Photography is all about light. Without light you have no picture. And getting the proper amount of light is what exposure is all about


Light and Exposure

In digital photography there are three factors that affect "exposure," and ISO is not really one of them (I'll explain that later). What does affect the digital exposure is the strength/quantity of  light, the shutter speed (how long we let the light in), and the aperture (how big of an opening we let the light in through).
The shutter speed affects image sharpness and motion blur by controlling how long the motion of the subject or camera is recorded.
The aperture affects sharpness and depth of focus. Depth of focus is normally noted as DOF or "depth of field," but it's really "Depth of Focus Field" or how much of an image is acceptably sharp. It's easiest to think of larger aperture numbers meaning more depth of focus at the expense of requiring more light or longer shutter speeds.

Here I have laid out the three factors in the shape of a rectangle. The height of the rectangle represents how strong the light is, and we cannot change that without adding light to the scene (i.e. using flash, time of day, etc). The two factors we *can* affect are aperture and shutter speed, indicated here in stops of light. And in this diagram we have a total of 12 stops of light being recorded (the area of the rectangle).

In this example we have reduced the aperture size (more sharpness/DOF) and reduced the shutter speed (more motion blur) in order to keep the same 12 stops of light. The size of the rectangle has not changed so the image brightness has not changed.

 And in this example we have reduced the shutter speed (sharper/less motion blur) and increased the aperture size (less lens sharpness/DOF) in order to keep the same 12 stops of light. The size of the rectangle still has not changed.

In this example we have increased the shutter speed without compensating with the aperture resulting in only 8 stops of light being recorded. Now the size of the rectangle has changed and the resulting image would be darker.

And in this example we have increased the size of the aperture (smaller aperture number) and reduced the shutter speed resulting in 18 stops of light being recorded. The size of the rectangle is larger and the resulting image would be much brighter than in the previous example.

So that's how the actual digital exposure works, but it is *not* how your exposure meter works.

The exposure meter also includes ISO. With film photography ISO represented the reactivity/sensitivity of the film to sunlight; it was a direct factor in the film image. If the sensitivity of the film we had was not adequate we would "push" or "pull" the film by exposing it (shutter speed/aperture) as if it had a different sensitivity. And then additional/different processing was required at a cost of dynamic range and noise.
With digital exposures ISO is more like pushing/pulling film and it has the same potential negative side effects. I like to correlate it as being a "brighness control" for the camera/computer display. With digital, ISO is amplification of the exposure as recorded above, amplification of both the noise and the signal. Changing the ISO does *not* change the camera's sensitivity/reactivity to light. And the affect of higher ISO's is not much different than increasing the brightness of a raw image with software after the fact.

I'm emphasizing this because photography is about light. And the decision as to when, how, and if a picture is taken should be based upon the light. This is what the very best photographers know and consider. And yet it is almost never discussed.
ISO is only about making the camera's meter (and you) happy. And that brings us to exposure metering and the exposure traingle. 

The Exposure Triangle

Above is an example of a viewfinder display. The shutter speed is displayed as 800 (1/800 of a second). The aperture (f-stop) is 8.0 (f/8). And the ISO is 200.
The meter needle is centered which means the camera believes the settings are right for the image. As long as the meter is centered when using the correct Metering Mode the exposure is correct.

Exposure metering is the combination of Aperture, ISO, and Shutter Speed, these three factors make up "the Exposure Triangle."  We will go into the details of these individual factors in separate articles, but let me try to make sense of the triangle.

I have never seen it explained particularly well, so I'll give it a try. Why is it even called a triangle? It's called the Exposure triangle because all three points are connected. If you move one you must move at least one other, but you could move all three. I'll use some more graphics to demonstrate.


I have laid out the three factors as they relate to each other in "stops of light". In this example the triangle occupies 18 stops of light (6 on each axis) to make a properly exposed image (note that the actual light is 12 stops). These are the settings the camera meter determined to be correct for this image. Now, we can change the shape of the triangle in any way we like, but it must always occupy a total of 18 stops in order to keep the exposure the same.

Here we have increased ISO and reduced Aperture size but the total is still 18 (actual light 10 stops).


Here we have Increased Shutter Speed time (longer), increased Aperture size, and reduced ISO. The total is still 18 stops (actual light 16 stops).

The total stops of light that need to be recorded is probably not going to be 18, but the numbers don't really matter. The camera will give you the starting point for a proper exposure. The only thing that matters is that if you move one setting you have to change something else in order to keep the exposure the same.
Also note that while all three examples contained 18 stops total; the first example contained 12 stops of actual light, the second contained 10 stops, and the last contained 16 stops. Because of this, the last image will give the best sensor performance resulting in the lowest noise and the greatest dynamic range/color. 

Below is a flash interaction based upon the "sunny 16 rule". The "sunny sixteen rule" is from the days of film and a lack of built in light meters. On a sunny day with hard shadows (light level set to maximum here) if the aperture is set to f/16 the ISO and shutter speed (SS) will be the same numbers (as close as possible) when the image is properly exposed. It's not something you really need to know.

This activity shows how each of the factors affect the others and the overall exposure. Spend some time playing around with it until you get the general idea.



As you may see, one stop of light is a big deal sometimes. When you get to the usable limit of ISO, or run out of usable shutter speed, the only option left is a faster lens. That's why optically excellent wide aperture lenses are so expensive and the chosen tool of the pro. It is also part of why the 35mm and 50mm primes are so popular.

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