Photography 101.5 – Aperture

Photo: Rainer Ebert used under CC license

The following post is from Australian photographer Neil Creek who is part of the Fine Art Photoblog, and is developing his blog as a resource for the passionate photographer.

Welcome to the fifth lesson in Photography 101 – A Basic Course on the Camera. In this series, we cover all the basics of camera design and use. We talk about the ‘exposure triangle’: shutter speed, aperture and ISO. We talk about focus, depth of field and sharpness, as well as how lenses work, what focal lengths mean and how they put light on the sensor. We also look at the camera itself, how it works, what all the options mean and how they affect your photos.

This week’s lesson is Aperture.

Here’s What We’ve Covered Previously in this series:

Lesson 1: Light and the Pinhole Camera
Lesson 2: Lenses and Focus
Lesson 3: Lenses, Light and Magnification
Lesson 4: Exposure and Stops

In previous lessons we have talked about the basic theory of how a camera works, including some basic optics, and introduced the idea of exposure and how we control it with the exposure triangle. In this lesson we will be drawing upon what we have learned to understand the first point on the exposure triangle – aperture – and how it works to create your photo.

Aperture

Fig 1.5.1 The iris opens and closes to change the aperture.
Based on (source-http://www.camerarepairer.co.uk/Glossary.htm)

The word aperture simply means “an opening” (reference.com). In the case of photography, the aperture is created by an adjustable iris that can be opened or closed to control how much light enters the camera. This iris is made of a series of thin metal blades that move together to create a roughly circular opening of variable size. In most DSLR cameras, the iris is built into the lens itself. It is the opening in this iris that is actually the aperture.

When adjusting the size of the aperture, we describe “opening” the aperture up and “stopping” it down. That simply refers to making the hole wider or narrower. A photographer may say they are “shooting wide”, meaning they have opened the aperture a lot, or they may say they are “stopped way down”, meaning the aperture has been closed a lot.

f ratio revisited

In Lesson 3, we discussed the f ratio, and described that as the focal length of the lens divided by its diameter. This is the focal ratio. For a single lens, the f ratio is always the same. However, with our adjustable aperture, we can do a very neat trick. The aperture acts on the lens as if it is cutting away the part of the lens which is covered. So as we stop down the aperture, we effectively make the lens smaller, and thus change the f ratio of the lens. As such, the size of the aperture is described by the f ratio that it creates. A wide aperture may be f2.8, a narrow aperture may be f22.

As we discussed in lesson four, we measure the change in exposure with stops – a doubling or halving of the light, and fractions of stops. As such, the same measure applies to aperture. To double the light getting through a lens, we need to double the area of that lens which is uncovered. The area of a circle is determined by the formula πr2, so a doubling of the area increases the diameter by approximately 1.41. From this figure we get the sequence of f ratios:

Depth of Field

Fig 1.5.2 A narrower aperture has a much greater depth of field.

If we look again at the exposure triangle diagram in lesson 4, you’ll see that the aperture influences the depth of field. The depth of field is the region of the photo which is in focus when the image is captured. It is a range of distance from the camera where objects look to be in focus. Aperture influences the depth of field by widening or narrowing this range, thus bringing more or less of the photo into focus, based on its distance from the camera.

Recalling what we learned back in lesson 1, the larger the hole that the light passes through a pinhole camera, the blurrier the image will be. When a lens is added to the camera, the image can be brought into focus, no matter the size of the hole, however, objects just either side of the focus point will be affected. If the hole, or aperture, is large, only objects very close to the focus point will be in focus. If the aperture is small, then the depth of the focus field is much wider, and more remote objects will be in focus.

Depth of Field can be pretty complex when you look deeply into it (such as: hyperfocal distance, airy discs and diffraction), but for an introductory course, the most important thing to remember is that a wide aperture (low value) means a shallow depth of field with less in focus and a narrow aperture (large value) means a broad depth of field with more in focus. We may return to discuss the more complicated issues in a future course.

Examples

Creative use of DOF isolates subject from background. Photo: annia316 used under CC license

The highlights on the strings clearly shows the changing DOF. Photo: Paul J. S. used under CC license

A narrow aperture gives a very wide depth of field. Photo: wili_hybrid used under CC license

These images show focus from the closest objects to the horizon. Photo: Neil creek all rights reserved.

Homework

• Find a scene with interesting stuff at many distances from you. Photograph the scene with different apertures (keeping the photo exposed correctly by changing shutter speed), and see the difference in the depth of field.
• Find a small subject that stands against a background a few meters away, like a flower. Try adjusting the aperture to isolate the subject from the background with depth of field.
• Find a view. Try and photograph a vista with something in the foreground, such as a rock at a lookout. Try and adjust the aperture to keep the whole photo in focus.

Resources

• Aperture – At wikipedia.com
• Understanding Depth of Field – At Cambridge in Colour
• Depth of Field Calculator – At dofmaster.com
• Introduction to Aperture – At digital-photography-school.com
• DoF group – At flickr.com