Table of Contents
How Exposure Works
Introduction
As with all articles on this site, I encourage you to read this a little bit at a time and return to it as often as needed.
Proper exposure is everything. If you cannot take a properly exposed photograph then you cannot take a photograph. It is at the end of the day the single biggest aspect of a photograph. I reiterate this idea because I see many young photographers who try to rush for results and do not dedicate enough time to learning the finer points to exposing a photograph properly. As a result, their images may never turn out and their real talent as a visual story teller will never be fully reached.
One of the reasons that learning exposure as I have laid out is useful is because the settings you change to obtain proper exposure always have two accompanying effects to the appearance of the photograph. I like to call this “the dual effects of settings”. This point is key, so tuck it in the back of your mind for a moment.
In hopes that I don’t scare you away, I will break exposure down to four basic principles (3 of which you have complete control over).
This photograph was taken at night, a situation in which there is very little available light.
Available Light
For a moment, close your eyes and then open them. When you close your eyes you see mostly black. This is because your eyelids have blocked off the majority of light that enters your eye. Your eyelids have changed the amount of light that is available to your eye. When you open your eyes you see what is around you. You see this because there is light in the room. But this light can change in intensity. If you turn the lights off in the room, you can likely still see because your body and brain have made adjustments to this loss of light.
This is a simple concept, I understand, but it is important to realize that your camera works much like your eye in that with a change in the available light you must make a change to your camera in order to still see everything. In humans, our brain does this for us automatically, primarily by changing the opening in our eye (the iris). And much like our eyes, our cameras too have a built in iris. We call this the aperture.
Aperture
The aperture is a small iris located in the lens (not in the camera) that opens and closes into bigger or smaller circular openings. The purpose of the aperture is to control the amount of light that is allowed to reach the shutter. A common metaphor for exposure involves a bucket of water with a hole in the bottom. If you think of water as being the light, the hole in the bottom can be thought of as the aperture. If that hole is bigger, more water will pour through it in a given amount of time.
This photograph shows the closed aperture of a 50mm lens.
To come back to a camera, the aperture is a hole in the lens that dictates how much light in a given amount of time comes through the lens, through the shutter, and onto your film or digital sensor.
Please Note
I would like to point out that this is often a setting that beginner photographers do not even realize takes place. Prior to taking a picture, a camera almost always has the aperture open all the way. This allows the most light possible through the camera which helps both the photographer and the camera see the scene. Once the shutter button is pushed, the camera closes (or “stops down”) the aperture to the setting selected.
A quick note before we move on
We have discussed the fact that every scene has a given amount of light. Keep in mind; this is not a hypothetical idea. We know that light is both a wave and a photon. A photon is literally a little particle of energy. So, when the amount of light changes in scene, there literally is a change in the amount of “light particles” (photons) that are available for your camera to capture.
Once this light—these particles—go through your lens, they pass through a hole. This hole is the aperture. And just like molecules of water passing through a hole in a bucket, tiny light particles pass through a hole in your lens. The bigger the hole, the more of these tiny light particles (known as photons) can pass through and eventually reach your film/sensor. Thus, regardless of how much light is available in a scene, your aperture is the single biggest factor in how much of that light is allowed to go through your camera. But for how long do we let that light pass through your camera? Read on…
Shutter Speed
The shutter is a mechanical device in you camera that sits just in front of your film/sensor. The shutter opens and closes when you push the shutter release button. When the shutter opens, light passing through the lens is exposed to the film/sensor. The duration that the shutter is open is referred to as shutter speed. This is a very important concept when exposing a photograph.
To refer back to the bucket metaphor for a moment, let’s pretend that the hole in the bucket allows 1 gallon of water per second to flow through. Now, beneath that hole lets assume there is a little flap that opens and closes. If the water is the light, the hole is the aperture, than the flap is our shutter. So with this flap closed, no water is flowing through but as soon as we open it water will flow through at 1 gallon per second because that is what the size of the hole allows for. But the amount of water that passes through that hole is going to ultimately be determined by how long the flap is open. If you only open it for half a second, only ½ of a gallon has passed through your hole.
The shutter in your camera works in exactly the same way. The aperture setting dictates how much light is able to come through your camera in a given amount of time but your shutter is what determines how much time light is passing through the aperture and therefore the shutter also dictates how much light is passing through your camera and onto your sensor/film. Think of the shutter as the stopwatch of the processes of taking a picture.
You may be thinking to yourself that there is therefore a direct relationship between the aperture and shutter settings, and you are correct. We will get to that momentarily.
ISO:isolating the final aspect of exposure…
You might be thinking that a bucket full of water with a hole in the bottom would create a pretty big mess that serves no purpose. Let me suggest this to you though; what if there was a hole in the bucket full of water because you needed to fill another bucket under it? What if we are just trying to fill another bucket up with water and we’re using the bucket with the hole and the flap to fill it up?
Well, then I would argue that we have completed the water bucket/photography exposure metaphor. If you think of the second bucket as your film/sensor then you may be able to think of your film/sensor as a “light gathering medium”. And that is all that film or digital sensors are. They are there to capture light. However, like a bucket designed to be filled fully with water, too little water does you no good, and too much creates a mess.
Just like buckets can change sizes (and therefore they can change the amount of water they are able to hold) film/sensors can change their sensitivity to light. This is dictated by a film/sensor’s ISO. ISO refers to how sensitive the medium is to light. The higher the ISO number, the more sensitive it is.
Lets take a moment to recap what we've learned so far...
Bringing it all together
A quick recap…
So far we have described a scene that contains a certain amount of light. We have described a lens that has an iris (or aperture) that allows certain amounts of light to pass through in a given time. We have described a device, the shutter, which determines how much time that hole is going to allow light to pass through and hit our film or digital sensor. Lastly, we have described the notion that different films and sensor settings (ISO) allows for different levels of sensitivity to the light that makes its way through the shutter. I hope you can begin to see that these four principles are all related to each other.
That is to say, if you are in a bright room, there is more light wanting to pass through the aperture. Therefore, you can reduce that light by closing the aperture. But you could also reduce the amount of time that the light passes through the sensor? Or perhaps you could merely reduce the sensitivity of your film/sensor.
A simplified example:
Let’s pretend that your aperture allows for 2 photons per second to pass through it and let’s say that your film/sensor’s ISO setting requires 6 photons of light. You would therefore need to have your shutter open for 3 seconds to capture the amount of light to obtain the desired exposure. Let’s pretend, though, that we don’t want to have our shutter open for 3 seconds. Let’s say that we only wanted it open for 1.5 seconds. At 2 photons per second for 1 second, our sensor would only see 2 photons instead of the required 6. What can we do? We could set our shutter to 1 second but instead open the aperture up to allow 6 photons per second to flow through. With this example, I hope you can see that you can adjust any of the 3 settings on your camera to obtain proper exposure (fortunately photographers don’t need to concern themselves about photons). So how do we decide which settings to change? To know this, we must first understand the dual effects of settings.