Tuesday, April 13, 2010

Exposure: Light, Aperture, Shutter Speed, and ISO - Part 1 - Aperture

In the next couple of posts, I'm going to try to explain how light, aperture, shutter and ISO combine to give you the right exposure for your pictures.

Here's how the system works:

fstops700

Light comes into your lens, reflected from the surface of your subject.

Your lens focuses the light.

Inside the lens is a diaphragm.

diaphragm
Picture by Mohylek (Creative Commons License)

The diaphragm is adjustable allowing you to control the aperture that the light passes through. Can you see that the aperture is bigger in the top picture? That means that more light is passing through the diaphragm on its way to the sensor.

When we're talking about the size of that aperture, we use the term f/stop. The number is the ratio of the diameter of the aperture to the focal length of the lens. For example, if we had a lens that had a focal length of 100mm, and the aperture had a diameter of 50mm, then the ratio would be 1/2. The f/stop would be f/2.

If the f/stop was f/8 on that 100 mm lens, then the diameter of the aperture would be 1/8 of the focal length.

(By the way, "focal length" means the optical length of the lens. In the old days, you actually could measure a lens with a ruler and come pretty close to knowing its "focal length".)

So the confusing part is that, as the f/stops values go up, it means that the size of the aperture gets smaller and the amount of light getting through goes down.

diaphragm2
picture by Chris Buckley (Creative Commons License)

This sequence shows, in photographs, how an aperture that is decreasing in size results in less light getting through.

For mathematical reasons that are beyond me, the volume of light passed through is halved every time you go "up" one full f/stop. So, f/2.8 allows half of the light as f/2.0. We went "up" a full f/stop when we went from f/2.0 to f/2.8.

This chart shows how the amount of light is decreased as the f/stops increase in "full" f/stops.
progression1

This is known, by the way, as "stopping down." The f/stop numbers are going up, but you're reducing the amount of light passing through the lens.

To complete the confusion, if I'm going from f/2.8 to f/2.0, I'm doubling the amount of light that is passing through the lens. The f/stop numbers went down, but the amount of light went up. This is known as "opening up."

Here's the summary:

Aperture, which is measured by f/stops, controls the volume of light passing through your lens.

As the f/stop number increases, the aperture gets smaller and the volume of light is reduced.

As the f/stop number decreases, the aperture gets bigger and the volume of light is increased.

And the f/stop number is really a fraction showing the ratio of the aperture to the focal length of the lens.

That's really all you need to remember about aperture for now.

Next, we'll talk about shutter speeds.


Jim Frazier

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