The Exposure L – Understanding the Correlation Between Shutter and Aperture
Numerous people, myself included, have written about the exposure triangle in order to explain how you need to balance ISO, Shutter Speed, and Aperture to get a good exposure. The confusing part is that using a triangle to explain this leads people to believe that a good exposure is an equal mix of shutter speed, ISO speed, and aperture, but this actually isnâ€™t a very clear way of looking at it. The problem is you canâ€™t actually draw a triangle and map what an exposure will be as any one or two of the other variables change. Instead of trying to use a triangle to explain it, we are going to simplify it like they used to do back in the days of film.
Whaaaat, Did he Say Film?
How can anything having to do with shooting film apply to an ultra-modern uber-pixel state-of-the-art modern and very non-film DSLR? When shooting film, you would install whatever ISO speed film and thatâ€™s what you were stuck with. The only thing you could adjust is aperture and shutter speed. With a current DSLR we do have the convenience of being able to dynamically adjust the ISO speed. While ISO does play an important role in the overall exposure, we generally do not adjust the ISO simply to increase or decrease or exposure. Instead, ISO Speed should be thought of as a way of getting us into a usable range where the Aperture and Shutter can then work properly, just like changing the film in your camera.
The Exposure L
So letâ€™s look at the Exposure L and see what it is telling us. Along one axis we have the common shutter speeds in one stop increments, along the horizontal axis we have the different aperture settings, also in one stop increments. Now, so long as we are using an ISO Speed setting that will allow us to get a good exposure somewhere on the chart. Once we have a point on the chart that will deliver a good exposure, we can then know exactly what other shutter/aperture combinations will also deliver a correct exposure by drawing a diagonal line between the points as shown on the chart.
For example, if we have a good exposure at f/16 and 1/250th of a second, we can also use the following:
- f/11 & 1/125th
- f/8 & 1/250th
- f/5.6 & 1/500th
By understanding what other aperture / shutter combinations will give us the same exposure, we can then make an artistic choice based on what kind of image style we are trying to create. If we want a shallower depth of field, we can go down a stop, but we have to compensate for moving up a stop in shutter speed.
Only if the scene is too bright or too dark to get an exposure somewhere on our chart do we need to change the ISO Speed setting.
If we look at some example images we will be able to see that once we have a usable exposure we can then slide diagonally up or down the chart to maintain that exposure.
As you can see from the example above, we slide diagonally down the scale so our exposure never changed so long as we adjusted the aperture completely opposite from how we set the shutter speed. As we adjust one up, the other has to go down at the same rate in order to maintain exposure. The effect is that our depth of field gets shallower and shallower as we open up the aperture.
Here is another example using subjects that are further apart.
In the last sample set we started with a large aperture and as we closed it down to increase our depth of field, we had to slow down the shutter speed accordingly in order to maintain the correct exposure. The only time we need to adjust the ISO is if the scene is too bright or too dark to get a good exposure within the range of settings the camera can handle.
If we look at our Exposure Chart and have shutter speed on one axis and aperture size on another axis, the ISO setting is not another axis like a triangle, it actually is a third dimension which can add or subtract overall brightness to the scene.
Hopefully this makes more sense than all the articles out there that talk about the â€œExposure Triangleâ€ because a good exposure does require balance of Shutter and Aperture, but ISO speed is a completely separate variable altogether.