To get a picture, it is just a matter of allowing the perfect amount of light to reach the sensor. Do that, and you’ve got something that is properly exposed. That’s it.

There are three ways to control the exposure.

  • Shutter Speed: the amount of time that the shutter is open. When the shutter is closed, no light reaches the sensor. This is measured in seconds or fractions of seconds, a smaller number is faster, allowing less light to reach the sensor.
  • Aperture: the size of the hole in the back of the lens that light must pass through to get to the sensor. This is measured in “f-stops”, larger numbers are a smaller hole, allowing less light to reach the sensor.
  • ISO Speed: the sensitivity of the sensor. Higher ISO numbers increase the sensor sensitivity, allowing it to be exposed with less light. That is, the higher the number, the less light that is needed.

What does this all mean?

Well, in the most simple case, let’s say you have a picture that is just a little too dark. How do we make it less dark? We can:

  1. slow down the shutter speed, which allows in light for a longer period of time
  2. make the aperture wider (smaller f number), allowing in more light in a given period of time
  3. boost the ISO, making the sensor more sensitive to a given amount of light

Each of these options has trade-offs, so which option (or combination) of options you choose varies based on the conditions. To make that choice, you need to know what each choice does, and what the trade-off is.

Shutter Speed<

A faster shutter speed allows in less light. However, a picture is a “snapshot of time”, and the timeframe it captures is the time that the shutter is open. The longer the shutter is open, the more movement that gets captured. If the shutter speed is “too long”, then the subject movement results in blurring. This can be good (water movement looks particularly nice with motion blur) or bad (kids don’t look good with blurry heads). If the shutter speed is low enough, even the movement of the camera itself shows up in the picture—the entire image is a blur of movement.

ISO Speed

A camera is most happy at what is called “base ISO.” The base ISO is the slowest (smallest ISO number) speed that the camera operates at. In the case of the D40, the base ISO is 200. ISO 400 is “twice as fast” as 200, meaning it takes half as much light to generate the same exposure. However, as the ISO is increased, the sensor is working harder to derive what is in the light it receives. This appears in the picture as increased noise and loss of detail. As a result, boosting ISO should be considered a “last resort” when other options are ruled out.

Aperture

I left aperture for last because it is the most complicated. While the shutter speed and ISO level are functions of the camera, aperture is a function of the lens. The key thing to realize here is that the f number is defined as a relationship between the focal length of the lens and the entrance pupil, that is the diaphragm of the lens as seen through the lens. The diaphragm is the little bladed whatsit in the back of the lens that controls how much light passes through it. It is actuated by a little lever on the back of the lens. Since the f number is defined as a relationship, all lenses set to the same f-number allow in the same amount of light. This means you can treat a $5000 300mm lens the same way you treat a $200 lens, at least when it comes to understanding how much light gets through the lens.

Confused yet? It gets worse. Lenses are sold with names like “Nikon 35mm AF-S f/1.8G.” What does this mean? Basically, it means that the lens has a 35mm focal length (which has little to do with the physical size of the lens), and that the ratio between the focal length and the width of the entrance pupil is at most, 1.8. Since smaller numbers are wider, that means this lens has the ability to gather more light than say the “Nikon 300mm f/2.8.” Since the diaphragm can be changed by the camera mucking with the lever on the back of the lens, this is the maximum aperture. The minimum aperture, typically something like f/22 is the same ratio, except measured when the diaphragm is completely closed.

Oh, but there is more. Light does funny things. A lens reshapes light, bending it in various ways. The diaphragm limits light by limiting where the light can come from. As the aperture is reduced (f number is increased), the light is increasingly restricted to come only from directly in front of the camera. Think of the incoming light in the shape of a cone, with the tip of the cone at the back of the camera. Wider apertures make a wider cone, allowing light to come in from more places. This is where physics get involved.

There are two aspects of aperture that need to be considered: Depth of Field and Refraction.

Depth of Field is a measure of how much of what you are taking a picture of is in focus. Depth of field is the result of the relationship between the size of the sensor, the focal length of the lens, the aperture, and how far away you are from what you are focusing on. It’s all quite complicated.

Fortunately, you don’t have to understand all of it. Just remember that when you are focusing on something, you are telling the lens to focus to a particular distance away. Large depth of field means that stuff that is not at the current focal length, but close enough is also in focus. The larger the depth of field, the more stuff that stays in focus.

Depth of field shrinks as:

  • you focus closer (close stuff has a smaller DoF than far away stuff)
  • you increase the aperture (wider aperture has a smaller DoF than narrow aperture when focusing on the same thing from the same spot)
  • you change the focal length (90mm has a smaller DoF than 35mm when focusing on the same thing from the same spot)

What does this all mean? A wide aperture brings in more light, but if you get wide enough, you might discover that great picture of a kid has his ear in focus, but not his nose.

On the opposite end is refraction. If you stop down the lens enough (that is, set the f-number to a high value), then the light actually starts to bend around the diaphragm. This shows up as a strange fuzziness in the picture.