EXPOSURE COMPENSATION
I will like to add a few words about Exposure Compensation, because I have seen there is lot of confusion regarding this among many people.
People think that when you dial a positive EC value it will somehow “brighten” the image and when you dial in a negative value, it will lower the amount of light entering the image.
But you have to stop and think why is this happening?
Do note that the amount of “exposure” in an image depends only on 3 factors: Shutter Speed, Aperture and ISO. These 3 things and only these 3 things determine the overall exposure.
So where does exposure compensation come from?
Exposure compensation is closely interlinked with “metering”. I will not elaborate “metering” in great details here, because that itself will amount to another separate post altogether, which not the topic of this discussion. I will only add the parts which are relevant.
Metering in simple words means the following: The camera tries to analyse the scene on its own and suggests values of shutter speed and aperture (provided you have set your ISO) which the camera thinks is ideal for the situation. There is complex algorithm involved and the type of metering chosen (Spot or Matrix etc) behind this decision. Now, metering suggests the values in a way that everything turns to a mid tone. So a completely white image will turn to mid grey, black will also turn to mid grey. Similarly if you fill a frame with dark blue, the camera will try to suggest exposure values for the image to look mid blue.
Now here I used the word “suggest”. It means that some amount of decision is taken by the camera and is not on your hand. So it implies that this thing will work in Automatic or Semi Automatic Modes: Aperture Priority mode, Shutter Priority Mode and Program mode.
I am explaining Exposure Compensation for Aperture Priority Mode now, and similar analogy can automatically be drawn for Shutter Priority mode as well.
When you are in Aperture Priority Mode, you decide the Aperture (and ISO) and the camera guesses the best possible shutter value with the help of metering so that the metered area can be turned into a mid tone. Now imagine you are in situation like ice clad region with lots of white ice. The camera will try to use a faster shutter so that less light enters and will try to make the image a mid tone one. You will not want this. What will you do? You dial some positive exposure compensation value to counter this. What does it do? It asks the camera to use a slower shutter speed of as many stops as you have dialled the EC value so that more light enters the camera. So your image will turn white.
You will see a scale like – |..|..0..|.. + in the screen. Here according to the camera, the 0 position is the ideal position where everything will be mid tone. When you dial in some positive exposure compensation, imagine the location of the 0 shifts that much points to the right and the camera things that as the new location of 0. So the camera will set the value of shutter speed accordingly.
When you are in a scene where everything is predominantly black and you want to preserve that tone, you will have to dial a Negative EC although it may seem opposite to what the common sense suggests. This is because here the camera will try to lighten the black to a mid grey with its metering and if you want the black tone to be preserved, you will have to lower the amount of light entering the camera.
When you are in Shutter Priority Mode, exact explanation prevails, just interchange Shutter Speed and Aperture in the above discussion.
So this Exposure Compensation is no magic that it magically brightens or darkens the image. How much “bright” or “dark” the image will be, rather in proper words, the exposure is entirely controlled by only 3 things shutter speed, aperture and ISO, and the exposure compensation value only controls one of the above thing.
In Program mode, the camera controls both the Shutter Speed and the Aperture, so you won’t be sure which thing the camera will control when you change the EC value from the 0 position. But be assured that new combination of aperture and shutter speed will be such that ‘x’ stop of extra light will enter the image (if you have given + ‘x’ EC) than in the case of 0 EC.
So when stating the EXIF data in an image, there is absolutely no use of giving the value of the exposure compensation dialled. Only Shutter Speed, Aperture and ISO will suffice. By providing the value of EC you are letting us know what the camera was originally thinking which is of no use to us.
Now in your mind the following question will arise:
What about the Full Manual mode? I see a – |..|..0..|.. + in the viewfinder in manual mode. And when I change the EC value, the change is reflected there in the scale too. What about that?
Notion buster:
In manual mode, you yourself set the shutter speed, aperture and ISO, the camera has no control over it and the amount of light entering the camera will depend only on these three, so whatever EC value you give, even if say +3, there is no magic by which it will allow more light to enter your camera.
The scale you see in the manual mode is just the work of the metering system of the camera to “help” you by giving you an idea of what the camera is thinking the correct exposure will be.
Suppose in Manual Mode you set Shutter Speed: 1/125, Aperture: F/5, ISO 200 and suppose the camera meter shows 0. Now if you dial +1 EC, the camera will now meter according to the new 0 to be set at +1. So it will think that the combination of the 3 things you have chosen actually will darken the photo by 1 stop. So in the scale it will show -1.
I hope you get the point.
If you have any more doubts feel free to ask.
Source: *sounava.wordpress.com/2011/07/03/exposure-compensation/
