What is “F” and why is it stopped?

In my estimation the f-value is one of the most confusing concepts in photography. F-value is the unit of measurement of the cameras aperture- the opening inside your lens that lets light through the lens. Inside your cameras lens is an iris that dilates and constricts, restricting or increasing the light flow through the lens. This iris is called the aperture. Understanding the aperture is one of the basic principles that have to be grasped to take great pictures. The f-value is a measurement of the ratio of the diameter of the aperture to the length of the lens. If you are interested in the technical details, a Google search will provide a whole lot of data about the how’s and why’s of this number. I am not sure that it is necessary to understand the reasoning behind the f-value, as long as you understand what the f-value means. It is the measurement of how open the lens aperture is. Many photographers refer to this measurement as the “f-stop” and they will talk about any adjustments they make in the exposure in terms of full stops, half stops and sometimes third stops- though the only short stops on your camera are the ones on your LCD. What is meant by a full stop is a change that either doubles or halves the light read by the cameras sensor.

full stopsThe standard f-values on point and shoot digital cameras range from f/1.8 at the wide end to f/16 at the narrow end. Most cameras that allow the user aperture control offer aperture settings in 1/2 or 1/3 stop increments. When shooting, you need to be aware of how changing the aperture settings changes your exposure. You should have some idea of how much light is let in or closed off as you change the f-value. Since a full stop change means the light is doubled or halved, you should know what aperture settings are considered full stops. There is no immediately apparent pattern to f-values, so the easiest way to remember full stops is memorize them Here are the full stops (and yes I typed them in from memory), f/2; f/2.8; f/4; f/5.6; f/8; f/11; f/16; f/22; f/32. Here’s how that works inside the camera. When the aperture is opened from f/5.6 to f/4, twice the light is able to come through the lens and hit the sensor. If it is closed from f/2.8 down to f/4, the aperture is constricted blocking off the light so that now half the light reaches the sensor. Each full stop change doubles or halves the light flowing through.

How does this come to play in the real world? The worst places in the world to shoot are school gymnasiums. The lighting is generally pretty poor and often times the action is very fast. To capture those moments, you need as much light hitting the sensor as possible. Open up the aperture as wide as it can go and you greatly increase your chances of getting a bright and sharp image. This is one of the reasons you see professional sports photographers with those huge lenses. If it was all about lens length, they could just mount a telescope on the front of their camera. Because they need a lot of light coming in quickly, they have lenses that open wide enough to give them an aperture of f/2.8 or larger. Any time you are taking pictures of animate objects in dim light, you are probably going to need to open the aperture.

Aside from the general flow of light, aperture exerts a great influence over two other things.Both these things greatly influence the artistic aspect of the photography, depth of field and light from the flash. Depth of field is the amount of the third dimension in the picture that is in focus. The depth of field is what makes a nice blurry background behind someone in a portrait and what makes a landscape image sharp from foreground to background. (Depth of field is influenced by more than just aperture, but you will have to read the upcoming article on depth of field to find out what). A larger aperture, which is indicated by a smaller f-value, creates a shallower depth of field. A smaller aperture, which is indicated by a larger f-value, helps create a larger depth of field in an image. This is most noticeable in macro photography, where the depth of field is already very narrow.
african violet f-5african violet f -16

Consider the two violets, the one on the left was taken with the aperture as wide as it would go on my macro lens, f/5. You can easily see the focal plane is not even deep enough to get the whole yellow part (I think its called an anther) in focus. The second image was taken with an aperture of f/16, over three stops smaller (which means three times less light is coming through the lens), and the whole flower is in focus, from the tip of the yellow do-hickey to the base of the petals. Understanding depth of field helps you to take control of the backgrounds of your images, giving you the ability to turn backgrounds into textures of color and light or leaving them clear and sharp all the way back.

The aperture setting also exerts great control over the impact of flash lighting in your image. A smaller aperture will restrict the influence of the flash over the image. Here’s why, when your flash lights, it does so for an incredibly brief fraction of time, usually much less time than your shutter is open. The impact of the flash on your picture has very little do with the shutter speed. Instead, it is controlled by how much of the flash light hits the sensor (aperture) and how sensitive the sensor is to that light (ISO). Lessening the influence of the flash by closing down the aperture is going to give your picture a greater feel of natural light. Opening up the aperture will help increase the light received from the flash to brighten up the subject and darken the ambient light.

Your camera’s aperture is a marvelous tool to control the light, feel and composition of any image. Don’t be afraid of its sometimes confusing measurements and terminology. Take charge of your aperture. Understanding and properly setting the f-stop will go along way to taking your photography to another level.


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