasian skin placed in the main light source (key light) is approximately one stop lighter than a midtone of 18% reflectance. Using this knowledge a photographer can take a reflected reading from their hand and increase the exposure by one stop to give an exposure equivalent to a reflected reading from an 18% gray card. Adjustments would be necessary for an SBR exceeding the latitude of the image sensor.

Re-framing - If the photographer is working quickly to record an unfolding event or activity the photographer may have little or no time to bracket or take an average midtone reading. In these circumstances the photographer may take a reading quickly from a scene of average reflectance close to the intended subject. This technique of re-framing may also include moving closer to the primary subject matter in order to remove the light source and the dominant light or dark tones from the framed area. Many modern cameras feature an exposure lock to enable the photographer to find a suitable exposure from the environment and lock off the metering system from new information as the camera is repositioned.

Judgement - The fastest technique for exposure compensation is that of judgement, gained from experience and knowledge. The photographer must previsualize the final image and estimate the degree of compensation required to produce the desired effect. 

Many cameras include a feature called auto exposure bracketing. With this control, you set up your camera to take a series of photos (most often three, though some cameras do more) where one is at the exposure chosen by the meter, one is at less exposure, and one is at more exposure. You choose how much of a change there is between exposures. Most of the time, you will find that a half to full stop or step change between exposures works well. 

There may be times when you want to shoot with more than the exposure compensation available on your camera. In those cases, check to see what exposure the camera is setting, then choose the manual shooting mode with an exposure more or less than the camera meter system recommends. Check your histogram to confirm the exposure.