In a previous blog post, we talked about exposure and how f/stop affected exposure by adjusting the amount of light enters the camera. The f/stop adjusts the opening (the aperture) with a diaphragm (groups of leafs that adjust to make a larger or smaller aperture). However, that is not all that an f/stop does. There are three functions of an f/stop and we will discuss them here.
1)f/stops are half of the control of exposure (the shutter speed is the other half).
2)f/stops allow the photographer to adjust the aperture for the best definition of the lens.
3)f/stops control depth of field.
As we mentioned before, exposure is determined by the quantity of light (intensity) and the length of time light is allowed into the camera. The shutter speed adjusts the time and the f/stop adjusts the quantity.
f/stops are half of the control of exposure (the shutter speed is the other half).
We have also discussed that when light passes through a different medium it changes speed. This means that when light passes through glass it moves slower than when it moves through the atmosphere. If the glass is crafted properly, the light bends in a very predictable way and you can recreate the light to form a focused image on the film plane.
When the light does not directly focus on the film plane that part of the image is represented as fuzzy spots. These fuzzy circles will translate to the image as out of focus. If the lens is focused on a particular point of a scene, there will be an area in front of and behind that focus point which will represent itself as “in focus” and everything closer and farther away will be “out of focus.” The distance of area that is in focus is called “depth of field” or DOF.
What does this have to do with the f/stop? Well, the f/stop determines the angle light that comes into the lens. With a small aperture there is less competition for light to get into the lens, and that will increase the depth of field. The larger the aperture the more light can come into the lens and the shallower the depth of field.
Now remember that the larger the aperture, the smaller the f/stop number. So f/4 will be a larger aperture than f/8. That also means that if you are focused on a single object at f/8 and open up the aperture to f/4 you will cut your depth of field and less of the image will be in focus.
f/stops allow the photographer to adjust the aperture for the best definition of the lens.
Before I go into detail on depth of field, I want to address what I mean by best definition of a lens. While lenses may have an indicated range of f/2.8 to f/32, that doesn’t mean that the lens will perform at its best at all of those f/stops. Some lenses lose their quality when you go to their extremes. While it is not true of all lenses, a good rule of thumb is to close a lens down or open it up at least one stop to start getting the best performance from a lens.
There are some newer lenses that perform amazingly well opened wide open, but you will have to research which lenses these are. Some lenses are designed only to work at optimum performance wide open.
f/stops control depth of field.
Depth of field is defined as the distancefrom the nearest point of acceptably sharp focus to thefarthest point of acceptably sharp focus of a scene beingphotographed. The depth of field isn’t the same at all f/stops and it also changes as the focal length of the lens changes. When you shoot at f/4 on a 50mm lens and a 180mm lens, the depth of field will be different.
Depth of field also changes with distance. The farther the subject is from the camera, the larger the depth of field. There is a focus point (called the hyperfocal distance) when everything behind the focus point will be in focus for infinity.
The depth of field can be measured with a DOF calculator. CLICK HERE
Adjusting the depth of field is an important to photography. It gives you control over your environment and isolate or integrate your subject with its environment.
Using a large depth of field with 14mm wide angle to integrate the environment.
Using a shallow depth of field with an 85mm lens to separate the subject from the environment.