Aperatures and f-stops

The aperature is the opening through which light passes through the lens and onto the film. The size of this opening is adjustable to alow more/less light through the lens depending on the circumstances. You control the size of the aperature by selecting an f-stop. The use of the term stop is purely historical as early cameras had several metal plates with holes of different diameters called stop plates. When the size of the aperature is decreased you are stopping down.

Here's the confusing part, each f-stop lets in exactly half/twice as much light as the f-stop next to it. However, they are usually numbered f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64. Depending on what type of lens you have, you'll probably have a different range of f-stops. They seem totally unrelated and give no clue as to their relationship. The number stems from a mathematical relationship which isn't important right now. Just try to remember that as you open the aperature up one stop you are letting in twice as much light (the area of the aperature becomes twice as big), as you stop down the opposite happens. Another easily confused fact about f-stops. f/1.4 is the largest aperature from the above list. As the value of the f-stops increase the aperature decreases. It is an inverse relationship. f/64 is a very small opening and actually has 1/2000th of the area of f/1.4.

A lens can either be called fast or slow. A fast lens is one that is capable of opening to a much wider aperature. These terms are relative of couse and depend on your application/oppinions/point-of-view.

Aperature in relation to depth of field

Depth of field is the distance within which objects appear sharp. Aperature and depth of field have an inverse relationship as well. As you increase the size of the aperature, the depth of field decreases to a point where only objects at a specific distance from the camera will appear sharp. As you decrease the size of the aperature you will notice that more and more of the objects in the image (from near to far) will appear sharp. The tradeoff however, is that as you decrease the aperature you must naturally increase the exposure time (shutter speed) to achieve the same effect on the film. This could easily result in a blurred image if you are shooting something in motion or are not using a tripod, etc.

Using your lens to figure out depth of field

Most lenses of manually operated cameras have two other sets of markings that would look something similar to the figure below depending on the lens.

The bottom most ring selects the f-stop. The top most set of numbers is a group of distances (usually in both feet and metres) that moves with the lens. Assuming that the distances above are measured in feet, you will notice that if you set the aperature to f4 all objects between 3 and 4 feet will appear sharp. If you reduce the aperature size to f11 however, everything between 2 - 5 feet will appear in focus.

The depth of field for each aperature is not a constant. As you focus on objects further and further away the range (in distance) for each aperature increases. At the end of the distance scale you will notice the symbol for infinity. If you can include this symbol within the f-stop range markers on your lens then you will be able to focus on eveything from the distance above the left f-stop range marker to infinity.

In this example you will notice that if you set your aperature to f/11 everything between 10 feet and infinity wil appear sharp in the image. If everything you want to appear sharp is within that range then there is no need to do any further focussing.

Measuring distance with you lens

You might think it's ridiculous to try to know the exact distance if every object in your image. It is. You can use your lens to find out the distance of the important objects in the scene you are shooting. You will be of course most interested in the closest and furthest object of interest. All you have to do is focus sharply on that particular object and then look at the distance that is marked above the centre arrow on your f-stop range. This will be the distance of that object. Be sure to include that distance within the range of the aperature you are using.

Next Topic: Shutter speeds and ASA/DIN

Return to the Main Page