ISO: The image sensor’s sensitivity to light
Shutter Speed: The length of time the shutter remains open to allow light to strike the image sensor
Aperture: The maximum size of the lens opening
In each of those articles, I alluded to the fact that each of these elements are not isolated. With your digital SLR camera each element effects the other and when you can begin to grasp how they relate to one another, you will be well on your way to more creative freedom. In attempting to understand exposure, it is best to look at ISO, shutter speed and aperture as “The Exposure Triangle”. The triangle brings these three exposure elements together whereby if you change one element you will have to change another element in order to keep the overall exposure constant.
The Exposure Triangle states that in order to maintain the same exposure, any change in one element of exposure (ISO, shutter speed or aperture) must be compensated with an opposite change in any one of the other elements of exposure.
As a simple example, once you have established the “correct” exposure, if you were to then move to a faster shutter speed which reduces the amount of light hitting the image sensor, in order to now get back to that same “correct” exposure you would then need to increase your aperture which compensates for the loss of light from the faster shutter speed by allowing more light to hit the image sensor.
- For each full stop increase in ISO (doubles the amount of light) —–> increase shutter speed one full stop OR decrease aperture one full stop (both halve the amount of light)
- For each full stop decrease in ISO (halves the amount of light) —–> decrease shutter speed one full stop OR increase aperture one full stop (both double the amount of light)
- For each full stop increase in shutter speed (halves the amount of light) —–> increase aperture one full stop OR increase ISO one full stop (both double amount of light)
- For each full stop decrease in shutter speed (doubles the amount of light) —–> decrease aperture one full stop OR decrease ISO one full stop (both halve the amount of light)
- For each full stop increase in aperture (doubles the amount of light) —–> increase shutter speed one full stop OR decrease ISO one full stop (both halve the amount of light)
- For each full stop decrease in aperture (halves the amount of light) —–> decrease shutter speed one full stop OR increase ISO one full stop (both double the amount of light)
The Exposure Triangle at work
While you should understand each of the relationships above fully, there will be no need to carry around a “cheat-sheet” to try and remember them all! Now here comes the beauty of digital SLR cameras – even in manual mode they still do a lot of the work for you! Here’s how:
The image below shows a typical in-viewfinder exposure level indicator you will find in your digital SLR camera. The numbers to the right of the zero represent an increase in exposure where “1” is an increase of 1 stop and “+2” is an increase in 2 stops. Similarly, the numbers to the left of the zero represent one and two stop decreases in exposure. Each of the dots between the numbers represents 1/3rd of a stop. Also note that the small black square under the scale will tell you where your exposure is set by sliding to the right or left under the scale. In the image below, the exposure is “correct” at ISO100, 1/60, f8
Let’s suppose the camera’s built in light meter has given us this exposure because we are not in a very bright area and we feel that a shutter speed of 1/60 is going to be too slow and that f8 is not going to bring enough of the scene into focus. Given the low light area we are in we therefore go ahead and change the ISO from 100 to 400. This makes the image sensor more sensitive to light therefore capturing more light at the same shutter speed and aperture. Because we are now capturing more light, this therefore makes us over-exposed by 2 stops as shown by the exposure level indicator below:
We now need to correct for this over-exposure. We can do this by playing with the shutter speed dial until our small black square under the exposure level indicator moves back to zero. If you were to do this you will find that the shutter speed will increase to 1/250 thereby reducing the amount of light to hit the image sensor:
Now we are happy with the shutter speed, but we are still not happy with the aperture which we find is reducing our depth of field and not bringing enough into focus. So we go ahead and adjust the aperture dial to f16 and find the level shows underexposed by 2 full stops because we have reduced the amount of light getting to the image sensor:
Now we’re happy with both the aperture and the shutter speed but we need to correct for the underexposure. Since we don’t want to move the aperture or shutter speed, the only option is to adjust the ISO. Because we are now not allowing enough light into the camera, we have to increase the ISO, making the image sensor more sensitive to light and therefore allowing it to capture more light without changing the shutter speed or aperture. In doing this we find that the ISO moves to from 400 to 1600, bringing the black square under the exposure level indicator to the zero mark signifying “correct” exposure:
We can see from this simple exercise of moving each separate exposure element around, that the overall exposure was affected but could be corrected by opposite changes in other elements. And we can see that by using the digital SLR camera’s built-in light meter and exposure level indicator we could make the necessary corrections with ease.
This example shows that in understanding exposure it is important to understand “The Exposure Triangle” and how ISO, shutter speed and aperture come together to effect each other. In order to maintain the “correct” exposure, any change in one element needs to be compensated with an opposite change in one of the other elements. Now this is why you purchased your digital SLR camera, is it not? To allow for more creativity?! By adjusting the shutter speed we can better freeze or express movement. By adjusting aperture we have play with depth of field thereby either highlighting a small element of our subject or expressing the grandness of our subject. But in doing so, we now know how to get back to that “correct” exposure! Once you start understanding exposure in this way you are well on your way to increasing your creativity!
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