Have you ever wondered if you meter is giving you the right exposure? Do you ever wonder how to figure out the proper exposure yourself? Well it is possible—with the f/16 rule.

The theory is simple. The basic exposure for a camera on a bright sunny day will be f/16 when the shutter speed matches the ISO. That is it … seriously!

If you have a great sunny day, set your ISO to 200 then set your shutter speed to 1/200 sec. The f/stop should be f/16. If you change the ISO then change the shutter speed accordingly. If weather conditions change, then you change the f/stop.

f/22- reflective sand or snow

f/16- bright sunny day

f/11- bright cloudy day

f/8- slightly overcast

f/5.6- heavy overcast or open shade

Now, this is to get a base exposure to work from. After you have your basic exposure you can make adjustments to fit the photograph you are creating.

In situations where you find that you are shooting with the sun on the back of your subject and their face in the shade (and assuming you are shooting a face on portrait), just make an adjustment by opening up two f/stops. So if the base exposure in the f/16 rule tells you that you should be shooting f/11, then open up two f/stops to f/5.6.

Even if you have a great light meter, there is some comfort in having the skills to know that your meter is in the right ball park. When you can work it out in your head, the chances between success and failure of a photograph is significantly reduced.

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.

There are two factors in creating exposure: The time light is allowed in and the amount of light that is allowed in to the camera. The math for exposure is exposure is equal to time multiplied by intensity.

E=T x I

To give a mental image, imagine that you have to fill a cup of water. There are two ways in which you can fill it at the sink: 1) open up the faucet so that a lot of water fills it up quickly or 2) let the water drip for a long time and fill up the glass that way. A camera works in a very similar way: that camera either lets in a lot of light or adjusts the time light is allowed to hit the film plane. 

How the camera measures the time of an exposure is easy. A shutter opens up for a moment and closes. This is called shutter speed. Shutter speeds can vary from the very fast (e.g. 1/8000 second) to very long (e.g. 1 minute).

What is more difficult to understand is how the camera determines the amount of light into the camera. This is determined by f/stop.

In a lens there is a device called a diaphragm (normally a series of adjustable leafs that make a smaller or larger opening in the lens). When more light is needed, the diaphragm is opened. When less light is needed the diaphragm is closed. f/stops measure the size of the opening, which photographers call the aperture.

So why are they called f/stops and how do they work?

First, f/stops aren’t fractions. It is a math equation where the f/stop number (represented as f/8, f/11, f/16 etc.) measures an amount of light. The f/stops are constant measurements. So while a small pocket camera f/8 setting may be a very small hole, and a large format camera f/8 may be a very large opening, the same amount of relative light is hitting the focal plane.

Full f/stops have specific numbers assigned to them. Each full f/stop represents a perfect measurement of light that is either double or half the amount of the next full f/stop. If you open a lens aperture one full f/stop then you have doubled the amount of light from the previous f/stop. If you close the aperture one full f/stop, you have cut the light in half.

The common full f/stops are – f/1, f/2, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/64

Many digital cameras now allow partial f/stops.

Once we know what our exposure should be (that will be covered in a different post), we can make adjustments to the shutter speed and f/stop to get the effects we want. Because shutter speeds are normally set to be in the same doubling principle as the f/stop (i.e. shutter speeds are normally 1 second, ½ sec, ¼ sec, 1/8 sec, 1/15 sec, 1/30 sec, etc.) each setting has the same effect of either adding or subtracting the amount of light to the film plan by a factor of 2.

Remember the formula for exposure (E= T x I). If you take your shutter speed and make it faster (less light), you can proportionally adjust your f/stop to allow in the equal amount of light. Adjust your shutter speed two settings from 1/125 sec to 1/500 sec (skipping by 1/250 sec), you have cut your light by 1/4th (You cut it in half from 1/125 to 1/250, and half again from 1/250 to 1/500). This means you must open up your aperture to allow four times the amount of light in, or two f/stop settings.

In another blog, I will discuss the effects you can create withf/stops.