Aperture

One of the most difficult things to understand about photography is the aperture. The aperture may also be referred to as the iris or the diaphragm.

Simply, a photographic aperture is the means of controlling the amount of light that strikes the sensor (or film). Aperture is basically a fancy word for a hole.


Exposure Control

The main use of the aperture is to control exposure. As you probably know, the smaller your aperture is set, the less light it will allow through. However, a smaller aperture is represented by a larger "f-number." I'll go into more detail on "f-numbers" or "f-stops" later. For now, just know that an aperture of f/1.4 is much larger than an aperture of f/22.

In early box cameras, such as the Kodak Brownie, a photographer had the ability to choose between two apertures: one for daylight (a small aperture) and one for darker subjects (a larger aperture). One would literally push or pull a small metal tab on the camera body to move the correct hole into place behind the lens.

Today, the aperture is much more advanced but still operates on the same principal.

The lenses on modern SLR cameras have what's called an iris diaphragm. The iris diaphragm is a mechanical means of creating an aperture. Because photographers face widely varying lighting conditions, from very bright to very dim, a mechanical iris diaphragm is an efficient method to change the aperture.

The iris diaphragm uses "blades" to create an opening that can be made larger and smaller.


Depth Of Field


Another effect that the adjustment of the aperture has on the final image is the depth of field. I'll go more into depth on DoF in another article but here's the gist.

Below is a little mock-up of a simple studio setup. The camera is in the center, our subject stands right in front of it. The field of view (what the camera can see) is represented by the red lines.

In the first image, the green lines represent our field. The field is the area in front of the camera that is in focus. So, the depth of field is the depth of the area that is in focus. In this diagram, the depth of field is large. The subject could move forward or backward and still remain in focus.
To achieve this, the aperture is set to a high aperture value (small aperture size).

© Nic Neufeld/Photo Basics

In the next diagram, we see a shallow depth of field. If the subject were to move forward or backward, he would be out of focus.

© Nic Neufeld/Photo Basics

The depth of field also varies depending on focal length. Further, the aperture has a direct affect on the appearance of the bokeh. I'll get to both of these at a later time.